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A new generation of subnanometer-sized materials reveals a general
surface polarons property: The recent advent of cutting-edge experimental techniques allows for a
precise synthesis of monodisperse subnanometer metal clusters composed by just
a few atoms, and opens new possibilities for subnanometer science. The
decoration of titanium dioxide surfaces with the Ag$_{5}$ atomic cluster
enables the stabilization of surface polarons. A new electron polarization
phenomenon accompanying surface polaron formation has thus been revealed. | cond-mat_mtrl-sci |
Current-constraining variational approaches to quantum transport: Presently, the main methods for describing a non-equilibrium
charge-transporting steady state are based on time-evolving it from the initial
zero-current situation. An alternative class of theories would give the
statistical non-equilibrium density operator from principles of statistical
mechanics, in a spirit close to Gibbs ensembles for equilibrium systems,
leading to a variational principle for the non-equilibrium steady state. We
discuss the existing attempts to achieve this using the maximum entropy
principle based on constraining the average current. We show that the
current-constrained theories result in a zero induced drop in electrostatic
potential, so that such ensembles cannot correspond to the time-evolved density
matrix, unless left- and right-going scattering states are mutually incoherent. | cond-mat_mtrl-sci |
Energy exchanges between atoms with a quartz crystal $μ$-balance: We propose an experimental method to fully characterize the energy exchange
of particles during the physical vapor deposition process of thin surface
layers. Our approach is based on the careful observation of perturbations of
the oscillation frequency of a Quartz Crystal $\mu$-balance induced by the
particles interaction. With this technique, it is possible to measure the
momentum exchange of the atoms during the evaporation process and determine the
ideal evaporation rate for an uniform energy distribution. We are able to
follow the desorption dynamics of particles immediately after the first layers
have been formed. These results are in close relation to the surface binding
energy of the evaporated material, they offer a better control to obtain the
desired properties of the thin surface layer. We applied our technique to
investigate the physical vapor evaporation process for diverse elements,
usually implemented in the development of film surface layers, such as Cu, W,
Au, Gd and In, and confirm that our results are in agreement with measurements
done previously with other techniques such as low-temperature
photoluminescence. | cond-mat_mtrl-sci |
Modeling intercalation chemistry with multi-redox reactions by sparse
lattice models in disordered rocksalt cathodes: Modern battery materials can contain many elements with substantial site
disorder, and their configurational state has been shown to be critical for
their performance. The intercalation voltage profile is a critical parameter to
evaluate the performance of energy storage. The application of commonly used
cluster expansion techniques to model the intercalation thermodynamics of such
systems from \textit{ab-initio} is challenged by the combinatorial increase in
configurational degrees of freedom as the number of species grows. Such
challenges necessitate efficient generation of lattice models without
over-fitting and proper sampling of the configurational space under charge
balance in ionic systems. In this work, we introduce a combined approach that
addresses these challenges by (1) constructing a robust cluster-expansion
Hamiltonian using the sparse regression technique, including
$\ell_0\ell_2$-norm regularization and structural hierarchy; and (2)
implementing semigrand-canonical Monte Carlo to sample charge-balanced ionic
configurations using the table-exchange method and an ensemble-average
approach. These techniques are applied to a disordered rocksalt oxyfluoride
Li$_{1.3-x}$Mn$_{0.4}$Nb$_{0.3}$O$_{1.6}$F$_{0.4}$ (LMNOF) which is part of a
family of promising earth-abundant cathode materials. The simulated voltage
profile is found to be in good agreement with experimental data and
particularly provides a clear demonstration of the Mn and oxygen contribution
to the redox potential as a function of Li content. | cond-mat_mtrl-sci |
How to Simulate Billiards and Similar Systems: An N-component continuous-time dynamic system is considered whose components
evolve autonomously all the time except for in discrete asynchronous instances
of pairwise interactions. Examples include chaotically colliding billiard balls
and combat models. A new efficient serial event-driven algorithm is described
for simulating such systems. Rather than maintaining and updating the global
state of the system, the algorithm tries to examine only essential events,
i.e., component interactions. The events are processed in a non-decreasing
order of time; new interactions are scheduled on the basis of the examined
interactions using preintegrated equations of the evolutions of the components.
If the components are distributed uniformly enough in the evolution space, so
that this space can be subdivided into small sectors such that only O(1)
sectors and O(1)$components are in the neighborhood of a sector, then the
algorithm spends time O (log N) for processing an event which is the
asymptotical minimum. The algorithm uses a simple strategy for handling data:
only two states are maintained for each simulated component. Fast data access
in this strategy assures the practical efficiency of the algorithm. It works
noticeably faster than other algorithms proposed for this model.
Key phrases: collision detection, dense packing, molecular dynamics, hard
spheres, granular flow | cond-mat_mtrl-sci |
Combined single crystal polarized XAFS and XRD at high pressure: probing
the interplay between lattice distortions and electronic order at multiple
length scales in high $T_c$ cuprates: Some of the most exotic material properties derive from electronic states
with short correlation length (~10-500 {\AA}), suggesting that the local
structural symmetry may play a relevant role in their behavior. Here we discuss
the combined use of polarized x-ray absorption fine structure and x-ray
diffraction at high pressure as a powerful method to tune and probe structural
and electronic orders at multiple length scales. Besides addressing some of the
technical challenges associated with such experiments, we illustrate this
approach with results obtained in the cuprate La$_{1.875}$Ba$_{0.125}$CuO$_4$,
in which the response of electronic order to pressure can only be understood by
probing the structure at the relevant length scales. | cond-mat_mtrl-sci |
Anomalous Nernst and Hall effects in magnetized platinum and palladium: We study the anomalous Nernst effect (ANE) and anomalous Hall effect (AHE) in
proximity-induced ferromagnetic palladium and platinum which is widely used in
spintronics, within the Berry phase formalism based on the relativistic band
structure calculations. We find that both the anomalous Hall ($\sigma_{xy}^A$)
and Nernst ($\alpha_{xy}^A$) conductivities can be related to the spin Hall
conductivity ($\sigma_{xy}^S$) and band exchange-splitting ($\Delta_{ex}$) by
relations $\sigma_{xy}^A =\Delta_{ex}\frac{e}{\hbar}\sigma_{xy}^S(E_F)'$ and
$\alpha_{xy}^A =
-\frac{\pi^2}{3}\frac{k_B^2T\Delta_{ex}}{\hbar}\sigma_{xy}^s(\mu)"$,
respectively. In particular, these relations would predict that the
$\sigma_{xy}^A$ in the magnetized Pt (Pd) would be positive (negative) since
the $\sigma_{xy}^S(E_F)'$ is positive (negative). Furthermore, both
$\sigma_{xy}^A$ and $\alpha_{xy}^A$ are approximately proportional to the
induced spin magnetic moment ($m_s$) because the $\Delta_{ex}$ is a linear
function of $m_s$. Using the reported $m_s$ in the magnetized Pt and Pd, we
predict that the intrinsic anomalous Nernst conductivity (ANC) in the magnetic
platinum and palladium would be gigantic, being up to ten times larger than,
e.g., iron, while the intrinsic anomalous Hall conductivity (AHC) would also be
significant. | cond-mat_mtrl-sci |
Multiscale Computation with Interpolating Wavelets: Multiresolution analyses based upon interpolets, interpolating scaling
functions introduced by Deslauriers and Dubuc, are particularly well-suited to
physical applications because they allow exact recovery of the multiresolution
representation of a function from its sample values on a finite set of points
in space. We present a detailed study of the application of wavelet concepts to
physical problems expressed in such bases. The manuscript describes algorithms
for the associated transforms which, for properly constructed grids of variable
resolution, compute correctly without having to introduce extra grid points. We
demonstrate that for the application of local homogeneous operators in such
bases, the non-standard multiply of Beylkin, Coifman and Rokhlin also proceeds
exactly for inhomogeneous grids of appropriate form. To obtain less stringent
conditions on the grids, we generalize the non-standard multiply so that
communication may proceed between non-adjacent levels. The manuscript concludes
with timing comparisons against naive algorithms and an illustration of the
scale-independence of the convergence rate of the conjugate gradient solution
of Poisson's equation using a simple preconditioning, suggesting that this
approach leads to an O(n) solution of this equation. | cond-mat_mtrl-sci |
Giant magnetic broadening of ferromagnetic resonance in a GMR
Co/Ag/Co/Gd quadlayer: Both magnetic-resonance damping and the giant magnetoresistance effect have
been predicted to be strongly affected by the local density of states in thin
ferromagnetic films. We employ the antiferromagnetic coupling between Co and Gd
to provide a spontaneous change from parallel to antiparallel alignment of two
Co films. A sharp increase in magnetic damping accompanies the change from
parallel to antiparallel alignment, analogous to resistivity changes in giant
magnetoresistance. | cond-mat_mtrl-sci |
Comparative study of Mo2Ga2C with superconducting MAX phase Mo2GaC: a
first-principles calculations: The structural, electronic, optical and thermodynamic properties of Mo2Ga2C
are investigated using density functional theory (DFT) within the generalized
gradient approximation (GGA). The optimized crystal structure is obtained and
the lattice parameters are compared with available experimental data. The
electronic density of states (DOS) is calculated and analyzed. The metallic
behavior for the compound is confirmed and the value of DOS at Fermi level is
4.2 states per unit cell per eV. Technologically important optical parameters
(e.g., dielectric function, refractive index, absorption coefficient, photo
conductivity, reflectivity, and loss function) have been calculated for the
first time. The study of dielectric constant (e1) indicates the Drude-like
behavior. The absorption and conductivity spectra suggest that the compound is
metallic. The reflectance spectrum shows that this compound has the potential
to be used as a solar reflector. The thermodynamic properties such as the
temperature and pressure dependent bulk modulus, Debye temperature, specific
heats, and thermal expansion coefficient of Mo2Ga2C MAX phase are derived from
the quasi-harmonic Debye model with phononic effect also for the first time.
Analysis of Tc expression using available parameter values (DOS, Debye
temperature, atomic mass etc.) suggests that the compound is less likely to be
superconductor. | cond-mat_mtrl-sci |
Long Range Magnetic order stabilized by acceptors: Tuning magnetic order in magnetic semiconductors is a long sought goal. A
proper concentration of acceptors can dramatically suppress local magnetic
order in favor of the long one. Using Mn and an acceptor codoped LiZnAs as an
example, we demonstrate, by first-principles calculation, the emergence of a
long-range magnetic order. This intriguing phenomenon can be understood from an
interplay between an acceptor-free magnetism and a band coupling magnetism. Our
observation thus lays the ground for a precise control of the magnetic order in
future spintronic devices. | cond-mat_mtrl-sci |
Synthesis, characterization and computational simulation of graphene
nanoplatelets stabilized in poly(styrene sulfonate) sodium salt: The production of large area interfaces and the use of scalable methods to
build-up designed nanostructures generating advanced functional properties are
of high interest for many materials science applications. Nevertheless, large
area coverage remains a major problem for pristine graphene and here we present
a hybrid, composite graphene-like material soluble in water, which can be
exploited in many areas, such as energy storage, electrodes fabrication,
selective membranes and biosensing. Graphene oxide (GO) was produced by the
traditional Hummers method being further reduced in the presence of
poly(styrene sulfonate) sodium salt (PSS), thus creating stable reduced
graphene oxide (rGO) nanoplateles wrapped by PSS (GPSS). Molecular dynamics
simulations were carried out of further clarify the interactions between PSS
molecules and rGO nanoplatelets, with calculations supported by FTIR analysis.
The intermolecular forces between rGO nanoplatelets and PSS lead to the
formation of a hybrid material (GPSS) stabilized by van der Waals forces,
allowing the fabrication of high quality layer-by-layer (LbL) films with
polyalillamine hydrochloride (PAH). Raman and electrical characterizations
corroborated the successful modifications in the electronic structures from GO
to GPSS after the chemical treatment, resulting in (PAH/GPSS) LbL films four
orders of magnitude more conductive than (PAH/GO). | cond-mat_mtrl-sci |
Reversible Band Gap Engineering in Carbon Nanotubes by Radial
Deformation: We present a systematic analysis of the effect of radial deformation on the
atomic and electronic structure of zigzag and armchair single wall carbon
nanotubes using the first principle plane wave method. The nanotubes were
deformed by applying a radial strain, which distorts the circular cross section
to an elliptical one. The atomic structure of the nanotubes under this strain
are fully optimized, and the electronic structure is calculated
self-consistently to determine the response of individual bands to the radial
deformation. The band gap of the insulating tube is closed and eventually an
insulator-metal transition sets in by the radial strain which is in the elastic
range. Using this property a multiple quantum well structure with tunable and
reversible electronic structure is formed on an individual nanotube and its
band-lineup is determined from first-principles. The elastic energy due to the
radial deformation and elastic constants are calculated and compared with
classical theories. | cond-mat_mtrl-sci |
Variational Methods For Phononic Calculations: Three fundamental variational principles used for solving elastodynamic
eigenvalue problems are studied within the context of elastic wave propagation
in periodic composites (phononics). We study the convergence of the eigenvalue
problems resulting from the displacement Rayleigh quotient, the stress Rayleigh
quotient and the mixed quotient. The convergence rates of the three quotients
are found to be related to the continuity and differentiability of the density
and compliance variation over the unit cell. In general, the mixed quotient
converges faster than both the displacement Rayleigh and the stress Rayleigh
quotients, however, there exist special cases where either the displacement
Rayleigh or the stress Rayleigh quotient shows the exact same convergence as
the mixed-method. We show that all methods converge faster for smoother
material property variations, but when density variation is rough, the
difference between the mixed quotient and stress Rayleigh quotient is higher
and similarly, when compliance variation is rough, the difference between the
mixed quotient and displacement Rayleigh quotient is higher. Since eigenvalue
problems such as those considered in this paper tend to be highly
computationally intensive, it is expected that these results will lead to fast
and efficient algorithms in the areas of phononics and photonics. | cond-mat_mtrl-sci |
Solution Processed Large-scale Multiferroic Complex Oxide Epitaxy with
Magnetically Switched Polarization: Complex oxides with tunable structures have many fascinating properties,
though high-quality complex oxide epitaxy with precisely controlled composition
is still out of reach. Here we have successfully developed solution-based
single crystalline epitaxy for multiferroic
(1-x)BiTi(1-y)/2FeyMg(1-y)/2O3-(x)CaTiO3 (BTFM-CTO) solid solution in large
area, confirming its ferroelectricity at atomic-scale with a spontaneous
polarization of 79~89uC/cm2. Careful compositional tuning leads to a bulk
magnetization of ~0.07uB/Fe at room temperature, enabling magnetically induced
polarization switching exhibiting a large magnetoelectric coefficient of
2.7-3.0X10-7s/m. This work demonstrates the great potential of solution
processing in large-scale complex oxide epitaxy and establishes novel
room-temperature magnetoelectric coupling in epitaxial BTFM-CTO film, making it
possible to explore a much wider space of composition, phase, and structure
that can be easily scaled up for industrial applications. | cond-mat_mtrl-sci |
Tunable Electronic Structure and Magnetic Coupling in Strained
Two-Dimensional Semiconductor MnPSe3: The electronic structures and magnetic properties of strained monolayer
MnPSe3 are investigated systematically by first-principles calculations. It is
found that the magnetic ground state (GS) of monolayer MnPSe3 can be
significantly affected by biaxial strain engineering, while the semiconducting
characteristics are well preserved. Owing to the sensitivity of the magnetic
coupling towards the structural deformation, a biaxial tensile strain about 13%
can lead to an antiferromagnetic-ferromagnetic (AFM-FM) transition. The
underlying physical mechanism of strain-dependent magnetic stability is mainly
attributed to the competition effect of direct AFM interaction and indirect FM
superexchange interaction between the nearest-neighbor (NN) two Mn atoms. In
addition, we find that FM MnPSe3 is an intrinsic half semiconductor with a
large spin exchange splitting in conduction bands, which is crucial for the
spin-polarized carrier injection and detection. The sensitive interdependence
among external stimuli, electronic structure and magnetic coupling suggests
that monolayer MnPSe3 can be a promising candidate in spintronics. | cond-mat_mtrl-sci |
CrTe$_2$ as a two-dimensional material for topological magnetism in
complex heterobilayers: The discovery of two-dimensional (2D) van der Waals magnetic materials and
their heterostructures provided an exciting platform for emerging phenomena
with intriguing implications in information technology. Here, based on a
multiscale modelling approach that combines first-principles calculations and a
Heisenberg model, we demonstrate that interfacing a CrTe$_2$ layer with various
Te-based layers enables the control of the magnetic exchange and
Dzyaloshinskii-Moriya interactions as well as the magnetic anisotropy energy of
the whole heterobilayer, and thereby the emergence of topological magnetic
phases such as skyrmions and antiferromagnetic Neel merons. The latter are
novel particles in the world of topological magnetism since they arise in a
frustrated Neel magnetic environment and manifest as multiples of intertwined
hexamer-textures. Our findings pave a promising road for proximity-induced
engineering of both ferromagnetic and long-sought antiferromagnetic chiral
objects in the very same 2D material, which is appealing for new information
technology devices employing quantum materials. | cond-mat_mtrl-sci |
Two Band Model Interpretation of the p to n Transition in Ternary
Tetradymite Topological Insulators: The requirement for large bulk resistivity in topological insulators has led
to the design of complex ternary and quaternary phases with balanced donor and
acceptor levels. A common feature of the optimized phases is that they lie
close to the p to n transition. The tetradymite Bi2Te3_xSex system exhibits
minimum bulk conductance at the ordered composition Bi2Te2Se. By combining
local and integral measurements of the density of states, we find that the
point of minimum electrical conductivity at x=1.0 where carriers change from
hole-like to electron-like is characterized by conductivity of the mixed type.
Our experimental findings, which are interpreted within the framework of a two
band model for the different carrier types, indicate that the mixed state
originates from different type of native defects that strongly compensate at
the crossover point. | cond-mat_mtrl-sci |
Investigation of thermal stability of hydrogenated amorphous Si/Ge
multilayers: Thermal stability of hydrogenated amorphous Si/Ge multilayers has been
investigated by Scanning Electron Microscopy (SEM), Transmission Electron
Microscopy (TEM) and Small-Angle X-Ray Diffraction (SAXRD) techniques.
Amorphous H-Si/Ge multilayers were prepared by RF sputtering with 1.5 and 6
ml/min H2 flow-rate. It is shown by Elastic Recoil Detection Analysis (ERDA)
that the hydrogen concentration increased by increasing H2 flow-rate. Annealing
of the samples was carried out at 400 and 450 oC for several hours. It has been
observed that samples prepared with 6 ml/min flow-rate at both annealing
temperatures underwent significant structural changes: the surface of the
samples was visibly roughened, gas bubbles were formed and craters were
created. The decay of the periodic structure of Si and Ge layers in these types
of multilayers was faster than in non-hydrogenated samples. Samples prepared
with 1.5 ml/min flow-rate have similar behaviour at 450 oC, but at 400 oC the
decay of the first order SAXRD peaks was slower than in case of the
non-hydrogenated multilayers. Qualitatively the observed behaviour can be
explained by the fast desorption of the saturated hydrogen, leading to the
formation of bubbles and craters at 450 oC, as well as, at 400oC in the sample
with lower H-content, by the possible passivation of the dangling bonds
resulting in a slowing down of the diffusion intermixing. | cond-mat_mtrl-sci |
Efficient thermal energy harvesting using nanoscale magnetoelectric
heterostructures: Thermomechanical cycles with a ferroelectric working substance convert heat
to electrical energy. As shown here, magnetoelectrically coupled
ferroelectric/ferromangtic composites (also called multiferroics) add new
functionalities and allow for an efficient thermal energy harvesting at room
temperature by exploiting the pyroelectric effect. By virtue of the
magnetoelectric coupling, external electric and magnetic fields can steer the
operation of these heat engines. Our theoretical predictions are based on a
combination of Landau-Khalatnikov-Tani approach (with a
Ginzburg-Landau-Devonshire potential) to simulate the ferroelectric dynamics
coupled to the magnetic dynamics. The latter is treated via the
electric-polarization-dependent Landau-Lifshitz-Gilbert equation. Performing an
adapted Olsen cycle we show that a multiferroic working substance is
potentially much more superior to sole ferroelectrics, as far as thermal energy
harvesting using pyroelectric effect is concerned. Our proposal holds promise
not only for low-energy consuming devices but also for cooling technology. | cond-mat_mtrl-sci |
Two-dimensional Weyl points and nodal lines in pentagonal materials and
their optical response: Two-dimensional pentagonal structures based on the Cairo tiling are the basis
of a family of layered materials with appealing physical properties. In this
work we present a theoretical study of the symmetry-based electronic and
optical properties of these pentagonal materials. We provide a complete
classification of the space groups that support pentagonal structures for
binary and ternary systems. By means of first-principles calculations, their
electronic band structures and the local spin textures in momentum space are
analyzed. Our results show that pentagonal structures can be realized in chiral
and achiral lattices with Weyl nodes pinned at high-symmetry points and nodal
lines along the Brillouin zone boundary; these degeneracies are protected by
the combined action of crystalline and time-reversal symmetries. Additionally,
we discuss the linear and nonlinear optical features of some penta-materials,
such as the shift current, which shows an enhancement due to the presence of
nodal lines and points, and their possible applications. | cond-mat_mtrl-sci |
Magnetic properties of Quantum Corrals from first principles
calculations: We present calculations for electronic and magnetic properties of surface
states confined by a circular quantum corral built of magnetic adatoms (Fe) on
a Cu(111) surface. We show the oscillations of charge and magnetization
densities within the corral and the possibility of the appearance of
spin--polarized states. In order to classify the peaks in the calculated
density of states with orbital quantum numbers we analyzed the problem in terms
of a simple quantum mechanical circular well model. This model is also used to
estimate the behaviour of the magnetization and energy with respect to the
radius of the circular corral. The calculations are performed fully
relativistically using the embedding technique within the
Korringa-Kohn-Rostoker method. | cond-mat_mtrl-sci |
Electronic band structure of ultimately thin silicon oxide on Ru(0001): Silicon oxide can be formed in a crystalline form, when prepared on a
metallic substrate. It is a candidate support catalyst and possibly the
ultimately-thin version of a dielectric host material for two-dimensional
materials (2D) and heterostructures. We determine the atomic structure and
chemical bonding of the ultimately thin version of the oxide, epitaxially grown
on Ru(0001). In particular, we establish the existence of two sub-lattices
defined by metal-oxygen-silicon bridges involving inequivalent substrate sites.
We further discover four electronic bands below Fermi level, at high binding
energies, two of them forming a Dirac cone at K point, and two others forming
semi-flat bands. While the latter two correspond to hybridized states between
the oxide and the metal, the former relate to the topmost silicon-oxygen plane,
which is not directly coupled to the substrate. Our analysis is based on high
resolution X-ray photoelectron spectroscopy, angle-resolved photoemission
spectroscopy, scanning tunneling microscopy, and density functional theory
calculations. | cond-mat_mtrl-sci |
TC++: First-principles calculation code for solids using the
transcorrelated method: TC++ is a free/libre open-source software of the transcorrelated (TC) method
for first-principles calculation of solids. Here, the TC method is one of the
promising wave-function theories that can be applied to periodic systems with
reasonable computational cost and satisfactory accuracy. We present our
implementation of TC++ including a detailed description of the divergence
correction technique applied to the TC effective interactions. We also present
the way to use TC++ and some results of application to simple periodic systems:
bulk silicon and homogeneous electron gas. | cond-mat_mtrl-sci |
Unveiling the complete dispersion of the giant Rashba split surface
states of ferroelectric $α$-GeTe(111) by alkali doping: $\alpha$-GeTe(111) is a non-centrosymmetric ferroelectric material, for which
a strong spin-orbit interaction gives rise to giant Rashba split states in the
bulk and at the surface. The detailed dispersions of the surface states inside
the bulk band gap remains an open question because they are located in the
unoccupied part of the electronic structure, making them inaccessible to static
angle-resolved photoemission spectroscopy. We show that this difficulty can be
overcome via in-situ potassium doping of the surface, leading to a rigid shift
of 80 meV of the surface states into the occupied states. Thus, we resolve in
great detail their dispersion and highlight their crossing at the
$\bar{\Gamma}$ point, which, in comparison with density functional theory
calculations, definitively confirms the Rashba mechanism. | cond-mat_mtrl-sci |
Electric Field Induced Phase Transition in KDP Crystal Near Curie Point:
Raman and X-ray Scattering Studies: X-ray scattering measurements are performed in order to verify % that the
mechanism leading to the DC electric field induced $C_{2v}^{19} \to
C_{2v}^{\neq 19}$ phase transition in KDP crystal at 119 K is the changing of
the local sites symmetries of phosphate group from $C_2$ in the $C_{2v}^{19}$
phase to $C_s$ in the $C_{2v}^{\neq 19}$ phase. It is shown by analyzing the
integrated intensity of the (800) and (080) reflections that under DC electric
field the density of oxygen atoms lying on these plane changes indicating that
phosphate group rotates around the [010] direction relative to the orthorhombic
$C_{2v}^{19}$ structure. Some Raman results are also discussed. | cond-mat_mtrl-sci |
Dielectric Engineering of Electronic Correlations in a van der Waals
Heterostructure: Heterostructures of van der Waals bonded layered materials offer unique means
to tailor dielectric screening with atomic-layer precision, opening a fertile
field of fundamental research. The optical analyses used so far have relied on
interband spectroscopy. Here we demonstrate how a capping layer of hexagonal
boron nitride (hBN) renormalizes the internal structure of excitons in a
WSe$_2$ monolayer using intraband transitions. Ultrabroadband terahertz probes
sensitively map out the full complex-valued mid-infrared conductivity of the
heterostructure after optical injection of $1s$ A excitons. This approach
allows us to trace the energies and linewidths of the atom-like $1s$-$2p$
transition of optically bright and dark excitons as well as the densities of
these quasiparticles. The fundamental excitonic resonance red shifts and
narrows in the WSe$_2$/hBN heterostructure compared to the bare monolayer.
Furthermore, the ultrafast temporal evolution of the mid-infrared response
function evidences the formation of optically dark excitons from an initial
bright population. Our results provide key insight into the effect of non local
screening on electron-hole correlations and open new possibilities of
dielectric engineering of van der Waals heterostructures. | cond-mat_mtrl-sci |
Aromatic Borozene: Based on our comprehensive theoretical investigation and known experimental
results for small boron clusters, we predict the existence of a novel aromatic
inorganic molecule, B12H6. This molecule, which we refer to as borozene, has
remarkably similar properties to the well-known benzene. Borozene is planar,
possesses a large first excitation energy, D3h symmetry, and more importantly
is aromatic. Furthermore, the calculated anisotropy of the magnetic
susceptibility of borozene is three times larger in absolute value than for
benzene. Finally, we can show that borozene molecules may be fused together to
give larger aromatic compounds with even larger anisotropic susceptibilities. | cond-mat_mtrl-sci |
Stability and electronic structure of covalently functionalized graphene
layers: We present exemplary results of extensive studies of mechanical, electronic
and transport properties of covalent functionalization of graphene monolayers
(GML) with -NH2. We report new results of ab initio studies of covalent
functionalization of GML with -NH2 groups up to 12.5% concentration. Our
studies are performed in the framework of the density functional theory (DFT)
and non-equilibrium Green's function (NEGF). We discuss the stability
(adsorption energy), elastic moduli, electronic structure, band gaps, and
effective electron masses as a function of the density of the adsorbed
molecules. We also show the conductance and I(V) characteristic of these
systems. Generally, the stability of the functionalized graphene layers
decreases with the growing concentration of attachments and we determine the
critical density of the molecules that can be chemisorbed on the surface of
GLs. Because of local deformations of GLs and sp3 rehybridization of the bonds
induced by fragments, elastic moduli decrease with increasing number of groups.
Simultaneously, we observe that the functionalizing molecules stretch the
graphenes lattice, the effect being more pronounced for higher concentration of
adsorbed molecules. We find out that the GLs functionalization leads in many
cases to the opening of the graphene band gap (up to 0.5302 eV for 12.5%
concentration) and can be therefore utilized in graphene devices. The new HOMO
and LUMO originate mostly from the impurity bands induced by the
functionalization and they exhibit parabolic dispersion with electron effective
masses comparable to ones in silicon or gallium nitride. | cond-mat_mtrl-sci |
Large Itinerant Electron Exchange Coupling in the Magnetic Topological
Insulator MnBi2Te4: Magnetism in topological materials creates phases exhibiting quantized
transport phenomena with applications in spintronics and quantum information.
The emergence of such phases relies on strong interaction between localized
spins and itinerant states comprising the topological bands, and the subsequent
formation of an exchange gap. However, this interaction has never been measured
in any intrinsic magnetic topological material. Using a multimodal approach,
this exchange interaction is measured in MnBi2Te4, the first realized intrinsic
magnetic topological insulator. Interrogating nonequilibrium spin dynamics,
itinerant bands are found to exhibit a strong exchange coupling to localized Mn
spins. Momentum-resolved ultrafast electron scattering and magneto-optic
measurements reveal that itinerant spins disorder via electron-phonon
scattering at picosecond timescales. Localized Mn spins, probed by resonant
X-ray scattering, disorder concurrently with itinerant spins, despite being
energetically decoupled from the initial excitation. Modeling the results using
atomistic simulations, the exchange coupling between localized and itinerant
spins is estimated to be >100 times larger than superexchange interactions.
This implies an exchange gap of >25 meV should occur in the topological surface
states. By directly quantifying local-itinerant exchange coupling, this work
validates the materials-by-design strategy of utilizing localized magnetic
order to create and manipulate magnetic topological phases, from static to
ultrafast timescales. | cond-mat_mtrl-sci |
Room temperature ferroic orders in Zr and (Zr, Ni) doped SrTiO$_3$: We synthesized strontium titanate SrTiO$_3$ (STO), Zr doped
$\text{Sr}_\text{1-x}\text{Zr}_\text{x}\text{Ti}\text{O}_3$ and (Zr, Ni)
co-doped
$\text{Sr}_\text{1-x}\text{Zr}_\text{x}\text{Ti}_\text{1-y}\text{Ni}_\text{y}\text{O}_3$
samples using solid state reaction technique to report their structural,
electrical and magnetic properties. The cubic $Pm$-$3m$ phase of the
synthesized samples has been confirmed using Rietveld analysis of the powder
X-ray diffraction pattern. The grain size of the synthesized materials was
reduced significantly due to Zr doping as well as (Zr, Ni) co-doping in STO.
The chemical species of the samples were identified using energy-dispersive
X-ray spectroscopy. We observed forbidden first order Raman scattering at 148,
547 and 797 cm$^{-1}$ which may indicate nominal loss of inversion symmetry in
cubic STO. The absence of absorption at 500 cm$^{-1}$ and within 600-700
cm$^{-1}$ band in Fourier Transform Infrared spectra corroborates Zr and Ni as
substitutional dopants in our samples. Due to 4% Zr doping in
$\text{Sr}_\text{0.96}\text{Zr}_\text{0.04}\text{Ti}\text{O}_3$ sample
dielectric constant, remnant electric polarization, remnant magnetization and
coercivity were increased. Notably, in the case of 4% Zr and 10% Ni co-doping
we have observed clearly the existence of both FE and FM hysteresis loops in
$\text{Sr}_{0.96}\text{Zr}_{0.04}\text{Ti}_{0.90}\text{Ni}_{0.10}\text{O}_3$
sample. In this co-doped sample, the remnant magnetization and coercivity were
increased by $\sim$1 and $\sim$2 orders of magnitude respectively as compared
to those of undoped STO. The coexistence of FE and FM orders in (Zr, Ni)
co-doped STO might have the potential for interesting multiferroic
applications. | cond-mat_mtrl-sci |
Comment on 'Hysteresis, Switching, and Negative Differential Resistance
in Molecular Junctions: a Polaron Model', by M. Galperin, M.A. Ratner, and A.
Nitzan, Nano Lett. 5, 125 (2005): It is shown that the ``hysteresis'' in a polaron model of electron transport
through the molecule found by M.Galperin et al. [Nano Lett. 5, 125 (2005)] is
an artefact of their ``mean-field'' approximation. The reason is trivial: after
illegitimate replacement $\hat{n}^{2}=\hat{n}n_{0},$ where \hat{n} is the
electron number operator, n_{0} the average molecular level occupation,
Galperin et al. obtained non-physical dependence of a renormalized molecular
energy level on the non-integer mean occupation number n_{0} (i.e. the electron
self-interaction) and the resulting non-linearity of current. The exact theory
of correlated polaronic transport through molecular quantum dots (MQDs) that we
proposed earlier [Phys. Rev. B67, 235312 (2003)] proved that there is no
hysteresis or switching in current-voltage characteristics of non-degenerate,
d=1, or double degenerate, d=2, molecular bridges, contrary to the mean-field
result. Switching could only appear in multiply degenerate MQDs with d>2 due to
electron correlations. Most of the molecular quantum dots are in the regime of
weak coupling to the electrodes addressed in our formalism. | cond-mat_mtrl-sci |
Theory of momentum-resolved magnon electron energy loss spectra: The
case of Yttrium Iron Garnet: We explore the inelastic spectra of electrons impinging in a magnetic system.
The methodology here presented is intended to highlight the charge-dependent
interaction of the electron beam in a STEM-EELS experiment, and the local
vector potential generated by the magnetic lattice. This interaction shows an
intensity $10^{-2}$ smaller than the purely spin interaction, which is taken to
be functionally the same as in the inelastic neutron experiment. On the other
hand, it shows a strong scattering vector dependence ($\kappa^{-4}$) and a
dependence with the relative orientation between the probe wavevector and the
local magnetic moments of the solid. We present YIG as a case study due to its
high interest by the community. | cond-mat_mtrl-sci |
A first-principles study of structural and elastic properties of bulk
SrRuO$_3$: We present a first-principles investigation of structural and elastic
properties of experimentally observed phases of bulk SrRuO$_3$ - namely
orthorhombic, tetragonal, and cubic - by applying density functional theory
(DFT) approximations. At first, we focus our attention on the accuracy of
calculated lattice constants in order to find out DFT approaches that best
represent the crystalline structure of SrRuO$_3$, since many important physical
quantities crucially depend on change in volume. Next, we evaluate
single-crystal elastic constants, mechanical stability, and macroscopic elastic
parameters trying to at least partially compensate for the existing lack of
information about these fundamental features of SrRuO$_3$. Finally, we analyze
the anomalous behavior of low-temperature orthorhombic phase under $C_{44}$
related shear deformation. It turns out that at critical strain values the
system exhibits a distinct deviation from the initial behavior which results in
an isosymmetric phase transition. Moreover, under $C_{44}$ related shear
deformation tetragonal SrRuO3 becomes mechanically unstable raising an open
question of what makes it experimentally observable at high temperatures. | cond-mat_mtrl-sci |
Vacancy-related color centers in twodimensional silicon carbide
monolayers: Basic vacancy defects in twodimensional silicon carbide (2D-SiC) are examined
by means of density functional theory calculations to explore their
magneto-optical properties as well as their potential in quantum technologies.
In particular, the characteristic hyperfine tensors and optical excited states
of carbon-vacancy, silicon-vacancy, and carbon antisite-vacancy pair defects in
2D-SiC are determined that are the key fingerprints of these defects that may
be observed in electron paramagnetic resonance and photoluminescence
experiments, respectively. Besides the fundamental characterization of the most
basic native defects, we show that the negatively charged carbon
antisite-vacancy defect is a promising candidate for realizing a near-infrared
single-photon quantum emitter with spin doublet ground state, where the
negative charge state may be provided by nitrogen doping of 2D-SiC. We find
that the neutral carbon-vacancy with spin triplet ground state might be used
for quantum sensing with a broad emission in the visible. | cond-mat_mtrl-sci |
FeRh groundstate and martensitic transformation: Cubic B2 FeRh exhibits a metamagnetic transition [(111) antiferromagnet (AFM)
to ferromagnet (FM)] around 353 K and remains structurally stable at higher
temperatures. However, the calculated zero-Kelvin phonons of AFM FeRh exhibit
imaginary modes at M-points in the Brillouin zone, indicating a premartensitic
instability, which is a precursor to a martensitic transformation at low
temperatures. Combining electronic-structure calculations with ab initio
molecular dynamics, conjugate gradient relaxation, and the solid-state
nudged-elastic band (SSNEB) methods, we predict that AFM B2 FeRh becomes
unstable at ambient pressure and transforms without a barrier to an AFM(111)
orthorhombic (martensitic) groundstate below 90K. We also consider competing
structures, in particular, a tetragonal AFM(100) phase that is not the global
groundstate, as proposed [Phys. Rev. B 94, 180407(R) (2016)], but a constrained
solution. | cond-mat_mtrl-sci |
Shift of Fermi level by substitutional impurity-atom doping in diamond
and cubic- and hexagonal-boron nitrides II. Generalized Gradient
Approximation: In succession to the first paper (arXiv 1406.6204v5), the impurity-atom
concentrations when the Fermi levels are either at the valence band maximum
(VBM) or the conduction band minimum (CBM) were identified for diamond, cubic
boron nitride (cBN), and hexagonal boron nitride (hBN) using the
Korringa-Kohn-Rostoker (KKR) scheme using the local density approximation
(LDA). In the present paper, the generalized gradient approximation (GGA) was
used instead of the LDA for exchange-correlation. The impurity atoms were B and
N for diamond, Be, Si, and C for cBN, and Be for hBN; these impurity atoms were
known in the first paper to form degenerate states by increased impurity-atom
concentrations. The impurity-atom concentrations when the Fermi level was
located either at the VBM or the CBM were as follows: (i) the B concentration
was 0.27 at.% in B-doped diamond, (ii) the N concentration was 0.25 at.% in
N-doped diamond, (iii) the concentration of Be substituting B was 0.88 at.% in
cBN, (iv) the concentration of Si substituting B was 0.06 at.% in cBN, (v) the
concentration of C substituting B was 0.07 at.% in cBN, (vi) the concentration
of C substituting N was 0.88 at.% in cBN, and (vii) the concentration of Be
substituting B was 1.80 at.% in hBN. The values of (iv) and (v) were
significantly smaller than the corresponding values in paper I, but it was
attributed to the input parameters used in the present paper, hence it was
concluded that the computed concentrations were not sensitive to the GGA used. | cond-mat_mtrl-sci |
Zak's Phase in Non-Symmetric One-Dimensional Crystals: In this work, we derive some analytical properties of Berry's phase in
one-dimensional quantum and classical crystals, also named Zak's phase, when
computed with a Fourier basis. We show that Zak's phase can be divided in two
terms: a global phase required to make the Bloch wave periodic in the Brillouin
zone and an internal phase which measures the relative delay of the different
Fourier terms within the Brillouin zone. While the former phase is dependent on
the origin of coordinates of the unit cell, the latter is independent of it, so
that it can be interpreted as an internal property of the band itself. We show
that this internal phase is always zero for a symmetric crystal while it can
take any value when this symmetry is broken, showing therefore that it can be
interpreted as a measure of the assymetry of the band. Since for a symmetric
crystal Zak's phase is entirely determined by the global part, we show that
this can be easily calculated by means of the parity of the Fourier terms at
the center and edge of the Brillouin zone, being therefore unnecessary the
integration of the modes through the unit cell and the entire Brillouin zone.
We provide numerical examples analyzing the internal part for both electronic
and classical waves (acoustic or photonic). We analyze the weakest electronic
potential capable of presenting asymmetry, as well as the double-Dirac delta
potential, and in both examples it is found that the internal phase varies
continuously as a function of a symmetry-control parameter, but it is zero when
the crystal is symmetric. For classical waves, the layered material is
analyzed. Although Zak's phase has been mainly studied in connection with the
existence of edge states in finite crystals, we consider that the study of the
internal phase can be more relevant to understand bulk properties of quantum
and classical crystals. | cond-mat_mtrl-sci |
Examining real-time TDDFT non-equilibrium simulations for the
calculation of electronic stopping power: In ion irradiation processes, electronic stopping power describes the energy
transfer rate from the irradiating ion to the target material's electrons. Due
to the scarcity and significant uncertainties in experimental electronic
stopping power data for materials beyond simple solids, there has been growing
interest in the use of first-principles theory for calculating electronic
stopping power. In recent years, advances in high-performance computing have
opened the door to fully first-principles nonequilibrium simulations based on
real-time time-dependent density functional theory (RT-TDDFT). While it has
been demonstrated that the RT-TDDFT approach is capable of predicting
electronic stopping power for a wide range of condensed matter systems, there
has yet to be an exhaustive examination of the physical and numerical
approximations involved and their effects on the calculated stopping power. We
discuss the results of such a study for crystalline silicon with protons as
irradiating ions. We examine the influences of key approximations in RT-TDDFT
nonequilibrium simulations on the calculated electronic stopping power,
including approximations related to basis sets, finite size effects,
exchange-correlation approximation, pseudopotentials, and more. Finally, we
propose a simple and efficient correction scheme to account for the
contribution from core-electron excitations to the stopping power, as it was
found to be significant for large proton velocities. | cond-mat_mtrl-sci |
Ultrafast spin dynamics in inhomogeneous systems: a density-matrix
approach applied to Co/Cu interfaces: Ultrafast spin dynamics on femto- to picosecond timescales is simulated
within a density-operator approach for a Co/Cu bilayer. The electronic
structure is represented in a tight-binding form; during the evolution of the
density operator, optical excitation by a femtosecond laser pulse, coupling to
a bosonic bath as well as dephasing are taken into account. Our simulations
corroborate the importance of interfaces for ultrafast transport phenomena and
demagnetisation processes. Moreover, we establish a reflow from Cu $d$ orbitals
across the interface into Co $d$ orbitals, which shows up prominently in the
mean occupation numbers. On top of this, this refilling manifests itself as a
minority-spin current proceeding several layers into the Cu region. The present
study suggests that the approach captures essential ultrafast phenomena and
provides insight into microscopic processes. | cond-mat_mtrl-sci |
A Microscopic Model of Ferroelectricity in Stress-free PbTiO3 Ultrathin
Films: The ground-state polarization of PbTiO3 thin films is studied using a
microscopic effective Hamiltonian with parameters obtained from
first-principles calculations. Under short-circuit electrical boundary
conditions, (001) films with thickness as low as three unit cells are found to
have a perpendicularly polarized ferroelectric ground state with significant
enhancement of the polarization at the surface. | cond-mat_mtrl-sci |
nanoNET: Machine Learning Platform for Predicting Nanoparticles
Distribution in a Polymer Matrix: Polymer nanocomposites (PNCs) offer a broad range of thermophysical
properties that are linked to their compositions. However, it is challenging to
establish a universal composition-property relation of PNCs due to their
enormous composition and chemical space. Here, we address this problem and
develop a new method to model the composition-microstructure relation of a PNC
through an intelligent machine learning pipeline named nanoNET. The nanoNET is
a nanoparticles (NPs) distribution predictor, built upon computer vision and
image recognition concepts. It integrates unsupervised deep learning and
regression in a fully automated pipeline. We conduct coarse-grained molecular
dynamics simulations of PNCs and utilize the data to establish and validate the
nanoNET. Within this framework, a random forest regression model predicts the
NPs distribution in a PNC in a latent space. Subsequently, a convolutional
neural network-based decoder converts the latent space representation to the
actual radial distribution function (RDF) of NPs in the given PNC. The nanoNET
predicts NPs distribution in many unknown PNCs very accurately. This method is
very generic and can accelerate the design, discovery, and fundamental
understanding of composition-microstructure relations of PNCs and other
molecular systems. | cond-mat_mtrl-sci |
Multi-State, Ultra-thin, BEOL-Compatible AlScN Ferroelectric Diodes: The growth in data generation necessitates efficient data processing
technologies to address the von Neumann bottleneck in conventional computer
architecture. Memory-driven computing, which integrates non-volatile memory
(NVM) devices in a 3D stack, is gaining attention, with CMOS back-end-of-line
(BEOL) compatible ferroelectric (FE) diodes being ideal due to their
two-terminal design and inherently selector-free nature, facilitating
high-density crossbar arrays. Here, we demonstrate BEOL-compatible,
high-performance FE-diodes scaled to 5, 10, and 20 nm FE
Al0.72Sc0.28N/Al0.64Sc0.36N films. Through interlayer (IL) engineering, we show
substantial improvements in the ON/OFF ratios (>166 times) and rectification
ratios (>176 times) in these scaled devices. The superlative characteristics
also enables 5-bit multi-state operation with a stable retention. We also
experimentally and theoretically demonstrate the counterintuitive result that
the inclusion of an IL can lead to a decrease in the ferroelectric switching
voltage of the device. An in-depth analysis into the device transport
mechanisms is performed, and our compact model aligns seamlessly with the
experimental results. Our results suggest the possibility of using scaled
AlxSc1-xN FE-diodes for high performance, low-power, embedded NVM. | cond-mat_mtrl-sci |
Anisotropy of Resonant Inelastic X-Ray Scattering at the K Edge of
Si:Theoretical Analysis: We investigate theoretically the resonant inelastic x-ray scattering (RIXS)
at the $K$ edge of Si on the basis of an ab initio calculation. We calculate
the RIXS spectra with systematically varying transfered-momenta,
incident-photon energy and incident-photon polarization. We confirm the
anisotropy of the experimental spectra by Y. Ma {\it et al}. (Phys. Rev. Lett.
74, 478 (1995)), providing a quantitative explanation of the spectra. | cond-mat_mtrl-sci |
Piezoelectricity in Two-Dimensional Group III Monochalcogenides: We find that several layer-phase group-III monochalcogenides, including GaS,
GaSe and InSe, are piezoelectric in the monolayer form. First-principles
calculations reveal that the piezoelectric coefficients of monolayer GaS, GaSe
and InSe are on the same order of magnitude as the earlier discovered
two-dimensional piezoelectric materials, such as BN and MoS2 monolayers. Our
study expands the family of two dimensional piezoelectric materials, suggesting
that strong piezoelectric response can occur in a wide range of two dimensional
materials with broken inversion symmetry. The co-existence of piezoelectricity
and superior photo-sensitivity in these two-dimensional semiconductors enables
the integration of electromechanical and optical sensors on the same material
platform. | cond-mat_mtrl-sci |
Charge carrier transport and lifetimes in n-type and p-type phosphorene
as 2D device active materials: an ab initio study: In this work, we provide a detailed analysis of phosphorene performance as
n-type and p-type active materials. The study is based on first principles
calculation of phosphorene electronic structure, and resulting electron and
hole scattering rates and lifetimes. Emphasis is put on extreme regimes
commonly found in semiconductor devices, i.e. high electric fields and heavy
doping, where impact ionization and Auger recombination can occur. We found
that electron-initiated impact ionization is weaker than the hole-initiated
process, when compared to carrier-phonon interaction rates, suggesting
resilience to impact ionization initiated breakdown. Moreover, calculated
minority electron lifetimes are limited by radiative recombination only, not by
Auger processes, suggesting that phosphorene could achieve good quantum
efficiencies in optoelectronic devices. The provided scattering rates and
lifetimes are critical input data for the modeling and understanding of
phosphorene-based device physics. | cond-mat_mtrl-sci |
Graphite and Hexagonal Boron-Nitride Possess the Same Interlayer
Distance. Why?: Graphite and hexagonal boron nitride (h-BN) are two prominent members of the
family of layered materials possessing a hexagonal lattice. While graphite has
non-polar homo-nuclear C-C intra-layer bonds, h-BN presents highly polar B-N
bonds resulting in different optimal stacking modes of the two materials in
bulk form. Furthermore, the static polarizabilities of the constituent atoms
considerably differ from each other suggesting large differences in the
dispersive component of the interlayer bonding. Despite these major differences
both materials present practically identical interlayer distances. To
understand this finding, a comparative study of the nature of the interlayer
bonding in both materials is presented. A full lattice sum of the interactions
between the partially charged atomic centers in h-BN results in vanishingly
small monopolar electrostatic contributions to the interlayer binding energy.
Higher order electrostatic multipoles, exchange, and short-range correlation
contributions are found to be very similar in both materials and to almost
completely cancel out by the Pauli repulsions at physically relevant interlayer
distances resulting in a marginal effective contribution to the interlayer
binding. Further analysis of the dispersive energy term reveals that despite
the large differences in the individual atomic polarizabilities the
hetero-atomic B-N C6 coefficient is very similar to the homo-atomic C-C
coefficient in the hexagonal bulk form resulting in very similar dispersive
contribution to the interlayer binding. The overall binding energy curves of
both materials are thus very similar predicting practically the same interlayer
distance and very similar binding energies. | cond-mat_mtrl-sci |
Observation of Anomalous Hall Effect in Noncollinear Antiferromagnetic
Mn3Sn Films: Magnetotransport is at the center of the spintronics. Mn3Sn, an
antiferromagnet that has a noncollinear 120{\deg} spin order, exhibits large
anomalous Hall effect (AHE) at room temperature. But such a behavior has been
remained elusive in Mn3Sn films. Here we report the observation of robust AHE
up to room temperature in quasi-epitaxial Mn3Sn thin films, prepared by
magnetron sputtering. The growth of both (11-20)- and (0001)-oriented Mn3Sn
films provides a unique opportunity for comparing AHE in three different
measurement configurations. When the magnetic field is swept along (0001)
plane, such as the direction of [01-10] and [2-1-10] the films show
comparatively higher anomalous Hall conductivity than its perpendicular
counterpart ([0001]), irrespective of their respectively orthogonal current
along [0001] or [01-10]. A quite weak ferromagnetic moment of 3 emu/cm^3 is
obtained in (11-20)-oriented Mn3Sn films, guaranteeing the switching of the
Hall signals with magnetization reversal. Our finding would advance the
integration of Mn3Sn in antiferromagnetic spintronics. | cond-mat_mtrl-sci |
Probing anisotropy in epitaxial Fe/Pt bilayers by spin-orbit torque
ferromagnetic resonance: We report the generation and detection of spin-orbit torque ferromagnetic
resonance (STFMR) in micropatterned epitaxial Fe/Pt bilayers grown by molecular
beam epitaxy. The magnetic field dependent measurements at an in-plane magnetic
field angle of 45 degrees with respect to the microwave-current direction
reveal the presence of two distinct voltage peaks indicative of a strong
magnetic anisotropy. We show that STFMR can be employed to probe the underlying
magnetic properties including the anisotropies in the Fe layer. We compare our
STFMR results with broadband ferromagnetic resonance spectroscopy of the
unpatterned bilayer thin films. The experimental STFMR measurements are
interpreted using an analytical formalism and further confirmed using
micromagnetic modeling, which shed light on the field-dependent magnetization
alignment in the microstructures responsible for the STFMR rectification. Our
results demonstrate a simple and efficient method for determining magnetic
anisotropies in microstructures by means of rf spectroscopy. | cond-mat_mtrl-sci |
Peach-Koehler forces within the theory of nonlocal elasticity: We consider dislocations in the framework of Eringen's nonlocal elasticity.
The fundamental field equations of nonlocal elasticity are presented. Using
these equations, the nonlocal force stresses of a straight screw and a straight
edge dislocation are given. By the help of these nonlocal stresses, we are able
to calculate the interaction forces between dislocations (Peach-Koehler
forces). All classical singularities of the Peach-Koehler forces are
eliminated. The extremum values of the forces are found near the dislocation
line. | cond-mat_mtrl-sci |
Modelling of epitaxial graphene functionalization: A new model for graphene, epitaxially grown on silicon carbide is proposed.
Density functional theory modelling of epitaxial graphene functionalization by
hydrogen, fluorine and phenyl groups has been performed with hydrogen and
fluorine showing a high probability of cluster formation in high adatom
concentration. It has also been shown that the clusterization of fluorine
adatoms provides midgap states in formation due to significant flat distortion
of graphene. The functionalization of epitaxial graphene using larger species
(methyl and phenyl groups) renders cluster formation impossible, due to the
steric effect and results in uniform coverage with the energy gap opening. | cond-mat_mtrl-sci |
Conductivity and Dissociation in Metallic Hydrogen: Implications for
Planetary Interiors: Liquid metallic hydrogen (LMH) was recently produced under static compression
and high temperatures in bench-top experiments. Here, we report a study of the
optical reflectance of LMH in the pressure region of 1.4-1.7 Mbar and use the
Drude free-electron model to determine its optical conductivity. We find static
electrical conductivity of metallic hydrogen to be 11,000-15,000 S/cm. A
substantial dissociation fraction is required to best fit the energy dependence
of the observed reflectance. LMH at our experimental conditions is largely
atomic and degenerate, not primarily molecular. We determine a plasma frequency
and the optical conductivity. Properties are used to analyze planetary
structure of hydrogen rich planets such as Jupiter. | cond-mat_mtrl-sci |
Continuous Frequency Controllable Nano-electromechanical Systems Based
on Multiwalled Carbon Nanotubes: We demonstrate a class of model nano-electromechanical systems (NEMS) based
on multiwalled carbon nanotubes (MWNTs) which has longer inner cores coaxially
oscillating inside their respective shorter outer shell holders and can operate
at continuously controllable frequencies up to the gigahertz range when fuelled
by AC electric fields. Its additional attributes include much larger
oscillation amplitudes and forces and much lower rates of thermal dissipation
(Q-factor = 10^5) and air damping (Q-factor = 10^4~10^5) than those of
nano-beam based NEMS. A crucial feature of the conceived model NEMS is that
after having tuned the electric field frequency to any prescribed value within
a permitted range, the NEMS will respond quickly (in sub-nanoseconds) at the
same oscillation frequency. These merits, when contrasted with the nano-beam
resonators developed so far, make it a better potential candidate for the
ongoing miniaturization progress from micro- to nano-electromechanical systems. | cond-mat_mtrl-sci |
Deep learning and the Schrödinger equation: We have trained a deep (convolutional) neural network to predict the
ground-state energy of an electron in four classes of confining two-dimensional
electrostatic potentials. On randomly generated potentials, for which there is
no analytic form for either the potential or the ground-state energy, the
neural network model was able to predict the ground-state energy to within
chemical accuracy, with a median absolute error of 1.49 mHa. We also
investigate the performance of the model in predicting other quantities such as
the kinetic energy and the first excited-state energy of random potentials. | cond-mat_mtrl-sci |
Multiscale insight into the Cd1-xZnxTe vibrational-mechanical properties
-- High-pressure experiments and ab initio calculations: The Cd1-xZnxTe semiconductor alloy is a regular system regarding its
macroscopic mechanic properties in that its experimental bulk modulus exhibits
a linear x-dependence, in line with ab initio predictions. Complexity arises at
the bond scale, referring to the intricate Cd1-xZnxTe percolation-type Raman
pattern [T. Alhaddad et al., Journal of Applied Physics 133, 065701 (2023)].
This offers an appealing benchmark to test various phonon coupling processes at
diverse length scales in a compact multi-oscillator assembly, presently tuned
by pressure. At x around 0, an inter-bond long-range/macro electric coupling
between the matrix and impurity polar phonons is detuned under pressure.
Inversely, at x around 1, an intra-bond short-range/nano mechanic coupling is
enforced between the two Zn Te apolar sub-phonons stemming from same and alien
percolation-type environments. The pressure-induced macro/nano polar/apolar
coupling/decoupling processes are compared within a model of two coupled
electric/mechanic harmonic oscillators in terms of a compromise between
proximity to resonance and strength of coupling, impacting the degree of mode
mixing, with ab initio (apolar case) and analytical (polar case) Raman
calculations in support. Notably, the free mechanic coupling at x around 1
opposes the achievement of a phonon exceptional point, manifesting the
inhibition of mechanic coupling, earlier evidenced with similar bonds for x
smaller than 0.5. Hence, the pressure dependence of a given bond vibration in a
disordered alloy basically differs depending on whether the bond is
matrix-like, i.e., self-connected in bulk (free coupling), or dispersed, i.e.,
self-connected in a chain (inhibited coupling). This features pressure-tunable
percolation-based on-off phonon switches in complex media. | cond-mat_mtrl-sci |
The deffect effect on electronic conductance in binomially tailored
quantum wire: The paper considers the effect of the defects on the electronic transmission
properties in binomially tailored waveguide quantum wires, in which each Dirac
delta function potential strength have been weight on the binomial distribution
law. We have assumed that a single free-electron channel is incident on the
structure and the scattering of electrons is solely from the geometric nature
of the problem. We have used the transfer matrix method to study the electron
transmission. We found this novel structure has a good defect tolerance. We
found the structure tolerate up to in strength defect and in position defect
for the central Dirac delta function in the binomial distribution. Also, we
found this structure can tolerate both defect up to in strength and in position
dislocation | cond-mat_mtrl-sci |
Phase stability of Au-Li binary systems studied using neural network
potential: The miscibility of Au and Li exhibits a potential application as an adhesion
layer and electrode material in secondary batteries. Here, to explore alloying
properties, we constructed a neural network potential (NNP) of Au-Li binary
systems based on density functional theory (DFT) calculations. To accelerate
construction of NNPs, we proposed an efficient and inexpensive method of
structural dataset generation. The predictions by the constructed NNP on
lattice parameters and phonon properties agree well with those obtained by DFT
calculations. We also investigated the mixing energy of Au$_{1-x}$Li$_{x}$ with
fine composition grids, showing excellent agreement with DFT verifications. We
found the existence of various compositions with structures on and slightly
above the convex hull, which can explain the lack of consensus on the Au-Li
stable phases in previous studies. Moreover, we newly found
Au$_{0.469}$Li$_{0.531}$ as a stable phase, which has never been reported
elsewhere. Finally, we examined the alloying process starting from the phase
separated structure to the complete mixing phase. We found that when multiple
adjacent Au atoms dissolved into Li, the alloying of the entire Au/Li interface
started from the dissolved region. This paper demonstrates the applicability of
NNPs toward miscible phases and provides the understanding of the alloying
mechanism. | cond-mat_mtrl-sci |
Hafnia for analog memristor: Influence of stoichiometry and crystalline
structure: The highly non-linear switching behavior of hafnia memristor actually hinders
its wide application in neuromorphic computing. Theoretical understanding into
its switching mechanism has been focused on the processes of conductive
filament generation and rupture, but possible phase transition and
crystallization around the region of conductive filaments (CFs) due to the
variation of O content have been paid less attention to. In this paper,
HfO$\mathrm{_x}$ structural models covering the full stoichiometries from Hf to
HfO$\mathrm{_2}$ were established, and the crystal structure evolution during
the reduction process of hafnia was obtained through first-principles
calculation. The electronic structures and O vacancy migration characteristics
of these structures were analyzed. A criterion was prescribed to predict the
mode of abrupt binary switching or gradual conductance modulation according to
the structure evolution of the CFs. In particular, factors that influence the
merging of tiny conductive channels into strong filaments are intensively
discussed, including the anisotropy of O vacancy migration and the size effect.
The feasibility of Mg doping to achieve robust gradual switching is discussed. | cond-mat_mtrl-sci |
Negative Temperature in Spin Dynamics Simulations: A simple and computationally efficient algorithm enables implementing
negative temperature values in a spin dynamics simulation. The algorithm uses a
Langevin spin dynamics thermostat with a negative damping parameter, enabling
the thermalization of an arbitrary interacting spin system to the Gibbs energy
distribution with a given negative temperature value. Canonical spin dynamics
simulations at a negative temperature are as robust as conventional positive
spin temperature simulations, providing a tool for quantitative dynamic studies
of the physics of highly excited magnetic states. Two simulation case studies
describing spin systems with antiferromagnetic and ferromagnetic ground states
are explored. The phase transitions occurring in the negative temperature range
do not necessarily exhibit similarities with their positive temperature
counterparts. The transition temperatures and the character of spin alignment
vary depending on the spatial range and strength of spin-spin interactions. | cond-mat_mtrl-sci |
Size-independent Shear Band Formation in Amorphous Nanowires made from
Simulated Casting: Molecular dynamics simulations indicate that surfaces strongly influence the
strain localization behavior of amorphous nanowires in tension. A sample
preparation routine that simulates casting was employed to facilitate the
relaxation of the sample surface. Samples as short as 15 nm (7.5 nm in
diameter) form dominant shear bands during deformation. The elastic energy
release during plastic deformation is sufficient to provide the excess
potential energy required for the shear band nucleation at rather small sample
sizes. The results show that shear band formation is almost size-independent
and is bounded only by its own length scale. | cond-mat_mtrl-sci |
Semiconducting layered blue phosphorus: A computational study: We investigate a previously unknown phase of phosphorus that shares its
layered structure and high stability with the black phosphorus allotrope. We
find the in-plane hexagonal structure and bulk layer stacking of this
structure, which we call `blue phosphorus', to be related to graphite. Unlike
graphite and black phosphorus, blue phosphorus displays a wide fundamental band
gap and should exfoliate easily to form quasi-2D structures suitable for
electronic applications. We study a likely transformation pathway from black to
blue phosphorus and discuss possible ways to synthesize the new structure. | cond-mat_mtrl-sci |
Microscopic mechanism of the non-crystalline anisotropic
magnetoresistance in (Ga,Mn)As: Starting with a microscopic model based on the Kohn-Luttinger Hamiltonian and
kinetic p-d exchange combined with Boltzmann formula for conductivity we
identify the scattering from magnetic Mn combined with the strong spin-orbit
interaction of the GaAs valence band as the dominant mechanism of the
anisotropic magnetoresistance (AMR) in (Ga,Mn)As. This fact allows to construct
a simple analytical model of the AMR consisting of two heavy-hole bands whose
charge carriers are scattered on the impurity potential of the Mn atoms. The
model predicts the correct sign of the AMR (resistivity parallel to
magnetization is smaller than perpendicular to magnetization) and identifies
its origin arising from the destructive interference between electric and
magnetic part of the scattering potential of magnetic ionized Mn acceptors when
the carriers move parallel to the magnetization. | cond-mat_mtrl-sci |
Mechanism of pressure induced amorphization of SnI4: a combined X-ray
diffraction -- X-ray absorption spectroscopy study: We have studied the amorphization process of SnI4 up to 26.8GPa with
unprecedented experimental details by combining Sn and I K edge X-ray
absorption spectroscopy and powder X-ray diffraction. Standards and reverse
Monte Carlo extended X-ray absorption fine structure (EXAFS) refinements
confirm that the SnI4 tetrahedron is a fundamental structural unit that is
preserved through the crystalline phase-I to crystalline phase-II transition
about 7 to 10GPa and then in the amorphous phase that appears above 20GPa. Up
to now unexploited Iodine EXAFS reveals to be extremely informative and
confirms the formation of iodine iodine short bonds close to 2.85{\AA} in the
amorphous phase at 26.8 GPa. A coordination number increase of Sn in the
crystalline phase-II appears to be excluded, while the deformation of the
tetrahedral units proceeds through a flattening that keeps the average I-Sn-I
angle close to 109.5{\deg}. Moreover, we put in evidence the impact of pressure
on the Sn near edge structure under competing geometrical and electronic
effects. | cond-mat_mtrl-sci |
Plasma-assisted fabrication of monolayer phosphorene and its Raman
characterization: There have been continuous efforts to seek for novel functional
two-dimensional semiconductors with high performance for future applications in
nanoelectronics and optoelectronics. In this work, we introduce a successful
experimental approach to fabricate monolayer phosphorene by mechanical cleavage
and the following Ar+ plasma thinning process. The thickness of phosphorene is
unambiguously determined by optical contrast combined with atomic force
microscope (AFM). Raman spectroscopy is used to characterize the pristine and
plasma-treated samples. The Raman frequency of A2g mode stiffens, and the
intensity ratio of A2g to A1g modes shows monotonic discrete increase with the
decrease of phosphorene thickness down to monolayer. All those phenomena can be
used to identify the thickness of this novel two-dimensional semiconductor
efficiently. This work for monolayer phosphorene fabrication and thickness
determination will facilitates the research of phosphorene. | cond-mat_mtrl-sci |
Edge chirality determination of graphene by Raman spectroscopy: Raman imaging on the edges of single layer micromechanical cleavage graphene
(MCG) was carried out. The intensity of disorder-induced Raman feature (D band
at ~1350 cm-1) was found to be correlated to the edge chirality: it is stronger
at the armchair edge and weaker at the zigzag edge. This shows that Raman
spectroscopy is a reliable and practical method to identify the chirality of
graphene edge and to help in determination of the crystal orientation. The
determination of graphene chirality is critically important for fundamental
study as well as for applications. | cond-mat_mtrl-sci |
Hardness of T-carbon: Density functional theory calculations: We revisit and interpret the mechanical properties of the recently proposed
allotrope of carbon, T-carbon [Sheng \emph{et al.}, Phys. Rev. Lett.,
\textbf{106}, 155703 (2011)], using density functional theory in combination
with different empirical hardness models. In contrast with the early estimation
based on the Gao's model, which attributes to T-carbon an high Vickers hardness
of 61 GPa comparable to that of superhard cubic boron nitride (\emph{c}-BN), we
find that T-carbon is not a superhard material, since its Vickers hardenss does
not exceed 10 GPa. Besides providing clear evidence for the absence of
superhardenss in T-carbon, we discuss the physical reasons behind the failure
of Gao's and \v{S}im$\rm\mathring{u}$nek and Vack\'a\v{r}'s (SV) models in
predicting the hardness of T-carbon, residing on their improper treatment of
the highly anisotropic distribution of quasi-\emph{sp}$^3$-like C-C hybrids. A
possible remedy to the Gao and SV models based on the concept of superatom is
suggest, which indeed yields a Vickers hardness of about 8 GPa. | cond-mat_mtrl-sci |
The Structural Phase Transition of the Relaxor Ferroelectric
68%PbMg1/3Nb2/3O3-32%PbTiO3: Neutron scattering techniques have been used to study the relaxor
ferroelectric 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 denoted in this paper as
0.68PMN-0.32PT. On cooling, these relaxor ferroelectrics have a long-range
ordered ferroelectric phase and the composition is close to that at which the
ferroelectric structure changes from rhombohedral to tetragonal. It was found
that above the Burns temperature of about 600K, the transverse optic mode and
the transverse acoustic mode are strongly coupled and a model was used to
describe this coupling that gave similar parameters to those obtained for the
coupling in PMN. Below the Burns temperature additional quasi-elastic
scattering was found which increased in intensity as the sample was cooled down
to the ferroelectric transition temperature but then decreased in intensity.
This behaviour is similar to that found in PMN. This scattering is associated
with the dynamic polar nano-regions that occur below the Burns temperature. In
addition to this scattering a strictly elastic resolution limited peak was
observed that was much weaker than the corresponding peak in pure PMN and which
decreased in intensity on cooling below the ferroelectric phase whereas for
PMN, which does not have a long-range ordered ferroelectric phase, the
intensity of this component increased monotonically as the sample was cooled.
The results of our study are compared with the recent measurements of Stock et
al. [PRB 73 064107] who studied 0.4PMN-0.6PT. The results are qualitatively
consistent with the random field model developed to describe the scattering
from PMN. | cond-mat_mtrl-sci |
Optical properties of exciton in two-dimensional transition metal
dichalcogenide nanobubbles: Strain in two-dimensional (2D) transition metal dichalcogenide (TMD) has led
to localized states with exciting optical properties, in particular in view of
designing one photon sources. The naturally formed of the MoS2 monolayer
deposed on hBN substrate leads to a reduction of the bandgap in the strained
region creating a nanobubble. The photogenerated particles are thus confined in
the strain-induced potential. Using numerical diagonalization, we simulate the
spectra of the confined exciton states, their oscillator strengths and
radiative lifetimes. We show that a single state of the confined exciton is
optically active, which suggests that the MoS2/hBN nanobubble is a good
candidate for the realisation of single-photon sources. Furthermore, the
exciton binding energy, oscillator strength and radiative lifetime are enhanced
due to the confinement effect. | cond-mat_mtrl-sci |
The Dzyaloshinskii-Moriya interaction is under control: an orchestrated
flip of the chiral link between structure and magnetism for
Fe$_{1-x}$Co$_x$Si: Monosilicides of 3d-metals frequently show a chiral magnetic ordering with
the absolute configuration defined by the chirality of the crystal structure
and the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and
magnetic chiralities are probed here for Fe$_{1-x}$Co$_x$Si series and their
mutual relationship is found to be dependent on the chemical composition. The
chirality of crystal structure was previously shown to be governed by crystal
growth, and the value of the DMI is nearly the same for all monosilicides of
Fe, Co and Mn. Our findings indicate that the sign of the DMI in
Fe$_{1-x}$Co$_x$Si is controlled by the Co composition $x$, thus, opening a
route towards controlled design of chiral spintronics devices. | cond-mat_mtrl-sci |
Liberation of slave modes inside domain walls in multiferroic Cu-Cl
boracite: Domain walls (DWs), the two-dimensional boundaries between symmetry
equivalent ferroic domains, are actively investigated due to their promise for
novel logic and memory devices. Moreover, they can be easily created, erased
and reshaped at a low energy cost due to their high mobility and large
electrical conductivity. Most work so far has been focused on DWs in proper
ferroelectrics, where the primary order parameter, ferroelectric polarization,
interpolates between the values in the domains by either reducing to zero (in
Ising-type DW) or rotating (Bloch type DW). Here we present a new member of DW
family with a complex inner texture of slave order parameters inside the wall
where the primary order parameter reduces to zero. Our first-principles-derived
model predicts the existence of monopolar and toroidal polarization patterns.
The results enable large-scale phase field simulations of complex domain
patterns in boracites and could inspire novel devices based on domain walls in
improper ferroelectrics. | cond-mat_mtrl-sci |
Ferroelectricity in the 1 $μ$C cm$^{-2}$ range induced by canted
antiferromagnetism in (LaMn$_{3}$)Mn$_{4}$O$_{12}$: Pyroelectric current and magnetoelectric coupling measurements on
polycrystalline samples of the quadruple perovskite
(LaMn$_{3}$)Mn$_{4}$O$_{12}$ give evidence of ferroelectricity driven by the
antiferromagnetic ordering of the $B$-site Mn$^{3+}$ ions at $T_{N,B}$=78 K
with record values of remnant electric polarization up to $P$=0.56 $\mu$C
cm$^{-2}$. X-ray diffraction measurements indicates an anomalous behavior of
the monoclinic $\beta$ angle at $T_{N,B}$, which suggests that $P$ lies in the
$ac$-plane, where the moments are collinear, so we conclude that exchange
striction is the mechanism of spin-driven ferroelectricity. Polarization values
$\sim$3 $\mu$C cm$^{-2}$ are expected in single crystals, which would open the
avenue towards practical multiferroic applications. | cond-mat_mtrl-sci |
Structure and optical properties of alpha- and gamma-cerium
sesquisulfide: Structural and electronic properties of the alpha- and gamma-phases of cerium
sesquisulfide, Ce2S3, are examined by first-principles calculations using the
GGA+U extension of density functional theory. The strongly correlated
f-electrons of Ce are described by a Hubbard-type on-site Coulomb repulsion
parameter. A single parameter of $U^/prime$=4 eV yields excellent results for
crystal structures, band gaps, and thermodynamic stability for both Ce2S3
allotropes. This approach gives insights in the difference in color of
brownish-black alpha-Ce2S3 and dark red gamma-Ce2S3. The calculations predict
that both Ce2S3 modifications are insulators with optical gaps of 0.8 eV
(alpha-phase) and 1.8 eV (gamma-phase). The optical gaps are determined by
direct electronic excitations at k=Gamma from localized and occupied Ce
4f-orbitals into empty Ce 5d-states. The f-states are situated between the
valence and conduction bands. The difference of 1 eV between the optical gaps
of the two Ce2S3 modifications is explained by different coordinations of the
cerium cations by sulfur anions. For both Ce2S3 modifications the calculations
yield an effective local magnetic moment of 2.6 $mu_B$ per cerium cation, which
is in agreement with measurements. The electronic energy of the alpha-phase is
computed to be 6 kJ/mol lower than that of the gamma-phase, which is consistent
with the thermodynamic stability of the two allotropes. | cond-mat_mtrl-sci |
Electronic thermal conductivity at high temperatures: Violation of the
Wiedemann-Franz law in narrow band metals: We study the electronic part of the thermal conductivity kappa of metals. We
present two methods for calculating kappa, a quantum Monte-Carlo (QMC) method
and a method where the phonons but not the electrons are treated
semiclassically (SC). We compare the two methods for a model of alkali-doped
C60, A3C60, and show that they agree well. We then mainly use the SC method,
which is simpler and easier to interpret. We perform SC calculations for Nb for
large temperatures T and find that kappa increases with T as kappa(T)=a+bT,
where a and b are constants, consistent with a saturation of the mean free
path, l, and in good agreement with experiment. In contrast, we find that for
A3C60, kappa(T) decreases with T for very large T. We discuss the reason for
this qualitatively in the limit of large T. We give a quantum-mechanical
explanation of the saturation of l for Nb and derive the Wiedemann-Franz law in
the limit of T much smaller than W, where W is the band width. In contrast, due
to the small W of A3C60, the assumption T much smaller than W can be violated.
We show that this leads to kappa(T) \sim T^{-3/2} for very large T and a strong
violation of the Wiedemann-Franz law. | cond-mat_mtrl-sci |
Optimal switching of a nanomagnet assisted by microwaves: We develop an efficient and general method for optimizing the microwave field
that achieves magnetization switching with a smaller static field. This method
is based on optimal control and renders an exact solution for the 3D microwave
field that triggers the switching of a nanomagnet with a given anisotropy and
in an oblique static field. Applying this technique to the particular case of
uniaxial anisotropy, we show that the optimal microwave field, that achieves
switching with minimal absorbed energy, is modulated both in frequency and in
magnitude. Its role is to drive the magnetization from the metastable
equilibrium position towards the saddle point and then damping induces the
relaxation to the stable equilibrium position. For the pumping to be efficient,
the microwave field frequency must match at the early stage of the switching
process the proper precession frequency of the magnetization, which depends on
the magnitude and direction of the static field. We investigate the effect of
the static field (in amplitude and direction) and of damping on the
characteristics of the microwave field. We have computed the switching curves
in the presence of the optimal microwave field. The results are in qualitative
agreement with micro-SQUID experiments on isolated nanoclusters. The strong
dependence of the microwave field and that of the switching curve on the
damping parameter may be useful in probing damping in various nanoclusters. | cond-mat_mtrl-sci |
Ill-Behaved Convergence of a Model of the Gd3Ga5O12 Garnet
Antiferromagnet with Truncated Magnetic Dipole-Dipole Interactions: Previous studies have found that calculations which consider long-range
magnetic dipolar interactions truncated at a finite cut-off distance Rc predict
spurious (unphysical) long-range ordered phases for Ising and Heisenberg
systems on the pyrochlore lattice. In this paper we show that, similar to these
two cases, calculations that use truncated dipolar interactions to model the
Gd3Ga5O12 garnet antiferromagnet also predict unphysical phases with
incommensurate ordering wave vector q_ord that is very sensitive to the dipolar
cut-off distance Rc. | cond-mat_mtrl-sci |
Magnon Dispersion and Anisotropies in SrCu$_2$(BO$_3$)$_2$: We study the dispersion of the magnons (triplet states) in
SrCu$_2$(BO$_3$)$_2$ including all symmetry-allowed Dzyaloshinskii-Moriya
interactions. We can reduce the complexity of the general Hamiltonian to a new
simpler form by appropriate rotations of the spin operators. The resulting
Hamiltonian is studied by both perturbation theory and exact numerical
diagonalization on a 32-site cluster. We argue that the dispersion is dominated
by Dzyaloshinskii-Moriya interactions. We point out which combinations of these
anisotropies affect the dispersion to linear-order, and extract their
magnitudes. | cond-mat_mtrl-sci |
Designing isoelectronic counterparts to layered group V semiconductors: In analogy to III-V compounds, which have significantly broadened the scope
of group IV semiconductors, we propose IV-VI compounds as isoelectronic
counterparts to layered group V semiconductors. Using {\em ab initio} density
functional theory, we study yet unrealized structural phases of silicon
mono-sulfide (SiS). We find the black-phosphorus-like $\alpha$-SiS to be almost
equally stable as the blue-phosphorus-like $\beta$-SiS. Both $\alpha$-SiS and
$\beta$-SiS monolayers display a significant, indirect band gap that depends
sensitively on the in-layer strain. Unlike 2D semiconductors of group V
elements with the corresponding nonplanar structure, different SiS allotropes
show a strong polarization either within or normal to the layers. We find that
SiS may form both lateral and vertical heterostructures with phosphorene at a
very small energy penalty, offering an unprecedented tunability in structural
and electronic properties of SiS-P compounds. | cond-mat_mtrl-sci |
Composites of FeCl3 and TiO2 with Bismaleimide resins: Ferric Chloride-Bismaleimide (FeCl3-BMI) and Titania-Bismaleimide (TiO2-BMI)
composite were synthesized using PVA as a binder. The composite systems were
deposited on glass slide as a homogenous coating. XRD and FTIR studies of the
composite system showed its crystalline and structural configuration. A mixed
phase of TiO2 and BMI as well as short range crystallinity was observed. An
increase in crystallinity with temperature was also seen. The percentage of N-H
symmetric stretching was also found to increase with temperature. | cond-mat_mtrl-sci |
Transferring MBE-grown topological insulator films to arbitrary
substrates and Metal-insulator transition via Dirac gap: Mechanical exfoliation of bulk crystals has been widely used to obtain thin
topological insulator (TI) flakes for device fabrication. However, such a
process produces only micro-sized flakes that are highly irregular in shape and
thickness. In this work, we developed a process to transfer the entire area of
TI Bi2Se3 thin films grown epitaxially on Al2O3 and SiO2 to arbitrary
substrates, maintaining their pristine morphology and crystallinity. Transport
measurements show that these transferred films have lower carrier
concentrations and comparable or higher mobilities than before the transfer.
Furthermore, using this process we demonstrated a clear metal-insulator
transition in an ultrathin Bi2Se3 film by gate-tuning its Fermi level into the
hybridization gap formed at the Dirac point. The ability to transfer large area
TI films to any substrate will facilitate fabrication of TI heterostructure
devices, which will help explore exotic phenomena such as Majorana fermions and
topological magnetoelectricity. | cond-mat_mtrl-sci |
Langmuir-Blodgett Monolayers of Cationic Dyes in the Presence and
Absence of Clay Mineral Layers: N,N'-Dioctadecyl Thiacyanine, Octadecyl
Rhodamine B and Laponite: Langmuir-Blodgett (LB) films of N,N'-dioctadecyl thiacyanine perchlorate (NK)
and octadecyl rhodamine B chloride (RhB18) and their mixtures in the presence
and absence of clay mineral layers were investigated by recording surface
pressure - area isotherms and by UV-Vis and fluorescence spectroscopies. The
isotherms of NK, RhB18 and their mixtures are characteristic for liquid
expanded state behaviour with repulsive interactions between the two cationic
dyes. In the presence of laponite the isotherms show liquid expanded and
condensed state behaviour. In laponite dispersions and in monolayers, NK has a
strong tendency to aggregate with formation of H- and J- aggregates. The
absorption and fluorescence maxima of the monomers in the films are at 435 nm
and at 480 nm; H-dimer have an absorption maximum around 410 nm and do not
fluoresce. J-dimers are present in all the films with absorption maximum at 461
nm and fluorescence at 463 nm. RhB18 is mainly present as monomers in the LB
films with an absorption maximum at 576 nm and fluorescence at 595 nm.
Fluorescence resonance energy transfer from NK to RhB18 has been observed in
clay dispersions and in films with and without laponite. The optimum condition
for NK RhB18 fluorescence energy transfer in the films is 90 mol% NK + 10 mol%
RhB18. | cond-mat_mtrl-sci |
Defect formation dynamics during CdTe overlayer growth: The presence of atomic-scale defects at multilayer interfaces significantly
degrades performance in CdTe-based photovoltaic technologies. The ability to
accurately predict and understand defect formation mechanisms during overlayer
growth is, therefore, a rational approach for improving the efficiencies of
CdTe materials. In this work, we utilize a recently developed CdTe bond-order
potential (BOP) to enable accurate molecular dynamics (MD) simulations for
predicting defect formation during multilayer growth. A detailed comparison of
our MD simulations to high-resolution transmission electron microscopy
experiments verifies the accuracy and predictive power of our approach. Our
simulations further indicate that island growth can reduce the lattice mismatch
induced defects. These results highlight the use of predictive MD simulations
to gain new insight on defect reduction in CdTe overlayers, which directly
addresses efforts to improve these materials. | cond-mat_mtrl-sci |
The Sign of Three: Spin/Charge Density Waves at the Boundaries of
Transition Metal Dichalcogenides: One-dimensional grain boundaries of two-dimensional semiconducting {\MX} (M=
Mo,W; X=S,Se) transition metal di-chalcogenides are typically metallic at room
temperature. The metallicity has its origin in the lattice polarization, which
for these lattices with $D_{3h}$ symmetry is a topological invariant, and leads
to one-dimenional boundary states inside the band gap. For boundaries
perpendicular to the polarization direction, these states are necessarily 1/3
occupied by electrons or holes, making them susceptible to a metal-insulator
transition that triples the translation period. Using density-functional-theory
calculations we demonstrate the emergence of combined one-dimensional spin
density/charge density waves of that period at the boundary, opening up a small
band gap of $\sim 0.1$ eV. This unique electronic structure allows for soliton
excitations at the boundary that carry a fractional charge of $\pm 1/3\ e$. | cond-mat_mtrl-sci |
Graphene -- Based Nanocomposites as Highly Efficient Thermal Interface
Materials: We found that an optimized mixture of graphene and multilayer graphene -
produced by the high-yield inexpensive liquid-phase-exfoliation technique - can
lead to an extremely strong enhancement of the cross-plane thermal conductivity
K of the composite. The "laser flash" measurements revealed a record-high
enhancement of K by 2300 % in the graphene-based polymer at the filler loading
fraction f =10 vol. %. It was determined that a relatively high concentration
of single-layer and bilayer graphene flakes (~10-15%) present simultaneously
with thicker multilayers of large lateral size (~ 1 micrometer) were essential
for the observed unusual K enhancement. The thermal conductivity of a
commercial thermal grease was increased from an initial value of ~5.8 W/mK to
K=14 W/mK at the small loading f=2%, which preserved all mechanical properties
of the hybrid. Our modeling results suggest that graphene - multilayer graphene
nanocomposite used as the thermal interface material outperforms those with
carbon nanotubes or metal nanoparticles owing to graphene's aspect ratio and
lower Kapitza resistance at the graphene - matrix interface. | cond-mat_mtrl-sci |
Cm2 Scale Synthesis of MoTe2 Thin Films with Large Grains and Layer
Control David: Owing to the small energy differences between its polymorphs, MoTe2 can
access a full spectrum of electronic states, from the 2H semiconducting state
to the 1T semimetallic state, and from the Td Weyl semimetallic state to the
superconducting state in the 1T and Td phase at low temperature. Thus, it is a
model system for phase transformation studies as well as quantum phenomena such
as the quantum spin Hall effect and topological superconductivity. Careful
studies of MoTe2 and its potential applications require large area MoTe2 thin
films with high crystallinity and thickness control. Here, we present cm2 scale
synthesis of 2H MoTe2 thin films with layer control and large grains that span
several microns. Layer control is achieved by controlling the initial thickness
of the precursor MoOx thin films, which are deposited on sapphire substrates by
atomic layer deposition and subsequently tellurized. Despite the van der Waals
epitaxy, the precursor-substrate interface is found to critically determine the
uniformity in thickness and grain size of the resulting MoTe2 films: MoTe2
grown on sapphire show uniform films while MoTe2 grown on amorphous SiO2
substrates form islands. This synthesis strategy decouples the layer control
from the variabilities of growth conditions for robust growth results, and is
applicable to grow other transition metal dichalcogenides with layer control. | cond-mat_mtrl-sci |
Influence of defects on the critical behaviour at the \boldmath{105} K
structural phase transition of SrTiO$_3$: II. The sharp component: The depth dependence of the crystallographic parameters mosaicity, lattice
parameter variation and integrated reflectivity and of the critical scattering
above the 105 K structural phase transition of SrTiO$_3$ have been studied in
five different single crystals by means of high resolution triple-crystal
diffractometry using 100-120 keV synchrotron radiation. Depth-dependent
impedance measurements indicate that the presence of oxygen vacancies is not
responsible for the two-length scale phenomenon. It is found that the sharp
component occurs only in surface near regions of highly perfect single crystals
and is coupled to an exponential inrease of the crystallographic quantities.
The second length scale is absent at a surface where the strain fields are able
to relax by a macroscopic bending of the lattice planes. The sharp component is
also strongly suppressed in crystals of relatively large mosaicity. The
combination of long range strain fields in highly perfect samples and the
vicinity of the surface seem to be necessary conditions for the observation of
the sharp component. The critical exponents for the second length scale are in
satisfying agreement with scaling laws if the intensity of the critical
scattering is assumed to be proportional to the square of the Lorentzian
susceptibility and not, as usual in the current convention, to a
Lorentzian-squared susceptibility. The critical exponents of the broad
component are independent of the appearance of the sharp component. | cond-mat_mtrl-sci |
Possible Kitaev Quantum Spin Liquid State in 2D Materials with S=3/2: Quantum spin liquids (QSLs) form an extremely unusual magnetic state in which
the spins are highly correlated and fluctuate coherently down to the lowest
temperatures, but without symmetry breaking and without the formation of any
static long-range-ordered magnetism. Such intriguing phenomena are not only of
great fundamental relevance in themselves, but also hold the promise for
quantum computing and quantum information. Among different types of QSLs, the
exactly solvable Kitaev model is attracting much attention, with most proposed
candidate materials, e.g., RuCl$_3$ and Na$_2$IrO$_3$, having an effective
$S$=1/2 spin value. Here, via extensive first-principle-based simulations, we
report the investigation of the Kitaev physics and possible Kitaev QSL state in
epitaxially strained Cr-based monolayers, such as CrSiTe$_3$, that rather
possess a $S$=3/2 spin value. Our study thus extends the playground of Kitaev
physics and QSLs to 3$d$ transition metal compounds. | cond-mat_mtrl-sci |
Bismuth-surfactant-induced growth and structure of InAs/GaAs(110)
quantum dots: We explore the Bi-surfactant-directed self-assembly and structure of InAs
quantum dots grown on GaAs(110) by molecular beam epitaxy. The addition of a Bi
flux during InAs deposition changes the InAs growth mode from two-dimensional
(2D) Frank-van der Merwe to Stranski-Krastanov, resulting in the formation of
three-dimensional (3D) InAs islands on the surface. Furthermore, exposing
static InAs 2D layers to Bi induces a rearrangement of the strained layer into
3D islands. We explore the effect of varying the InAs thickness and Bi flux for
these two growth approaches, observing a critical thickness for 3D island
formation in both cases. Characterization of (110) InAs quantum dots with
high-resolution transmission electron microscopy reveals that larger islands
grown by the Stranski-Krastanov mode are plastically relaxed, while small
islands grown by the on-demand approach are coherent. Strain relaxation along
the [1-10] direction is achieved by 90 degree pure-edge dislocations with
dislocation lines running along [001]. In contrast, strain relief along [001]
is by 60 degree misfit dislocations. This behaviour is consistent with
observations of planar (In,Ga)As/GaAs(110) layers. These results illustrate how
surfactant Bi can provoke and control quantum dot formation where it normally
does not occur. | cond-mat_mtrl-sci |
Magnetic properties of Sn-substituted Ni-Zn ferrite:synthesized from
nano-sized powders of NiO, ZnO, Fe2O3 and SnO2: A series of Ni0.6-x/2Zn0.4-x/2SnxFe2O4 (x = 0.0, 0.05, 0.1, 0.15, 0.2 and
0.3) (NZSFO) ferrite composites have been synthesized from nano powders using
standard solid state reaction technique. The spinel cubic structure of the
investigated samples has been observed by the X-ray diffraction (XRD). The
magnetic properties such as saturation magnetization (Ms), remanent
magnetization (Mr), coercive field (Hc) and Bohr magneton (B) are calculated
from the hysteresis loops. The value of Ms is found to decrease with increasing
Sn content in the samples. This change has been successfully explained by the
variation of A-B interaction strength due to Sn substitution in different
sites. The compositional stability and quality of the prepared ferrite
composites have also been endorsed by the fairly constant initial permeability
(/) over a wide range of frequency region. The decreasing trend of / with
increasing Sn content has been observed. Curie temperature (TC) has found to
increase with the increase in Sn content. Wide spread frequency utility zone
indicates that the NZSFO can be considered as a good candidate for use in
broadband pulse transformer and wide band read-write heads for video recording.
The abnormal behavior for x = 0.05 has been explained with existing theory. | cond-mat_mtrl-sci |
The Effect of short-range order on the viscosity and crystallization of
Al-Mg melts: In this work, using the methods of viscosimetry and thermal analysis, the
concentration changes in the values of the supercooling viscosity of Al-Mg
melts with Mg content from 2.5 to 95 at.% are studied. It is shown that the
temperature dependences of viscosity are well described by an exponential
dependence. The concentration dependence of viscosity is not monotonous and
reflects a change in the chemical short-range order in the liquid phase. The
concentration dependence of supercooling of Al-Mg melts is determined by the
type of solid phase formed during solidification, and also reflects the most
significant changes in the chemical short-range order in the liquid phase at 20
and 80 at.% Mg. Al-Mg alloys in the concentration ranges: 0-10, 40-50 and
90-100 at.% Mg are prone to non-equilibrium crystallization, the formation of
quasi-eutectics and solidification without intermediate intermetallic phases. | cond-mat_mtrl-sci |
Theoretical investigation on the ferromagnetic two-dimensional scandium
monochloride sheet that has a high Curie temperature and could be exfoliated
from a known material: A two-dimensional scandium monochloride sheet was investigated by using
density functional theory. It could be exfoliated from a known bulk material
with a cleavage energy slightly lower than that of graphene. The sheet has a
ferromagnetic ground state with a Curie temperature of 100 K. Moreover, the
sheet becomes a half-metal under hole doping. The Curie temperature increases
to 250 K with the doping amount of 0.4 per primitive cell, which is close to
the ice point. The two-dimensional scandium monochloride sheet should be a good
candidate for two-dimensional spintronics. | cond-mat_mtrl-sci |
A DFT based first-principles investigation of the physical properties of
Bi2Te2Se topological insulator: A topological insulator possesses a bulk energy gap splitting the lowest
empty band from the highest occupied electronic band. The electronic states at
the surface (or edge in two dimensions), on the other hand, of a topological
insulator are gapless and are protected by the time reversal symmetry. Such
systems are promising for variety of optoelectronic, superconducting,
thermoelectric and quantum computation related applications. We have studied
elastic, mechanical, electronic, optical properties, bonding character and the
electronic charge density distribution of ternary Bi2Te2Se topological
insulator. The compound under study is mechanically stable and elastically
anisotropic. The electronic band structure calculations reveal high degree of
anisotropy in the energy dispersion. Electronic effective mass is high in the
c-direction compared to that in the ab-plane. The optical constants show
moderate level of variation with respect to the polarization of the electric
field of the incident radiation. The optical spectra are consistent with the
electronic band structure and electronic density of states features. Both
electronic band structure and optical constants show clear indications of a
direct band gap of 0.610 eV for Bi2Te2Se. It is also found that Bi2Te2Se
possesses high refractive index at low photon energies in the infrared and
visible region. It has low reflectivity in the ultraviolet region. Bi2Te2Se
absorbs photons strongly in the ultraviolet energies. All these features make
Bi2Te2Se suitable for diverse class of optoelectronic device applications. | cond-mat_mtrl-sci |
Colossal room-temperature electrocaloric strength aided by hydrostatic
pressure in lead-free multiferroic solid solutions: Solid-state cooling applications based on the electrocaloric (EC) effect are
particularly promising from a technological point of view due to their downsize
scalability and natural implementation in circuitry. However, EC effects
typically occur far from room temperature, involve materials that contain toxic
substances and require relatively large electric fields ($\sim 100$-$1000$ kV
cm$^{-1}$) that cause fateful leakage current and dielectric loss problems.
Here, we propose a possible solution to these practical issues that consists in
concertedly applying hydrostatic pressure and electric fields on lead-free
multiferroic materials. We theoretically demonstrate this strategy by
performing first-principles simulations on supertetragonal
BiFe$_{1-x}$Co$_{x}$O$_{3}$ solid solutions (BFCO). It is shown that
hydrostatic pressure, besides adjusting the occurrence of EC effects to near
room temperature, can reduce enormously the intensity of the driving electric
fields. For pressurized BFCO, we estimate a colossal room-temperature EC
strength, defined like the ratio of the adiabatic EC temperature change by the
applied electric field, of $\sim 1$ K cm kV$^{-1}$, a value that is several
orders of magnitude larger than those routinely measured in uncompressed
ferroelectrics. | cond-mat_mtrl-sci |
Review of Theoretical and Computational Methods for 2D Materials
Exhibiting Charge Density Waves: Two-dimensional (2D) materials that exhibit charge density waves (CDWs) have
generated many research endeavors in the hopes of employing their exotic
properties for various quantum-based technologies. Early investigations
surrounding CDWs were mostly focused on bulk materials. However, applications
for quantum devices have since required devices to be constructed from
few-layer material to fully utilize the material's properties. This field has
greatly expanded over the decades, warranting a focus on the computational
efforts surrounding CDWs in 2D materials. In this review, we will cover ground
in the following relevant, theory-driven subtopics for TaS2 and TaSe2: summary
of general computational techniques and methods, atomic structures, Raman
modes, and effects of confinement and dimensionality. Through understanding how
the computational methods have enabled incredible advancements in quantum
materials, one may anticipate the ever-expanding directions available for
continued pursuit as the field brings us through the 21st century. | cond-mat_mtrl-sci |
Hund's physics and the magnetic ground state of CrOX (X = Cl, Br): To understand the magnetic property of layered van der Waals materials CrOX
(X = Cl, Br), we performed the detailed first-principles calculations for both
bulk and monolayer. We found that the charge-only density functional theory
combined with the explicit on-site interaction terms (so-called cDFT$+U$) well
reproduces the experimental magnetic ground state of bulk CrOX, which is not
the case for the use of spin-dependent density functional (so-called sDFT$+U$).
Unlike some of the previous studies, our results show that CrOX monolayers are
antiferromagnetic as in the bulk. It is also consistent with our magnetic force
linear response calculation of exchange couplings $J_{\rm ex}$. The result of
orbital-decomposed $J_{\rm ex}$ calculations shows that the Cr
$t_\textrm{2g}$-$t_\textrm{2g}$ component mainly contributes to the
antiferromagnetic order in both bulk and monolayer. Our result and analysis
show that taking the correct Hund's physics into account is of key importance
to construct the magnetic phase diagram and to describe the electronic
structure. | cond-mat_mtrl-sci |
Data based constitutive modelling of rate independent inelastic effects
in composite cables using Preisach hysteresis operators: This contribution aims at introducing first steps to develop hysteresis
operator type inelastic constitutive laws for Cosserat rods for the simulation
of cables composed of complex interior components. Motivated by the basic
elements of Cosserat rod theory, we develop a specific approach to constitutive
modelling adapted for this application. Afterwards, we describe the
hysteretical behaviour arising from cyclic bending experiments on cables by
means of the Preisach operator. As shown in pure bending experiments, slender
structures such as electric cables behave inelastically, and open hysteresis
loops arise with noticeable difference between the first load cycle and the
following ones. The Preisach operator plays an important role in describing the
input-output relation in hysteresis behaviours, and it can be expressed as a
superposition of relay operators. Hence, a mathematical formulation of the
problem is introduced, and a first attempt is made to determine the hysteresis
behaviour that describes the relation between curvature and bending moment.
Therefore, a suitable kernel function is identified in a way that its
integration over the Preisach plane results in the bending moment of the
specimen, and a comparison between different kernel functions is performed. | cond-mat_mtrl-sci |
Vibration Damping of Carbon Nanotube Assembly Materials: Vibration reduction is of great importance in various engineering
applications, and a material that exhibits good vibration damping along with
high strength and modulus has become more and more vital. Owing to the superior
mechanical property of carbon nanotube (CNT), new types of vibration damping
material can be developed. This paper presents recent advancements, including
our progresses, in the development of high-damping macroscopic CNT assembly
materials, such as forests, gels, films, and fibers. In these assemblies,
structural deformation of CNTs, zipping and unzipping at CNT connection nodes,
strengthening and welding of the nodes, and sliding between CNTs or CNT bundles
are playing important roles in determining the viscoelasticity, and elasticity
as well. Towards the damping enhancement, strategies for micro-structure and
interface design are also discussed. | cond-mat_mtrl-sci |
A Cosserat crystal plasticity and phase field theory for grain boundary
migration: The microstructure evolution due to thermomechanical treatment of metals can
largely be described by viscoplastic deformation, nucleation and grain growth.
These processes take place over different length and time scales which present
significant challenges when formulating simulation models. In particular, no
overall unified field framework exists to model concurrent viscoplastic
deformation and recrystallization and grain growth in metal polycrystals. In
this work a thermodynamically consistent diffuse interface framework
incorporating crystal viscoplasticity and grain boundary migration is
elaborated. The Kobayashi--Warren--Carter (KWC) phase field model is extended
to incorporate the full mechanical coupling with material and lattice rotations
and evolution of dislocation densities. The Cosserat crystal plasticity theory
is shown to be the appropriate framework to formulate the coupling between
phase field and mechanics with proper distinction between bulk and grain
boundary behaviour. | cond-mat_mtrl-sci |
Handedness manipulation of propagating antiferromagnetic magnons: Antiferromagnetic magnons possess a distinctive feature absent in their
ferromagnetic counterparts: the presence of two distinct handedness modes, the
right-handed (RH) and left-handed (LH) precession modes. The magnon handedness
determines the sign of spin polarization carried by the propagating magnon,
which is indispensable for harnessing the diverse functionalities. However, the
control of coherently propagating magnon handedness in antiferromagnets has
remained elusive so far. Here we demonstrate the manipulation and electrical
readout of propagating magnon handedness in perpendicularly magnetized
synthetic antiferromagnets (SAF). We find that the antiferromagnetic magnon
handedness can be directly identified by measuring the inverse spin Hall effect
(ISHE) voltage, which arises from the spin pumping effect caused by the
propagating antiferromagnetic magnons in the SAF structure. The RH and LH modes
of the magnon can be distinguishable particularly when the SAF structure is
sandwiched by heavy metals with the same sign of spin Hall angle. Moreover, we
succeed in controlling the handedness of propagating antiferromagnetic magnons
by tuning the excitation microwave frequency. This work unveils promising
avenues for harnessing magnon unique properties in antiferromagnet-based
magnonic applications. | cond-mat_mtrl-sci |
Ab Initio Study of the Structural Phase Transition in Cubic Pb_3GeTe_4: In the substitutionally disordered narrow-gap semiconductor Pb_{1-x}Ge_xTe, a
finite-temperature cubic-rhombohedral transition appears above a critical
concentration $x \approx 0.005$. As a first step towards a first-principles
investigation of this transition in the disordered system, a (hypothetical)
ordered cubic Pb_3GeTe_4 supercell is studied. First principles
density-functional calculations of total energies and linear response functions
are performed using the conjugate-gradients method with ab initio
pseudopotentials and a plane-wave basis set. Unstable modes in Pb_3GeTe_4 are
found, dominated by off-centering of the Ge ions coupled with displacements of
their neighboring Te ions. A model Hamiltonian for this system is constructed
using the lattice Wannier function formalism. The parameters for this
Hamiltonian are determined from first principles. The equilibrium
thermodynamics of the model system is studied via Metropolis Monte Carlo
simulations. The calculated transition temperature, T_c, is approximately 620K
for the cubic Pb_3GeTe_4 model, compared to the experimental value of T_c
\approx 350K for disordered Pb_{0.75}Ge_{0.25}Te. Generalization of this
analysis to the disordered Pb_{1-x}Ge_xTe system is discussed. | cond-mat_mtrl-sci |
Dislocation scattering in a two-dimensional electron gas: A theory of scattering by charged dislocation lines in a two-dimensional
electron gas (2DEG) is developed. The theory is directed towards understanding
transport in AlGaN/GaN high-electron-mobility transistors (HEMT), which have a
large number of line dislocations piercing through the 2DEG. The scattering
time due to dislocations is derived for a 2DEG in closed form. This work
identifies dislocation scattering as a mobility-limiting scattering mechanism
in 2DEGs with high dislocation densities. The insensitivity of the 2DEG (as
compared to bulk) to dislocation scattering is explained by the theory. | cond-mat_mtrl-sci |
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