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A fully ab initio approach to inelastic atom-surface scattering: We introduce a fully ab initio theory for inelastic scattering of any atom
from any surface exciting single phonons, and apply the theory to helium
scattering from Nb(100). The key aspect making our approach general is a direct
first-principles evaluation of the scattering atom-electron vertex. By
correcting misleading results from current state-of-the-art theories, this
fully ab initio approach will be critical in guiding and interpreting
experiments that adopt next-generation, non-destructive atomic beam scattering. | cond-mat_mtrl-sci |
Metalloboranes from first-principles calculations: A candidate for
high-density hydrogen storage: Using first principles calculations, we show the high hydrogen storage
capacity of a new class of compounds, metalloboranes. Metalloboranes are
transition metal (TM) and borane compounds that obey a novel-bonding scheme. We
have found that the transition metal atoms can bind up to 10 H2 molecules. | cond-mat_mtrl-sci |
Modelling the Nonlinear Response of Fibre-reinforced Bending Fluidic
Actuators: Soft actuators are receiving increasing attention from the engineering
community, not only in research but even for industrial applications. Among
soft actuators, fibre-reinforced Bending Fluidic Actuators (BFAs) became very
popular thanks to features such as robustness and easy design and fabrication.
However, an accurate modelling of these smart structures, taking into account
all the nonlinearities involved, is a challenging task. In this effort, we
propose an analytical mechanical model to capture the quasi-static response of
fibre-reinforced BFAs. The model is fully 3D and for the first time includes
the effect of the pressure on the lateral surface of the chamber as well as the
non-constant torque produced by the pressure at the tip. The presented model
can be used for design and control, while providing information about the
mechanics of these complex actuators. | cond-mat_mtrl-sci |
On microcontinuum field theories: the Eshelby stress tensor and
incompatibility conditions: We investigate linear theories of incompatible micromorphic elasticity,
incompatible microstretch elasticity, incompatible micropolar elasticity and
the incompatible dilatation theory of elasticity (elasticity with voids). The
incompatibility conditions and Bianchi identities are derived and discussed.
The Eshelby stress tensor (static energy momentum) is calculated for such
inhomogeneous media with microstructure. Its divergence gives the driving
forces for dislocations, disclinations, point defects and inhomogeneities which
are called configurational forces. | cond-mat_mtrl-sci |
Excitonic Photoluminescence properties of nanocrystalline GaSb and
Ga0.62In0.38Sb embedded in silica films: The GaSb and Ga0.62In0.38Sb nanocrystals were embedded in the SiO2 films by
radio-frequency magnetron co-sputtering and were grown on GaSb and Si
substrates at different temperatures. We present results on the 10K excitonic
photoluminescence (PL) properties of nanocrystalline GaSb and Ga0.62In0.38Sb as
a function of their size. The measurements show that the PL of the GaSb and
Ga0.62In0.38Sb nanocrystallites follows the quantum confinement model very
closely. By using deconvolution of PL spectra, origins of structures in
photoluminescence were identified. | cond-mat_mtrl-sci |
Excitons and Many-Electron Effects in the Optical Response of
Single-Walled Boron Nitride Nanotubes: We report first-principles calculations of the effects of quasiparticle
self-energy and electron-hole interaction on the optical properties of
single-walled BN nanotubes. Excitonic effects are shown to be even more
important in BN nanotubes than in carbon nanotubes. Electron-hole interactions
give rise to complexes of bright (and dark) excitons, which qualitatively alter
the optical response. Excitons with binding energy larger than 2 eV are found
in the (8,0) BN nanotubes. Moreover, unlike the carbon nanotubes, theory
predicts that these exciton states are comprised of coherent supposition of
transitions from several different subband pairs, giving rise to novel
behaviors. | cond-mat_mtrl-sci |
The roles of adhesion, internal heat generation and elevated
temperatures in normally loaded, sliding rough surfaces: The thermal effects of plastic and frictional heat generation and elevated
temperature were examined along with the role of adhesion in the context of
galling wear, using a representative crystal plasticity, normally loaded,
sliding surface model. Galling frequency behaviour was predicted for 316L
steel. Deformation of the surfaces was dominated by the surface geometry, with
no significant effect due to variations in frictional models. Plastic and
frictional heating were found to have a minimal effect on the deformation of
the surface, with the rapid conduction of heat preventing any highly localised
heating. There was no corresponding effect on the predicted galling frequency
response.
Isothermal, elevated temperature conditions caused a decrease in galling
resistance, driven by the temperature sensitivity of the critical resolved
shear stress. The extent of deformation, as quantified by the area of
plastically deformed material and plastic reach, increased with temperature.
Comparisons were made with literature results for several surface amplitude and
wavelength conditions. Model results compared favourably with those in the
literature. However, the reduction in predicted galling resistance with
elevated temperature for a fixed surface was not as severe as observations in
the literature, suggesting other mechanisms (e.g. phase transformations,
surface coatings and oxides) are likely important. | cond-mat_mtrl-sci |
Buckled honeycomb lattice and unconventional magnetic response: We study the magnetic response of buckled honeycomb-lattice materials. The
buckling breaks the sublattice symmetry, enhances the spin-orbit coupling, and
allows the tuning of a topological quantum phase transition. As a result, there
are two doubly degenerate spin-valley coupled massive Dirac bands, which
exhibit an unconventional Hall plateau sequence under strong magnetic fields.
We show how to externally control the splitting of anomalous zeroth Landau
levels, the prominent Landau level crossing effects, and the polarizations of
spin, valley, and sublattice degrees of freedom. In particular, we reveal that
in a p-n junction, spin-resolved fractionally quantized conductance appears in
a two-terminal measurement with a spin-polarized current propagating along the
interface. In the low-field regime where the Landau quantization is not
applicable, we provide a semiclassical description for the anomalous Hall
transport. We comment briefly on the effects of electron-electron interactions
and Zeeman couplings to electron spins and to atomic orbitals. | cond-mat_mtrl-sci |
Tunable mechanical and thermal properties of ZnS/CdS core/shell
nanowires: Using all atom molecular dynamics (MD) simulations, we have studied the
mechanical properties of ZnS/CdS core/shell nanowires. Our results show that
the coating of a few atomic layer CdS shell on the ZnS nanowire leads to a
significant change in the stiffness of the core/shell nanowires compared to the
stiffness of pure ZnS nanowires. The binding energy between the core and shell
region decreases due to the lattice mismatch at the core-shell interface. This
reduction in binding energy plays an important role in determining the
stiffness of a core/shell nanowire. We have also investigated the effects of
the shell on the thermal conductivity and melting behavior of the nanowires. | cond-mat_mtrl-sci |
Unusually High and Anisotropic Thermal Conductivity in Amorphous Silicon
Nanostructures: Amorphous Si (a-Si) nanostructures are ubiquitous in numerous electronic and
optoelectronic devices. Amorphous materials are considered to possess the lower
limit to the thermal conductivity (k), which is ~1 W/m-K for a-Si. However,
recent work suggested that k of micro-thick a-Si films can be greater than 3
W/m-K, which is contributed by propagating vibrational modes, referred to as
"propagons". However, precise determination of k in a-Si has been elusive.
Here, we used novel structures of a-Si nanotubes and suspended a-Si films that
enabled precise in-plane k measurement within a wide thickness range of 5 nm to
1.7 um. We showed unexpectedly high in plane k in a-Si nanostructures, reaching
~3.0 and 5.3 W/m-K at 100 nm and 1.7 um, respectively. Furthermore, the
measured in plane k is significantly higher than the cross-plane k on the same
films. This usually high and anisotropic k in the amorphous Si nanostructures
manifests the surprising broad propaganda mean free path distribution, which is
found to range from 10 nm to 10 um, in the disordered and atomically isotropic
structure. This result provides an unambiguous answer to the century-old
problem regarding the mean free path distribution of propagons and also shed
light on the design and performance of numerous a-Si based electronic and
optoelectronics devices. | cond-mat_mtrl-sci |
Designing of Organic Bridging Linkers of Metal-Organic Frameworks for
Enhanced Carbon Dioxide Adsorption: The global rate of anthropogenic CO2 emission is rising, which urges the
development of efficient carbon capture and storage (CCS) technologies. Among
the various CO2 capture methods, adsorption by the linkers of the Metal-Organic
Frameworks (MOFs) materials has received more interest as excellent CO2
adsorbents because of their important role in understanding the interaction
mechanism for CO2 adsorption. Here, we investigate the adsorption of CO2
molecules at the center and side positions of several MOF-linkers using
molecular cluster models. The interaction between CO2 and the linkers is
approximated by computing the binding enthalpy ({\Delta}H) through the first
principles-based Density Functional Theory with Grimmes dispersion correction
(i.e., B3LYP-D3) and second-order Moller Plesset Theory (MP2). The computed
values of {\Delta}H indicate the weak nature of CO2 adsorption on the pristine
linkers, hence the strategy of lithium decoration is used to see its impact on
the binding strength. Among the various linkers tested, CO2 adsorbing at the
side position of the DFBDC-2 linker has strong adsorption with {\Delta}H value
of about -35.32 kJ/mol computed by the B3LYP-D3 method. The Energy
Decomposition Analysis (EDA) study reveals that among all the energy terms, the
contribution of electrostatic and polarization energy terms to the {\Delta}H
value are the most dominant one. Furthermore, the results of Frontier Molecular
Orbital Analysis (FMO) revealed that all the linkers remained stable even after
Li-decoration. The results of our investigations will direct towards the
development and synthesis of novel adsorbents with enhanced CO2 adsorption. | cond-mat_mtrl-sci |
Novel experimental design for high pressure - high temperature
electrical resistance measurements in a 'Paris-Edinburgh' large volume press: We present a novel experimental design for high sensitivity measurements of
the electrical resistance of samples at high pressures (0-6GPa) and high
temperatures (300-1000K) in a 'Paris-Edinburgh' type large volume press.
Uniquely, the electrical measurements are carried out directly on a small
sample, thus greatly increasing the sensitivity of the measurement. The
sensitivity to even minor changes in electrical resistance can be used to
clearly identify phase transitions in material samples. Electrical resistance
measurements are relatively simple and rapid to execute and the efficacy of the
present experimental design is demonstrated by measuring the electrical
resistance of Pb, Sn and Bi across a wide domain of temperature-pressure phase
space and employing it to identify the loci of phase transitions. Based on
these results, the phase diagrams of these elements are reconstructed to high
accuracy and found to be in excellent agreement with previous studies. In
particular, by mapping the locations of several well-studied reference points
in the phase diagram of Sn and Bi, it is demonstrated that a standard
calibration exists for the temperature and pressure, thus eliminating the need
for direct or indirect temperature and pressure measurements. The present
technique will allow simple and accurate mapping of phase diagrams under
extreme conditions and may be of particular importance in advancing studies of
liquid state anomalies. | cond-mat_mtrl-sci |
Electronic and optical properties in graphane: We develop the tight-binding model to study electronic and optical properties
of graphane. The strong sp3 chemical bondings among the carbon and hydrogen
atoms induce a special band structure and thus lead to the rich optical
excitations. The absorption spectrum hardly depends on the direction of
electric polarization. It ex- hibits a lot of shoulder structures and
absorption peaks, which arise from the extreme points and the saddle points of
the parabolic bands, respectively. The threshold op- tical excitations, only
associated with the 2px and 2py orbitals of the carbon atoms, are revealed in a
shoulder structure at ?3.5 eV. The first symmetric absorption peak, appearing
at ~11 eV, corresponds to energy bands due to the considerable hybridiza- tion
of carbon 2pz orbitals and H 1s orbitals. Also, some absorption peaks at higher
frequencies indicate the bonding of 2s and 1s orbitals. These results are in
sharp contrast to those of the sp2 graphene systems. | cond-mat_mtrl-sci |
Inherent heating instability of direct microwave sintering process:
Sample analysis for porous 3Y-ZrO2: Direct microwave heating of 3Y-ZrO 2 is studied at frequency of 2.45 GHz.
Different conditions of input power, sample position and size are tested. For
the first time, the experimentally known instability of microwave sintering is
explained coupling the effective medium approximation and finite-element
method. We show how the material dielectric permittivity imaginary part which
increases with temperature and relative density encourages high hot spot
phenomena. It is shown that the sample location has a great impact on the | cond-mat_mtrl-sci |
NiCl3 Monolayer: Dirac Spin-Gapless Semiconductor and Chern Insulator: The great obstacle for practical applications of the quantum anomalous Hall
(QAH) effect is the lack of suitable QAH materials (Chern insulators) with
large non-trivial band gap, room-temperature magnetic order and high carrier
mobility. The Nickle chloride (NiCl3) monolayer characteristics are
investigated herein using first-principles calculations. It is reported that
NiCl3 monolayers constitute a new class of Dirac materials with Dirac
spin-gapless semiconducting and high-temperature ferromagnetism (~400K). Taking
into account the spin-orbit coupling, the NiCl3 monolayer becomes an intrinsic
insulator with a large non-trivial band gap of ~24 meV, corresponding to an
operating temperature as high as ~280K at which the quantum anomalous Hall
effect could be observed. The calculated large non-trivial gap, high Curie
temperature and single-spin Dirac states reported herein for the NiCl3
monolayer lead us to propose that this material give a great promise for
potential realization of a near-room temperature QAH effect and potential
applications in spintronics. Last but not least the calculated Fermi velocities
of Dirac fermion of about 4x105 m/s indicate very high mobility in NiCl3
monolayers. | cond-mat_mtrl-sci |
Combined experimental and theoretical investigation of the
premartensitic transition in Ni$_2$MnGa: Ultraviolet-photoemission (UPS) measurements and supporting specific-heat,
thermal-expansion, resistivity and magnetic-moment measurements are reported
for the magnetic shape-memory alloy Ni$_2$MnGa over the temperature range $100K
< T < 250K$. All measurements detect clear signatures of the premartensitic
transition ($T_\mathrm{PM}\sim 247K$) and the martensitic transition
($T_\mathrm{M} \sim 196K$). Temperature-dependent UPS shows a dramatic
depletion of states (pseudogap) at $T_\mathrm{PM}$ located 0.3eV below the
Fermi energy. First-principles electronic structure calculations show that the
peak observed at 0.3eV in the UPS spectra for $T > T_\mathrm{PM}$ is due to the
Ni-d minority-spin electrons. Below $T_\mathrm{M}$ this peak disappears,
resulting in an enhanced density of states at energies around 0.8eV. This
enhancement reflects Ni-d and Mn-d electronic contributions to the
majority-spin density of states and is accompanied by significant
reconstruction of the Fermi surface. | cond-mat_mtrl-sci |
Advanced calculations of x-ray spectroscopies with FEFF10 and Corvus: The real-space Green's function code FEFF has been extensively developed and
used for calculations of x-ray and related spectra, including x-ray absorption
(XAS), x-ray emission (XES), inelastic x-ray scattering, and electron energy
loss spectra (EELS). The code is particularly useful for the analysis and
interpretation of the XAS fine-structure (EXAFS) and the near-edge structure
(XANES) in materials throughout the periodic table. Nevertheless, many
applications, such as non-equilibrium systems, and the analysis of ultra-fast
pump-probe experiments, require extensions of the code including
finite-temperature and auxiliary calculations of structure and vibrational
properties. To enable these extensions, we have developed in tandem, a new
version FEFF10, and new FEFF based workflows for the Corvus workflow manager,
which allow users to easily augment the capabilities of FEFF10 via auxiliary
codes. This coupling facilitates simplified input and automated calculations of
spectra based on advanced theoretical techniques. The approach is illustrated
with examples of high temperature behavior, vibrational properties, many-body
excitations in XAS, super-heavy materials, and fits of calculated spectra to
experiment. | cond-mat_mtrl-sci |
Analysis of the linear relationship between asymmetry and magnetic
moment at the M-edge of 3d transition metals: The magneto-optical response of Fe and Ni during ultrafast demagnetization is
studied experimentally and theoretically. We have performed pump-probe
experiments in the transverse magneto-optical Kerr effect (T-MOKE) geometry
using photon energies that cover the M-absorption edges of Fe and Ni between 40
to 72 eV. The asymmetry was detected by measuring the reflection of light for
two different orientations of the sample magnetization. Density functional
theory (DFT) wasused to calculate the magneto-optical response of different
magnetic configurations, representing different types of excitations:
long-wavelength magnons, short wavelength magnons, and Stoner excitations. In
the case of Fe, we find that the calculated asymmetry is strongly dependent on
the specific type of magnetic excitation. Our modelling also reveals that
during remagnetization Fe is, to a reasonable approximation, described by
magnons, even though small non-linear contributions could indicate some degree
of Stoner excitations as well. In contrast, we find that the calculated
asymmetry in Ni is rather insensitive to the type of magnetic excitations.
However, there is a weak non-linearity in the relation between asymmetry and
the off-diagonal component of the dielectric tensor, which does not originate
from the modifications of the electronic structure. Our experimental and
theoretical results thus emphasize the need of considering a coupling between
asymmetry and magnetization that may be more complex that a simple linear
relationship. This insight is crucial for the microscopic interpretation of
ultrafast magnetization experiments. | cond-mat_mtrl-sci |
Quantitative Phase Field Model for Electrochemical Systems: Modeling microstructure evolution in electrochemical systems is vital for
understanding the mechanism of various electrochemical processes. In this work,
we propose a general phase field framework that is fully variational and thus
guarantees that the energy decreases upon evolution in an isothermal system.
The bulk and interface free energies are decoupled using a grand potential
formulation to enhance numerical efficiency. The variational definition of the
overpotential is used, and the reaction kinetics is incorporated into the
evolution equation for the phase field to correctly capture capillary effects
and eliminate additional model parameter calibrations. A higher-order kinetic
correction is derived to accurately reproduce general reaction models such as
the Butler-Volmer, Marcus, and Marcus-Hush-Chidsey models. Electrostatic
potentials in the electrode and the electrolyte are considered separately as
independent variables, providing additional freedom to capture the interfacial
potential jump. To handle realistic materials and processing parameters for
practical applications, a driving force extension method is used to enhance the
grid size by three orders of magnitude. Finally, we comprehensively verify our
phase field model using classical electrochemical theory. | cond-mat_mtrl-sci |
Real time optical observation and control of atomically thin transition
metal dichalcogenide synthesis: Understanding the mechanisms involved in chemical vapour deposition (CVD)
synthesis of atomically thin transition metal dichalcogenides (TMDCs) requires
the precise control of numerous growth parameters. All the proposed mechanisms
and their relation to the growth conditions are inferred from characterising
intermediate formations obtained by stopping the growth blindly. To fully
understand the reaction routes that lead to the monolayer formation, real time
observation and control of the growth are needed. Here, we demonstrate how a
custom-made CVD chamber that allows real time optical monitoring can be
employed to study the reaction routes that are critical to the production of
the desired layered thin crystals in salt assisted TMDC synthesis. Our real
time observations reveal the reaction between the salt and the metallic
precursor to form intermediate compounds which lead to the layered crystal
formation. We identified that both the vapour-solid-solid and
vapour-liquid-solid growth routes are in an interplay. Furthermore, we
demonstrate the role H$_{2}$ plays in the salt-assisted WSe$_{2}$ synthesis.
Finally, we guided the crystal formation by directing the liquid intermediate
compound through pre-patterned channels. The methods presented in this article
can be extended to other materials that can be synthesized via CVD. | cond-mat_mtrl-sci |
Shack-Hartmann wavefront sensing: A new approach to time-resolved
measurement of stress intensity during dynamic fracture of small brittle
specimens: The stress intensity factor is important for understanding crack initiation
and propagation. Because it cannot be measured directly, the characterization
of the stress intensity factor relies on the measurement of deformation around
a crack tip. Such measurements are challenging for dynamic fracture of brittle
materials where the deformation is small and the crack tip velocity can be high
(>1 km/s). Digital gradient sensing (DGS) is capable of full-field measurement
of surface deformation with sub-microsecond temporal resolution, but it is
limited to centimeter-scale specimens and has a spatial resolution of only
$\sim 1$mm. This limits its ability to measure deformations close to the crack
tip. Here, we demonstrate the potential of Shack-Hartmann wavefront sensing
(SHWFS), as an alternative to DGS, for measuring surface deformation during
dynamic brittle fracture of millimeter-scale specimens. Using an commercial
glass ceramic as an example material, we demonstrate the capability of SHWFS to
measure the surface slope evolution induced by a propagating crack on
millimeter-scale specimens with a micrometer-scale spatial resolution and a
sub-microsecond temporal resolution. The SHWFS apparatus has the additional
advantage of being physically more compact than a typical DGS apparatus. We
verify our SHWFS measurements by comparing them with analytical predictions and
phase-field simulations of the surface slope around a crack tip. Then, fitting
the surface slope measurements to the asymptotic crack-tip field solution, we
extract the evolution of the apparent stress intensity factor associated with
the propagating crack tip. We conclude by discussing potential future
enhancements of this technique and how its compactness could enable the
integration with other characterization techniques including x-ray
phase-contrast imaging (XPCI) toward a multi-modal characterization. | cond-mat_mtrl-sci |
Ultrafast electronic and lattice dynamics in laser-excited crystalline
bismuth: Femtosecond spectroscopy is applied to study transient electronic and lattice
processes in bismuth. Components with relaxation times of 1 ps, 7 ps and ~ 1 ns
are detected in the photoinduced reflectivity response of the crystal. To
facilitate the assignment of the observed relaxation to the decay of particular
excited electronic states we use pump pulses with central wavelengths ranging
from 400 nm to 2.3 mum. Additionally, we examine the variation of parameters of
coherent A1g phonons upon the change of excitation and probing conditions. Data
analysis reveals a significant wavevector dependence of electron-hole and
electron- phonon coupling strength along \Gamma--T direction of the Brillouin
zone. | cond-mat_mtrl-sci |
Random sequential adsorption of tetramers: Adsorption of tetramer built of four identical spheres was studied
numerically using the Random Sequential Adsorption (RSA) algorithm. Tetramers
were adsorbed on a two dimensional, flat and homogeneous surface. Two different
models of the adsorbate were investigated: a rhomboid and a square one; monomer
centres were put on vertices of rhomboids and squares, respectively. Numerical
simulations allow to establish the maximal random coverage ratio as well as the
Available Surface Function (ASF), which is crucial for determining kinetics of
the adsorption process. These results were compared with data obtained
experimentally for KfrA plasmid adsorption. Additionally, the density
autocorrelation function was measured. | cond-mat_mtrl-sci |
Hybrid Optical Modes in Hexagonal Crystals: In nanostructure electronic devices, it is well-known that the optical
lattice waves in the constituent semiconductor crystals have to obey both
mechanical and electrical boundary conditions at an interface. The theory of
hybrid optical modes, established for cubic crystals, is here applied to
hexagonal crystals. In general, the hybrid is a linear combination of a
longitudinally-polarized (LO) mode, an interface mode (IF), and an interface TO
mode. It is noted that the dielectric and elastic anisotropy of these crystals
add significant complications to the assessment of the electro-phonon
interaction. We point out that, where extreme accuracy is not needed, a cubic
approximation is available. The crucial role of lattice dispersion is
emphasised. In the extreme long-wavelength limit, where lattice dispersion is
unimportant, the polar optical hybrid consists of an LO component plus an IF
component only. In his case no fields are induced in the barrier, and there are
no remote-phonon effects. | cond-mat_mtrl-sci |
Twist-bend heliconical chiral nematic liquid crystal phase of an achiral
rigid bent-core mesogen: The chiral, heliconical (twist-bend) nematic ground state is reported in an
achiral, rigid, bent-core mesogen (UD68). Similar to the nematic twist-bend
(NTB) phase observed in bent molecular dimers, the NTB phase of UD68 forms
macroscopic, smectic-like focal-conic textures and exhibits nanoscale, periodic
modulation with no associated modulation of the electron density, i.e., without
a detectable lamellar x-ray reflection peak. The NTB helical pitch is pTB ~ 14
nm. When an electric field is applied normal to the helix axis, a weak
electroclinic effect is observed, revealing 50 um-scale left- and right-handed
domains in a chiral conglomerate. | cond-mat_mtrl-sci |
Spin dynamics from a constrained magnetic Tight-Binding model: A dynamics of the precession of coupled atomic moments in the tight-binding
(TB) approximation is presented. By implementing an angular penalty functional
in the energy that captures the magnetic effective fields self-consistently,
the motion of the orientation of the local magnetic moments is observed faster
than the variation of their magnitudes. This allows the computation of the
effective atomic magnetic fields that are found consistent with the
Heisenberg's exchange interaction, by comparison with classical atomistic spin
dynamics on Fe, Co and Ni magnetic clusters. | cond-mat_mtrl-sci |
Improvement of the poly-3-hexylthiophene transistor performance using
small molecule contact functionalization: We demonstrate an approach to improve poly-3-hexylthiophene field effect
transistors by modifying the gold contacts with monolayer thick
pentacenequinone (PQ) or naphthalene (NL). The effective contact resistance is
reduced by a factor of two and sixteen for interlayers of PQ and NL,
respectively. The observation is attributed to different injection barriers at
the metal-organic interface caused by the functionalization and to an
additional tunneling barrier enhancing the on/off ratios. This barrier yields
to activation energies of 37meV (NL) and 104meV (PQ) below 190K, which are
smaller than without functionalization, 117meV. | cond-mat_mtrl-sci |
Drude weight in presence of nonlocal potentials: The nonlocal potential contributes an extra term to the velocity operator; I
show here that such term affects the formal expression of the Drude weight in a
nontrivial way. Notably, the present main result fixes a disturbing discrepancy
in the Dreyer-Coh-Stengel sum rule [Phys. Rev. Lett. {\bf 128}, 095901 (2022)]. | cond-mat_mtrl-sci |
Phase Transition of Iron-based Single Crystals at Extreme Strain Rates
under Dynamic Loadings: Phase transition of iron, as a prototype of martensite phase transition under
dynamic loadings, exhibits huge diverges in its TP among experiments with
different pressure medium and loading rates, even in the same initial samples.
Great achievements are made in understanding strain or stress dependence of the
TP under dynamic loadings. However, present understandings on the strain rate
dependence of the TP are far from clear, even a virgin for extreme high strain
rates. In this work, large scale NEMD simulations are conducted to study the
effects of strain rates on the phase transition of iron-based single crystals.
Our results show that the phase transition is preceded by lattice instabilities
under ramp compressions, but present theory, represented by modified Born
criteria, cannot correctly predict observed onsets of the instability. Through
considering both strain and strain gradient disturbances, new instability
criteria are proposed, which could be generally applied for studying
instabilities under either static or dynamic loadings. For the ramp with a
strain rate smaller than about 1010s-1, the observed onset of instabilities is
indeed equal to the one predicted by the new instability criteria under small
gradient disturbances. The observed onsets deviates from the predicted one at
lager strain rates because of finite strain gradient effect. Interestingly, the
strain rate dependence of the TP also exhibits an obvious change at the same
strain rate, i.e., 1010 s-1. When 1010 s-1, a certain power law is obeyed, but
it is not applicable at larger strain rates. This strain rate effect on the TP
is well interpreted with nucleation time and the finite strain gradient effect.
According to these basic understandings, the roles of strain rates on
nucleation and growth of the phase transition are studied. | cond-mat_mtrl-sci |
Temperature-Modulated Differential Scanning Calorimetry Analysis of
High-Temperature Silicate Glasses: Differential scanning calorimetry (DSC) is one of the most versatile probes
for silicate glasses, allowing determination of, e.g., transition temperatures
(glass, crystallization, melting) and the temperature dependence of heat
capacity. However, complications arise for glasses featuring overlapping
transitions and low sensitivity, e.g., arising from SiO2-rich compositions with
small change in heat capacity during glass transition or the low sensitivity of
thermocouples at high temperature. These challenges might be overcome using
temperature-modulated DSC (TM-DSC), which enables separation of overlapping
signals and improved sensitivity at the expense of increased measurement
duration. | cond-mat_mtrl-sci |
Construction of Multi-Dimensional Functions for Optimization of
Additive-Manufacturing Process Parameters: The authors present a generic framework for parameter optimization of
additive manufacturing (AM) processes, one tailored to a high-throughput
experimental methodology (HTEM). Given the large number of parameters, which
impact the quality of AM-metallic components, the authors advocate for
partitioning the AM parameter set into stages (tiers), based on their relative
importance, modeling one tier at a time until successful, and then
systematically expanding the framework. The authors demonstrate how the
construction of multi-dimensional functions, based on neural networks (NN), can
be applied to successfully model relative densities and Rockwell hardness
obtained from HTEM testing of the Inconel 718 superalloy fabricated, using a
powder-bed approach. The authors analyze the input data set, assess its
suitability for predictions, and show how to optimize the framework for the
multi-dimensional functional construction, such as to obtain the highest degree
of fit with the input data. The novelty of the research work entails the
versatile and scalable NN framework presented, suitable for use in conjunction
with HTEM, for the AM parameter optimization of superalloys, and beyond. | cond-mat_mtrl-sci |
Large Exotic Spin Torques in Antiferromagnetic Iron Rhodium: Spin torque is a promising tool for driving magnetization dynamics for novel
computing technologies. These torques can be easily produced by spin-orbit
effects, but for most conventional spin source materials, a high degree of
crystal symmetry limits the geometry of the spin torques produced. Magnetic
ordering is one way to reduce the symmetry of a material and allow exotic
torques, and antiferromagnets are particularly promising because they are
robust against external fields. We present spin torque ferromagnetic resonance
measurements and second harmonic Hall measurements characterizing the spin
torques in antiferromagnetic iron rhodium alloy. We report extremely large,
strongly temperature-dependent exotic spin torques with a geometry apparently
defined by the magnetic ordering direction. We find the spin torque efficiency
of iron rhodium to be (330$\pm$150) % at 170 K and (91$\pm$32) % at room
temperature. We support our conclusions with theoretical calculations showing
how the antiferromagnetic ordering in iron rhodium gives rise to such exotic
torques. | cond-mat_mtrl-sci |
Oxygen Reduction Activity of Carbon Nitride Supported on Carbon
Nanotubes: Fuel cells offer an alternative to burning fossil fuels, but use platinum as
a catalyst which is expensive and scarce. Cheap, alternative catalysts could
enable fuel cells to become serious contenders in the green energy sector. One
promising class of catalyst for electrochemical oxygen reduction is
iron-containing, nanostructured, nitrogen-doped carbon. The catalytic activity
of such N-doped carbons has improved vastly over the years bringing industrial
applications ever closer. Stoichiometric carbon nitride powder has only been
observed in recent years. It has nitrogen content up to 57% and as such is an
extremely interesting material to work with. The electrochemical activity of
carbon nitride has already been explored, confirming that iron is not a
necessary ingredient for 4-electron oxygen reduction. Here, we synthesize
carbon nitride on a carbon nanotube support and subject it to high temperature
treatment in an effort to increase the surface area and conductivity. The
results lend insight into the mechanism of oxygen reduction and show the
potential for carbon nanotube-supported carbon nitride to be used as a catalyst
to replace platinum in fuel cells. | cond-mat_mtrl-sci |
Fractional Skyrme lines in ferroelectric barium titanate: We predict a topological defect in ferroelectric barium titanate which we
call a skyrme line. These are line-like objects characterized by skyrmionic
topological charge. As well as configurations with integer charge, the charge
density can split into well-localized fractional parts. We show that under
certain conditions the fractional skyrme lines are stable. We discuss a
mechanism to create fractional topological charge objects and investigate their
stability. | cond-mat_mtrl-sci |
Semi-supervised machine learning model for analysis of nanowire
morphologies from transmission electron microscopy images: In the field of materials science, microscopy is the first and often only
accessible method for structural characterization. There is a growing interest
in the development of machine learning methods that can automate the analysis
and interpretation of microscopy images. Typically training of machine learning
models requires large numbers of images with associated structural labels,
however, manual labeling of images requires domain knowledge and is prone to
human error and subjectivity. To overcome these limitations, we present a
semi-supervised transfer learning approach that uses a small number of labeled
microscopy images for training and performs as effectively as methods trained
on significantly larger image datasets. Specifically, we train an image encoder
with unlabeled images using self-supervised learning methods and use that
encoder for transfer learning of different downstream image tasks
(classification and segmentation) with a minimal number of labeled images for
training. We test the transfer learning ability of two self-supervised learning
methods: SimCLR and Barlow-Twins on transmission electron microscopy (TEM)
images. We demonstrate in detail how this machine learning workflow applied to
TEM images of protein nanowires enables automated classification of nanowire
morphologies (e.g., single nanowires, nanowire bundles, phase separated) as
well as segmentation tasks that can serve as groundwork for quantification of
nanowire domain sizes and shape analysis. We also extend the application of the
machine learning workflow to classification of nanoparticle morphologies and
identification of different type of viruses from TEM images. | cond-mat_mtrl-sci |
Dynamic transverse magnetic susceptibility in the projector
augmented-wave method. Application to Fe, Ni, and Co: We present a first principles implementation of the dynamic transverse
magnetic susceptibility in the framework of linear response time-dependent
density functional theory. The dynamic susceptibility allows one to obtain the
magnon dispersion as well as magnon lifetimes for a particular material, which
strongly facilitates the interpretation of inelastic neutron scattering
experiments as well as other spectroscopic techniques. We apply the method to
Fe, Ni, and Co and perform a thorough convergence analysis with respect the
basis set size, $k$-point sampling, spectral smearing and unoccupied bands. In
particular, it is shown that while the gap error (acoustic magnon energy at
$\mathbf{q}=\mathbf{0}$) is highly challenging to converge, the spin-wave
stiffness and the dispersion relation itself are much less sensitive to
convergence parameters. Our final results agrees well with experimentally
extracted magnon dispersion relations except for Ni, where it is well-known
that the exchange splitting energy is poorly represented in the local density
approximation. We also find good agreement with previous first principles
calculations and explain how differences in the calculated dispersion relations
can arise from subtle differences in computational approaches. | cond-mat_mtrl-sci |
Thermodynamic dislocation theory: Finite deformations: The present paper extends the thermodynamic dislocation theory initiated by
Langer, Bouchbinder and Lookman [2010] to non-uniform finite plastic
deformations. The equations of motion are derived from the variational equation
involving the free energy density and the positive definite dissipation
function. We also consider the simplified theory by neglecting the excess
dislocations. For illustration, the problem of finite strain constrained shear
of single crystals with one active slip system is solved within the proposed
theory. | cond-mat_mtrl-sci |
Prediction of Glass Elasticity from Free Energy Density of Topological
Constraints: Despite the critical importance of the elastic properties of modern
materials, there is not a singular model that can predict the modulus to an
accuracy needed for industrial glass design. To address this problem, we
propose an approach to calculate the elastic modulus based on the free energy
density of topological constraints in the glass-forming network. Our approach
shows quantitatively accurate agreement with glasses across a variety of
compositional families. Moreover, using temperature-dependent constraint
theory, the temperature dependence of the modulus can also be predicted. Our
approach is general and theoretically can be applied to any network glass. | cond-mat_mtrl-sci |
Expression and interactions of stereo-chemically active lone pairs and
their relation to structural distortions and thermal conductivity: Stereo-chemically active lone pairs are typically described as an important
non-bonding effect, and large interest has centered on understanding the
derived effect of lone pair expression on physical properties such as the
thermal conductivity. To manipulate such properties, it is essential to
understand the conditions that lead to lone pair expression and to provide a
quantitative chemical description. Here we first use density functional theory
calculations to establish the presence of stereo-chemically active lone pairs
on antimony in $\text{MnSb}_{2}\text{O}_{4}$. The lone pairs are formed through
a similar mechanism to those in binary post-transition metal compounds in an
oxidation state of two less than their main group number, where the degree of
orbital interaction determines the expression of the lone pair. In
$\text{MnSb}_{2}\text{O}_{4}$ the Sb lone pairs interact through a void space
in the crystal structure, and they minimize their mutual repulsion by
introducing a deflection angle. This angle increases significantly with
decreasing Sb-Sb distance, thus showing the highly destabilizing nature of the
lone pair interactions. Analysis of the chemical bonding in the structure shows
that it is dominated by polar covalent interactions. A database search of
related ternary chalcogenide structures shows that for structures with a lone
pair the degree of lone pair expression is largely determined by whether the
antimony-chalcogen units are connected or not, suggesting a cooperative effect.
Isolated $\text{SbX}_3$ units have larger X-Sb-X bond angles, and therefore
weaker lone pair expression than connected units. Since increased lone pair
expression is equivalent to an increased orbital interaction (covalent
bonding), which typically leads to increased heat conduction, this can explain
the previously established correlation between larger bond angles and lower
thermal conductivity. | cond-mat_mtrl-sci |
Quantum size effect on the dissociation of O2 molecules on ultrathin
Pb(111) films: Using first-principles calculations, we systematically study the dissociation
of O$_2$ molecules on different ultrathin Pb(111) films. Based on our previous
work revealing the molecular adsorption precursor states for O$_2$, we further
explore that why there are two nearly degenerate adsorption states on Pb(111)
ultrathin films, but no precursor adsorption states exist at all on the
Mg(0001) and Al(111) surfaces. And the reason is concluded to be the different
surface electronic structures. For the O$_2$ dissociation, we consider both the
reaction channels from gas-like and molecularly adsorbed O$_2$ molecules. We
find that the energy barrier for O$_2$ dissociation from the molecular
adsorption precursor states is always smaller than from O$_2$ gases. The most
energetically favorable dissociation process is found to be the same on
different Pb(111) films, and the energy barriers are found to be modulated by
the quantum size effects of Pb(111) films. | cond-mat_mtrl-sci |
Ultrahigh ion diffusion in oxide crystal by engineering the interfacial
transporter channels: The mass storage and removal in solid conductors always played vital role on
the technological applications such as modern batteries, permeation membranes
and neuronal computations, which were seriously lying on the ion diffusion and
kinetics in bulk lattice. However, the ions transport was kinetically limited
by the low diffusional process, which made it a challenge to fabricate
applicable conductors with high electronic and ionic conductivities at room
temperature. It was known that at essentially all interfaces, the existed space
charge layers could modify the charge transport, storage and transfer
properties. Thus, in the current study, we proposed an acid solution/WO3/ITO
structure and achieved an ultrafast hydrogen transport in WO3 layer by
interfacial job-sharing diffusion. In this sandwich structure, the transport
pathways of the protons and electrons were spatially separated in acid solution
and ITO layer respectively, resulting the pronounced increasing of effective
hydrogen diffusion coefficient (Deff) up to 106 times. The experiment and
theory simulations also revealed that this accelerated hydrogen transport based
on the interfacial job-sharing diffusion was universal and could be extended to
other ions and oxide materials as well, which would potentially stimulate
systematic studies on ultrafast mixed conductors or faster solid-state
electrochemical switching devices in the future. | cond-mat_mtrl-sci |
Electronic and optical properties of metal-doped TiO$_2$ nanotubes:
Spintronic and photocatalytic applications: Due to their characteristic geometry, TiO$_2$ nanotubes (TNTs), suitably
doped by metal-substitution to enhance their photocatalytic properties, have a
high potential for applications such as clean fuel production. In this context,
we present a detailed investigation of the magnetic, electronic, and optical
properties of transition-metal doped TNTs, based on hybrid density functional
theory. In particular, we focus on the $3d$, the $4d$, as well as selected $5d$
transition-metal doped TNTs. Thereby, we are able to explain the enhanced
optical activity and photocatalytic sensitivity observed in various
experiments. We find, for example, that Cr- and W-doped TNTs can be employed
for applications like water splitting and carbon dioxide reduction, and for
spintronic devices. The best candidate for water splitting is Fe-doped TNT, in
agreement with experimental observations. In addition, our findings provide
valuable hints for future experimental studies of the ferromagnetic/spintronic
behavior of metal-doped titania nanotubes. | cond-mat_mtrl-sci |
Modes of Kink Motion on Dislocations in Semiconductors: Analysis is given of the changes of dislocation motion modes with stress and
temperature variation. Different regimes of dislocation kink pair formation and
spreading (motion in the random potential, in the field of random forces, the
quasi-localization) are considered. Discrepancies are discussed between the
theory and experimental data on dislocation velocities. | cond-mat_mtrl-sci |
Carbon Decorated TiO2 Nanotube Membranes: A Renewable Nanofilter for
Size- and Charge Selective Enrichment of Proteins: In this work, we design a TiO2 nanomembrane (TiNM) that can be used as a
nanofilter platform for a selective enrichment of specific proteins. After use
the photocatalytic properties of TiO2 allow to decompose unwanted remnant on
the substrate and thus make the platform reusable. To construct this platform
we fabricate a free-standing TiO2 nanotube array and remove the bottom oxide to
form a both-end open TiNM. By pyrolysis of the natural tube wall contamination
(C/TiNM), the walls become decorated with graphitic carbon patches. Owing to
the large surface area, the amphiphilic nature and the charge adjustable
character, this C/TiNM can be used to extract and enrich hydrophobic and
charged biomolecules from solutions. Using human serum albumin (HSA) as a model
protein as well as protein mixtures, we show that the composite membrane
exhibits a highly enhanced loading capacity and protein selectivity and is
reusable after a short UV treatment. | cond-mat_mtrl-sci |
Large Anomalous Hall Effect at Room Temperature in a Fermi-Level-Tuned
Kagome Antiferromagnet: The recent discoveries of surperisingly large anomalous Hall effect in chiral
antiderromagnets have triggered extensive research efforts in various fields,
ranging from topological condensed-matter physics to antiferromagnetic
spintronics, and energy harvesting technology. However, such AHE-hosting
antiferromagnetic materials are rare in nature. Herein, we demonstrate that
Mn2.4Ga, a Fermi-level-tuned kagome antiferromagnet, has a large anomalous Hall
conductivity of about 150 {\Omega}-1cm-1 at room temperature that surpasses the
usual high values (i.e.,20-50 {\Omega}-1cm-1) observed so far in two
outstanding kagome antiferromagnets, Mn3Sn and Mn3Ge. The spin triangular
structure of Mn2.4Ga guarantees a nonzero Berry curvature while generates only
a weak net moment in the kagome plane.Moreover, the anomalous Hall conductivity
exhibits a sign reversal with the rotation of a small magnetic field, which can
be ascribed to the field-controlled chirality of the spin triangular structure.
Our theoretical calculation indicate that the large AHE in Mn2.4Ga originates
from a significantly enhanced Berry curvature associated wiht the tuning of the
Fermi level close to the Weyl points. These properties, together with the
ability to manipulate moment orientations using a moderate external magnetic
field, make Mn2.4Ga extremely exciting for future antiferromagnetic
spintronics. | cond-mat_mtrl-sci |
Probing the Electron States and Metal-Insulator Transition Mechanisms in
Atomically Thin MoS2 Based on Vertical Heterostructures: The metal-insulator transition (MIT) is one of the remarkable electrical
transport properties of atomically thin molybdenum disulphide (MoS2). Although
the theory of electron-electron interactions has been used in modeling the MIT
phenomena in MoS2, the underlying mechanism and detailed MIT process still
remain largely unexplored. Here, we demonstrate that the vertical
metal-insulator-semiconductor (MIS) heterostructures built from atomically thin
MoS2 (monolayers and multilayers) are ideal capacitor structures for probing
the electron states in MoS2. The vertical configuration of MIS heterostructures
offers the added advantage of eliminating the influence of large impedance at
the band tails and allows the observation of fully excited electron states near
the surface of MoS2 over a wide excitation frequency (100 Hz-1 MHz) and
temperature range (2 K- 300 K). By combining capacitance and transport
measurements, we have observed a percolation-type MIT, driven by density
inhomogeneities of electron states, in the vertical heterostructures built from
monolayer and multilayer MoS2. In addition, the valence band of thin MoS2
layers and their intrinsic properties such as thickness-dependence screening
abilities and band gap widths can be easily accessed and precisely determined
through the vertical heterostructures. | cond-mat_mtrl-sci |
Dependence of magnetic domain patterns on plasma-induced differential
oxidation of CoPd thin films: We demonstrate the evolution of the micro-patterned magnetic domains in CoPd
thin films pretreated with e-beam lithography and O2 plasma. During the
days-long oxidation, significantly different behaviors of the patterned
magnetic domains under magnetization reversal are observed via magneto-optic
Kerr effect microscopy on different days. The evolution of the magnetic
behaviors indicate critical changes in the local magnetic anisotropy energies
due to the Co oxides that evolve into different oxide forms, which are
characterized by micro-area X-ray absorption spectroscopy and X-ray
photoelectron spectroscopy. The coercive field of the area pre-exposed to
plasma can decrease to a value 10 Oe smaller than that unexposed to plasma,
whereas after a longer duration of oxidation the coercive field can instead
become larger in the area pre-exposed to plasma than that unexposed, leading to
an opposite magnetic pattern. Various forms of oxidation can therefore provide
an additional dimension for magnetic-domain engineering to the current
conventional lithographies. | cond-mat_mtrl-sci |
Tuning the valence and concentration of europium and luminescence
centers in GaN through co-doping and defect association: Defect physics of europium (Eu) doped GaN is investigated using
first-principles hybrid density-functional defect calculations. This includes
the interaction between the rare-earth dopant and native point defects (Ga and
N vacancies) and other impurities (O, Si, C, H, and Mg) unintentionally present
or intentionally incorporated into the host material. While the trivalent
Eu$^{3+}$ ion is often found to be predominant when Eu is incorporated at the
Ga site in wurtzite GaN, the divalent Eu$^{2+}$ is also stable and found to be
predominant in a small range of Fermi-level values in the band-gap region. The
Eu$^{2+}$/Eu$^{3+}$ ratio can be tuned by tuning the position of Fermi level
and through defect association. We find co-doping with oxygen can facilitate
the incorporation of Eu into the lattice. The unassociated Eu$_{\rm Ga}$ is an
electrically and optically active defect center and its behavior is profoundly
impacted by local defect--defect interaction. Defect complexes such as Eu$_{\rm
Ga}$-O$_{\rm N}$, Eu$_{\rm Ga}$-Si$_{\rm Ga}$, Eu$_{\rm Ga}$-H$_i$, Eu$_{\rm
Ga}$-Mg$_{\rm Ga}$, and Eu$_{\rm Ga}$-O$_{\rm N}$-Mg$_{\rm Ga}$ can efficiently
act as deep carrier traps and mediate energy transfer from the host into the
Eu$^{3+}$ $4f$-electron core which then leads to sharp red intra-$f$
luminescence. Eu-related defects can also give rise to defect-to-band
luminescence. The unassociated Eu$_{\rm Ga}$, for example, is identified as a
possible source of the broad blue emission observed in n-type,
Eu$^{2+}$-containing GaN. This work calls for a re-assessment of certain
assumptions regarding specific defect configurations previously made for
Eu-doped GaN and further investigation into the origin of the photoluminescence
hysteresis observed in (Eu,Mg)-doped samples. | cond-mat_mtrl-sci |
Ab-initio prediction of the electronic and optical excitations in
polythiophene: isolated chains versus bulk polymer: We calculate the electronic and optical excitations of polythiophene using
the GW approximation for the electronic self-energy, and include excitonic
effects by solving the electron-hole Bethe-Salpeter equation. Two different
situations are studied: excitations on isolated chains and excitations on
chains in crystalline polythiophene. The dielectric tensor for the crystalline
situation is obtained by modeling the polymer chains as polarizable line
objects, with a long-wavelength polarizability tensor obtained from the
ab-initio polarizability function of the isolated chain. With this model
dielectric tensor we construct a screened interaction for the crystalline case,
including both intra- and interchain screening. In the crystalline situation
both the quasi-particle band gap and the exciton binding energies are
drastically reduced in comparison with the isolated chain. However, the optical
gap is hardly affected. We expect this result to be relevant for conjugated
polymers in general. | cond-mat_mtrl-sci |
Flexomagnetic effect in Mn-based antiperovskites: We report appearance of the net magnetization in Mn-based antiperovskite
compounds as a result of the external strain gradient (flexomagnetic effect).
In particular, we describe the mechanism of the magnetization induction in the
Mn_{3}GaN at the atomic level in terms of the behavior of the local magnetic
moments (LMM) of the Mn atoms. We show that the flexomagnetic effect is linear
and results from the non-uniformity of the strain, i.e. it is absent not only
in the ground state but also when the applied external strain is uniform. We
estimate the flexomagnetic coefficient to be 1.95 mu_{B}*angstrom. We show that
at the moderate values of the strain gradient (~ 0.1%) the flexomagnetic
contribution is the only non-vanishing input to the induced magnetization. | cond-mat_mtrl-sci |
Quasi-harmonic temperature dependent elastic constants: applications to
silicon, aluminum, and silver: We present ab-initio calculations of the quasi-harmonic temperature dependent
elastic constants. The isothermal elastic constants are calculated at each
temperature as second derivatives of the Helmholtz free energy with respect to
strain and corrected for finite pressure effects. This calculation is repeated
for a grid of geometries and the results interpolated at the minimum of the
Helmholtz free energy. The results are compared with the quasi-static elastic
constants. Thermodynamic relationships are used to derive the adiabatic elastic
constants that are compared with the experimental measurements. These
approaches are implemented for cubic solids in the $\texttt{thermo_pw}$ code
and are validated by applications to silicon, aluminum, and silver. | cond-mat_mtrl-sci |
Significant elastic anisotropy in Ti$_{1-x}$Al$_x$N alloys: Strong compositional-dependent elastic properties have been observed
theoretically and experimentally in Ti$_{1-x}$Al$_x$ N alloys. The elastic
constant, C$_{11}$, changes by more than 50% depending on the Al-content.
Increasing the Al-content weakens the average bond strength in the local
octahedral arrangements resulting in a more compliant material. On the other
hand, it enhances the directional (covalent) nature of the nearest neighbor
bonds that results in greater elastic anisotropy and higher sound velocities.
The strong dependence of the elastic properties on the Al-content offers new
insight into the detailed understanding of the spinodal decomposition and age
hardening in Ti$_{1-x}$Al$_x$N alloys. | cond-mat_mtrl-sci |
Flatten the Li-ion Activation in Perfectly Lattice-matched MXene and
1T-MoS2 Heterostructures via Chemical Functionalization: MXene and its derivatives have attracted considerable attention for potential
application in energy storage like batteries and supercapacitors owing to its
ultrathin metallic structures. However, the complexity of the ionic and
electronic dynamics in MXene based hybrids, which are normally needed for
device integration, triggers both challenges and opportunities for its
application. In this paper, as a prototype of metallic hybrids of MXene,
heterostructures consisting of Ti3C2T2 (T= None, O and F atoms) and metallic
MoS2 (1T phase) are investigated. Through density functional theory, we
investigate the interfacial electronic variation, thermal activation, and anode
performance in the lithium-ion battery (LIB) of Ti3C2T2/1T-MoS2. We found that
different surface atomic groups in MXene can significantly alter the affinity,
redox reaction and kinetics of Li atoms in the interface of the Ti3C2T2 and
1T-MoS2. Through examining the three possible pathways of Li by climbing
image-nudged elastic band (CI-NEB) and ab-initio molecular dynamics (AIMD)
simulation, the diffusion curve becomes significantly flattened from the naked
to O- and F-terminated Ti3C2 MXene with activation barriers reducing from 0.80
to 0.22 and 0.29 eV, respectively, and room-temperature diffusion coefficients
increasing from 1.20x10-6 to 2.75x10-6, 1.70x10-4 cm2 s-1, respectively. The
functionalization with O or F eliminates the steric hindrance of Li
intercalation by breaking the strong interaction between two layers and
provides additional adsorption sites for Li diffusion in the meantime. Our work
suggests that surface functional groups play a significant role in
Ti3C2T2/1T-MoS2 modification and Ti3C2F2/1T-MoS2 with the high diffusion
coefficient and theoretical capacity could be a promising anode material for
LIBs. | cond-mat_mtrl-sci |
Conductance through atomic point contacts between fcc(100) electrodes of
gold: Electrical conductance through various nanocontacts between gold electrodes
is studied by using the density functional theory, scalar-relativistic
pseudopotentials, generalized gradient approximation for the
exchange-correlation energy and the recursion-transfer-matrix method along with
channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips
and 1 to 5 gold atoms between the tips. Upon elongation of the contact by
adding gold atoms between the tips, the conductance at Fermi energy E_F evolves
from G ~ 3 G_0 to G ~ 1 G_0 (G_0 = 2e/h^2). Formation of a true one-atom point
contact, with G ~ 1 G_0 and only one open channel, requires at least one atom
with coordination number 2 in the wire. Tips that share a common vertex atom or
tips with touching vertex atoms have three partially open conductance channels
at E_F, and the symmetries of the channels are governed by the wave functions
of the tips. The long 5-atom contact develops conductance oscillations and
conductance gaps in the studied energy range -3 < E-E_F < 5 eV, which reflects
oscillations in the local density of electron states in the 5-atom linear "gold
molecule" between the electrodes, and a weak coupling of this "molecule" to the
tips. | cond-mat_mtrl-sci |
Non-Linear Beam Splitter in Bose-Einstein Condensate Interferometers: A beam splitter is an important component of an atomic/optical Mach-Zehnder
interferometer. Here we study a Bose Einstein Condensate beam splitter,
realized with a double well potential of tunable height. We analyze how the
sensitivity of a Mach Zehnder interferometer is degraded by the non-linear
particle-particle interaction during the splitting dynamics. We distinguish
three regimes, Rabi, Josephson and Fock, and associate to them a different
scaling of the phase sensitivity with the total number of particles. | cond-mat_mtrl-sci |
High-temperature stability and grain boundary complexion formation in a
nanocrystalline Cu-Zr alloy: Nanocrystalline Cu-3 at.% Zr powders with ~20 nm average grain size were
created with mechanical alloying and their thermal stability was studied from
550-950 {\deg}C. Annealing drove Zr segregation to the grain boundaries, which
led to the formation of amorphous intergranular complexions at higher
temperatures. Grain growth was retarded significantly, with 1 week of annealing
at 950 {\deg}C, or 98% of the solidus temperature, only leading to coarsening
of the average grain size to 54 nm. The enhanced thermal stability can be
connected to both a reduction in grain boundary energy with doping as well as
the precipitation of ZrC particles. High mechanical strength is retained even
after these aggressive heat treatments, showing that complexion engineering may
be a viable path toward the fabrication of bulk nanostructured materials with
excellent properties. | cond-mat_mtrl-sci |
Putting Density Functional Theory to the Test in
Machine-Learning-Accelerated Materials Discovery: Accelerated discovery with machine learning (ML) has begun to provide the
advances in efficiency needed to overcome the combinatorial challenge of
computational materials design. Nevertheless, ML-accelerated discovery both
inherits the biases of training data derived from density functional theory
(DFT) and leads to many attempted calculations that are doomed to fail. Many
compelling functional materials and catalytic processes involve strained
chemical bonds, open-shell radicals and diradicals, or metal-organic bonds to
open-shell transition-metal centers. Although promising targets, these
materials present unique challenges for electronic structure methods and
combinatorial challenges for their discovery. In this Perspective, we describe
the advances needed in accuracy, efficiency, and approach beyond what is
typical in conventional DFT-based ML workflows. These challenges have begun to
be addressed through ML models trained to predict the results of multiple
methods or the differences between them, enabling quantitative sensitivity
analysis. For DFT to be trusted for a given data point in a high-throughput
screen, it must pass a series of tests. ML models that predict the likelihood
of calculation success and detect the presence of strong correlation will
enable rapid diagnoses and adaptation strategies. These "decision engines"
represent the first steps toward autonomous workflows that avoid the need for
expert determination of the robustness of DFT-based materials discoveries. | cond-mat_mtrl-sci |
Controlling the energy of defects and interfaces in the amplitude
expansion of the phase-field crystal model: One of the major difficulties in employing phase field crystal (PFC) modeling
and the associated amplitude (APFC) formulation is the ability to tune model
parameters to match experimental quantities. In this work we address the
problem of tuning the defect core and interface energies in the APFC
formulation. We show that the addition of a single term to the free energy
functional can be used to increase the solid-liquid interface and defect
energies in a well-controlled fashion, without any major change to other
features. The influence of the newly added term is explored in two-dimensional
triangular and honeycomb structures as well as bcc and fcc lattices in three
dimensions. In addition, a finite element method (FEM) is developed for the
model that incorporates a mesh refinement scheme. The combination of the FEM
and mesh refinement to simulate amplitude expansion with a new energy term
provides a method of controlling microscopic features such as defect and
interface energies while simultaneously delivering a coarse-grained examination
of the system. | cond-mat_mtrl-sci |
Growth-sequence-dependent interface magnetism of SrIrO$_3$ -
La$_{0.7}$Sr$_{0.3}$MnO$_3$ bilayers: Bilayers of the oxide 3d ferromagnet La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and
the 5d paramagnet SrIrO$_{3}$ (SIO) with large spin-orbit coupling (SOC) have
been investigated regarding the impact of interfacial SOC on magnetic order.
For the growth sequence of LSMO on SIO, ferromagnetism is strongly altered and
large out-of-plane-canted anisotropy associated with lacking magnetic
saturation up to 4 T has been observed. Thin bilayer films have been grown
coherently in both growth sequences on SrTiO$_3$ (001) by pulsed laser
deposition and structurally characterized by scanning transmission electron
microscopy (STEM) and x-ray diffraction (XRD). Measurements of magnetization
and field-dependent Mn L$_{2,3}$ edge x-ray magnetic circular dichroism (XMCD)
reveal changes of LSMO magnetic order which are strong in LSMO on SIO and weak
in LSMO underneath of SIO. We attribute the impact of the growth sequence to
the interfacial lattice structure/symmetry which is known to influence the
interfacial magnetic coupling. | cond-mat_mtrl-sci |
Synthesis and Characterization of Sodium Iron Antimonate Na2FeSbO5
One-Dimensional Antiferromagnetic Chain Compound with a Spin Glass Ground
State: A new oxide, sodium iron antimonate, Na2FeSbO5, was synthesized and
structurally characterized, and its static and dynamic magnetic properties were
comprehensively studied both experimentally by dc and ac magnetic
susceptibility, magnetization, specific heat, electron spin resonance (ESR) and
Moessbauer measurements, and theoretically by density functional calculations.
The resulting single-crystal structure (a = 15.6991(9) A; b = 5.3323 (4) A; c =
10.8875(6) A; S.G. Pbna) consists of edge-shared SbO6 octahedral chains, which
alternate with vertex-linked, magnetically active FeO4 tetrahedral chains. The
57Fe Moessbauer spectra confirmed the presence of high-spin Fe3+ (3d5) ions in
a distorted tetrahedral oxygen coordination. The magnetic susceptibility and
specific heat data show the absence of a long-range magnetic ordering in
Na2FeSbO5 down to 2 K, but ac magnetic susceptibility unambigously demonstrates
spin-glass-type behavior with a unique two-step freezing at Tf1 about 80 K and
Tf2 about 35 K. Magnetic hyperfine splitting of 57Fe Moessbauer spectra was
observed below T* about 104 K (Tf1 < T*). The spectra just below T* (Tf1 < T <
T*) exhibit a relaxation behavior caused by critical spin fluctuations,
indicating the existence of short-range correlations. The stochastic model of
ionic spin relaxation was used to account for the shape of the Moessbauer
spectra below the freezing temperature. A complex slow dynamics is further
supported by ESR data revealing two different absorption modes presumably
related to ordered and disordered segments of spin chains. The data imply a
spin-cluster ground state for Na2FeSbO5. | cond-mat_mtrl-sci |
Accelerated Design of Chalcogenide Glasses through Interpretable Machine
Learning for Composition Property Relationships: Chalcogenide glasses possess several outstanding properties that enable
several ground breaking applications, such as optical discs, infrared cameras,
and thermal imaging systems. Despite the ubiquitous usage of these glasses, the
composition property relationships in these materials remain poorly understood.
Here, we use a large experimental dataset comprising approx 24000 glass
compositions made of 51 distinct elements from the periodic table to develop
machine learning models for predicting 12 properties, namely, annealing point,
bulk modulus, density, Vickers hardness, Littleton point, Youngs modulus, shear
modulus, softening point, thermal expansion coefficient, glass transition
temperature, liquidus temperature, and refractive index. These models, by far,
are the largest for chalcogenide glasses. Further, we use SHAP, a game theory
based algorithm, to interpret the output of machine learning algorithms by
analyzing the contributions of each element towards the models prediction of a
property. This provides a powerful tool for experimentalists to interpret the
models prediction and hence design new glass compositions with targeted
properties. Finally, using the models, we develop several glass selection
charts that can potentially aid in the rational design of novel chalcogenide
glasses for various applications. | cond-mat_mtrl-sci |
Metastability and anharmonicity enhance defect-assisted nonradiative
recombination in low-symmetry semiconductors: Strong nonradiative recombination has been observed in quasi-one-dimensional
antimony selenide, which runs counter to the simple intuition that claims high
defect tolerance exists in semiconductors with antibonding state in the valence
band and bonding state in the conduction band. Here we reveal such a defect
intolerance actually stems from the richness of structural metastability and
vibrational anharmonicity owing to the low-symmetry atomic structure. Taking
the deep defect V$_{\rm Se}$ as a benchmark, we show the defect with its
ground-state configuration alone does not act as a recombination center.
Instead, we identify three different configurations with different formation
energies, such richness of metastability offers a higher probability to
accomplish a rapid recombination cycle. Another contributing factor is the
anharmonicity in the potential energy surfaces that is caused by the large
atomic relaxation, which elevates the total capture coefficient by 2-3 orders
of magnitude compared with harmonic approximation. Therefore, the unique
properties from both crystals and phonons in quasi-one-dimensional system
enhance the nonradiative recombination, making the traditional intuition of
defect tolerance invalid. These results highlight the importance of the correct
identification of metastable defects and phonon anharmonicity in the
nonradiative recombination in low-symmetry semiconductors. | cond-mat_mtrl-sci |
Spin Modulation in Semiconductor Lasers: We provide an analytic study of the dynamics of semiconductor lasers with
injection (pump) of spin-polarized electrons, previously considered in the
steady-state regime. Using complementary approaches of quasi-static and small
signal analyses, we elucidate how the spin modulation in semiconductor lasers
can improve performance, as compared to the conventional (spin-unpolarized)
counterparts. We reveal that the spin-polarized injection can lead to an
enhanced bandwidth and desirable switching properties of spin-lasers. | cond-mat_mtrl-sci |
Size-dependence of non-empirically tuned DFT starting points for
$G_0W_0$ applied to $π$-conjugated molecular chains: $G_0W_0$ calculations for predicting vertical ionization potentials (IPs) and
electron affinities of molecules and clusters are known to show a significant
dependence on the density functional theory (DFT) starting point. A number of
non-empirical procedures to find an optimal starting point have been proposed,
typically based on tuning the amount of HF exchange in the underlying hybrid
functional specifically for the system at hand. For the case of
$\pi$-conjugated molecular chains, these approaches lead to a significantly
different amount of HF exchange for different oligomer sizes. In this study, we
analyze if and how strongly this size dependence affects the ability of
non-empirical tuning approaches to predict accurate IPs for $\pi$-conjugated
molecular chains of increasing chain length. To this end, we employ three
different non-empirical tuning procedures for the $G_0W_0$ starting point to
calculate the IP of polyene oligomers up to 22 repeat units and compare the
results to highly accurate coupled-cluster calculations. We find that, despite
its size dependence, using an IP-tuned hybrid functional as a starting point
for $G_0W_0$ yields excellent agreement with the reference data for all chain
lengths. | cond-mat_mtrl-sci |
Carbon mono and dioxide hydrogenation over pure and metal oxide
decorated graphene oxide substrates: insight from DFT: Based on first principles density functional theory calculations we explore
the energetics of the conversion of carbon mono and dioxide to methane over
graphene oxide surfaces. Similar to the recently discovered hydration of
various organic species over this catalyst, the transfer of hydrogen atoms from
hydroxyl groups of graphene oxide provide a step by step transformation
hydrogenation of carbon oxides. Estimated yields of modeled reactions at room
temperature are about 0.01% for the carbon mono and dioxide. For the modeling
of graphene oxide/metal oxide composites, calculations in the presence of MO_2
(where M = V, Cr, Mn, Fe) have been performed. Results of these calculations
demonstrate significant decreases of the energy costs and increases of reaction
yields to 0.07%, which is comparable to the efficiency of these reactions over
platinum and ruthenium-based photocatalysts. Increasing the temperature to the
value 100C should provide the total conversion of carbon mono and dioxides. | cond-mat_mtrl-sci |
Isotropic or anisotropic screening in black phosphorous: can doping tip
the balance?: Black phosphorus (BP), a layered van der Waals (vdW) crystal, has unique
in-plane band anisotropy and many resulting anisotropy properties such as the
effective mass, electron mobility, optical absorption, thermal conductivity and
plasmonic dispersion. However, whether anisotropic or isotropic charge
screening exist in BP remains a controversial issue. Based on first-principles
calculations, we study the screening properties in both of single-layer and
bulk BP, especially concerning the role of doping. Without charge doping, the
single-layer and bulk-phase BP show slight anisotropic screening. Electron and
hole doping can increase the charge screening of BP and significantly change
the relative static dielectric tensor elements along two different in-plane
directions. We further study the charge density change induced by potassium (K)
adatom near the BP surface, under different levels of charge doping. The
calculated two-dimensional (2D) charge redistribution patterns also confirm
that doping can greatly affect the screening feature and tip the balance
between isotropic and anisotropic screening. We corroborate that screening in
BP exhibit slight intrinsic anisotropy and doping has significant influence on
its screening property. | cond-mat_mtrl-sci |
Effect of wear particles and roughness on nanoscale friction: Frictional contacts lead to the formation of a surface layer called the third
body, consisting of wear particles and structures resulting from their
agglomerates. Its behavior and properties at the nanoscale control the
macroscopic tribological performance. It is known that wear particles and
surface topography evolve with time and mutually influence one another.
However, the formation of the mature third body is largely uncharted territory
and the properties of its early stages are unknown. Here we show how a third
body initially consisting of particles acting as roller bearings transitions
into a shear-band-like state by forming adhesive bridges between the particles.
Using large-scale atomistic simulations on a brittle model material, we find
that this transition is controlled by the growth and increasing disorganization
of the particles with increasing sliding distance. Sliding resistance and wear
rate are at first controlled by the surface roughness, but upon agglomeration
wear stagnates and friction becomes solely dependent on the real contact area
in accordance with the plasticity theory of contact by Bowden and Tabor. | cond-mat_mtrl-sci |
Revealing the ultra-fast domain wall motion in Manganese Gold through
permalloy capping: Antiferromagnets offer much faster dynamics compared to their ferromagnetic
counterparts but their order parameter is extremely difficult to detect and
control. So far, controlling the N\'eel order parameter electrically is limited
to only very few materials where N\'eel spin-orbit torques are allowed by
symmetry. In this work, we show that coupling a thin ferromagnet (permalloy)
layer on top of an antiferromagnet (Mn$_2$Au) solves a major roadblock -- the
controlled reading, writing, and manipulation of antiferromagnetic domains. We
confirm by atomistic spin dynamics simulations that the domain wall patterns in
the Mn$_2$Au are imprinted on the permalloy, therefore allowing for indirect
imaging of the N\'eel order parameter. Our simulations show that the coupled
domain wall structures in Mn$_2$Au-Py bilayers can be manipulated by either
acting on the N\'eel order parameter via N\'eel spin-orbit torques or by acting
on the magnetisation (the ferromagnetic order parameter) via magnetic fields.
In both cases, we predict ultra-high domain wall speeds on the order of 8.5
km/s. Thus, employing a thin ferromagnetic layer has the potential to easily
control the N\'eel order parameter in antiferromagnets even where N\'eel
spin-orbit torques are forbidden by symmetry. The controlled manipulation of
the antiferromagnetic order parameter provides a promising basis for the
development of high-density storage and efficient computing technologies
working in the THz regime. | cond-mat_mtrl-sci |
The Band-Gap Problem in Semiconductors Revisited: Effects of Core States
and Many-Body Self-Consistency: A novel picture of the quasiparticle (QP) gap in prototype semiconductors Si
and Ge emerges from an analysis based on all-electron, self-consistent, GW
calculations. The deep-core electrons are shown to play a key role via the
exchange diagram --if this effect is neglected, Si becomes a semimetal.
Contrary to current lore, the Ge 3d semicore states (e.g., their polarization)
have no impact on the GW gap. Self-consistency improves the calculated gaps --a
first clear-cut success story for the Baym-Kadanoff method in the study of
real-materials spectroscopy; it also has a significant impact on the QP
lifetimes. Our results embody a new paradigm for ab initio QP theory. | cond-mat_mtrl-sci |
Theoretical determination of the Raman spectra of MgSiO3 perovskite and
post-perovskite at high pressure: We use the density functional perturbation theory to determine for the first
time the pressure evolution of the Raman intensities for a mineral, the two
high-pressure structures of MgSiO3 perovskite and post-perovskite. At high
pressures, the Raman powder spectra reveals three main peaks for the perovskite
structure and one main peak for the post-perovskite structure. Due to the large
differences in the spectra of the two phases Raman spectroscopy can be used as
a good experimental indication of the phase transition. | cond-mat_mtrl-sci |
Drastic Changes in Dielectric Function of Silver Under dc Voltage: Significant changes of the relative permittivity of a silver film have been
detected using the surface plasmon resonance (SPR) method when a constant
electric field is applied to a MDM (metal-dielectric-metal) nanostructure. The
structure looks like a capacitor with a 177-nm dielectric corundum film placed
between two silver films 49nm and 37nm thick. The effect manifests itself as a
noticeable change of the reflectivity of the structure when the voltage of up
to 30V is applied to the electrodes. We have a good agreement between the
theory and experiment only if we suppose that the optical parameters of anode
and cathode silver films change differently and the Al_2O_3 film absorbs the
incident light. The refraction coefficient of the cathode silver layer is shown
to become zero when the applied voltage is above 16V. | cond-mat_mtrl-sci |
First-Principles-Based Insight into Electrochemical Reactivity in a
Cobalt-Carbonate-Hydrate Pseudocapacitor: Cobalt carbonate hydroxide (CCH) is a pseudocapacitive material with
remarkably high capacitance and cycle stability. Previously, it was reported
that CCH pseudocapacitive materials are orthorhombic in nature. Recent
structural characterization has revealed that they are hexagonal in nature;
however, their H positions still remain unclear. In this work, we carried out
first-principles simulations to identify the H positions. Through the
simulations, we could consider various fundamental deprotonation reactions
inside the crystal and computationally evaluate the electromotive forces (EMF)
of the deprotonation ($V_\mathrm{dp}$). Compared with the experimental
potential window of the reaction ($< 0.6$ V (vs. saturated calomel electrode
(SCE))), the computed $V_\mathrm{dp}$ (vs. SCE) value ($3.05$ V) was beyond the
potential window, indicating that deprotonation never occurred inside the
crystal. This may be attributed to the strongly formed hydrogen-bonds (H-bonds)
in the crystal, thereby leading to the structural stabilization. We further
investigated the crystal anisotropy in an actual capacitive material by
considering the growth mechanism of the CCH crystal. By associating our X-ray
diffraction (XRD) peak simulations with experimental structural analysis, we
found that the H-bonds formed between CCH $\{(\bar{1}\bar{1}\bar{1}),
(2\bar{1}\bar{1}), (2\bar{1}1)\}$ planes (approximately parallel to $ab$-plane)
can result in 1-D growth (stacked along with $c$-axis). | cond-mat_mtrl-sci |
Calculation of Gilbert damping and magnetic moment of inertia using
torque-torque correlation model within ab initio Wannier framework: Magnetization dynamics in magnetic materials are well described by the
modified semiclassical Landau-Lifshitz-Gilbert (LLG) equation, which includes
the magnetic damping $\alpha$ and the magnetic moment of inertia $\mathrm{I}$
tensors as key parameters. Both parameters are material-specific and physically
represent the time scales of damping of precession and nutation in
magnetization dynamics. $\alpha$ and $\mathrm{I}$ can be calculated quantum
mechanically within the framework of the torque-torque correlation model. The
quantities required for the calculation are torque matrix elements, the real
and imaginary parts of the Green's function and its derivatives. Here, we
calculate these parameters for the elemental magnets such as Fe, Co and Ni in
an ab initio framework using density functional theory and Wannier functions.
We also propose a method to calculate the torque matrix elements within the
Wannier framework. We demonstrate the effectiveness of the method by comparing
it with the experiments and the previous ab initio and empirical studies and
show its potential to improve our understanding of spin dynamics and to
facilitate the design of spintronic devices. | cond-mat_mtrl-sci |
Lattice dynamics study in PbWO4 under high pressure: Room-temperature Raman scattering has been measured in lead tungstate up to
17 GPa. We report the pressure dependence of all the Raman modes of the
tetragonal scheelite phase (PbWO4-I, space group I41/a), which is stable at
ambient conditions. Upon compression the Raman spectrum undergoes significant
changes around 6.2 GPa due to the onset of a partial structural phase
transition to the monoclinic PbWO4-III phase (space group P21/n). Further
changes in the spectrum occur at 7.9 GPa, related to a scheelite-to-fergusonite
transition. This transition is observed due to the sluggishness and kinetic
hindrance of the I-to-III transition. Consequently, we found the coexistence of
the scheelite, PbWO4-III, and fergusonite phases from 7.9 to 9 GPa, and of the
last two phases up to 14.6 GPa. Further to the experiments, we have performed
ab initio lattice dynamics calculations which have greatly helped us in
assigning the Raman modes of the three phases and discussing their pressure
dependence. | cond-mat_mtrl-sci |
Anisotropy effects on the magnetic excitations of a ferromagnetic
monolayer below and above the Curie temperature: The field-driven reorientation transition of an anisotropic ferromagnetic
monolayer is studied within the context of a finite-temperature Green's
function theory. The equilibrium state and the field dependence of the magnon
energy gap $E_0$ are calculated for static magnetic field $H$ applied in plane
along an easy or a hard axis. In the latter case, the in-plane reorientation of
the magnetization is shown to be continuous at T=0, in agreement with free spin
wave theory, and discontinuous at finite temperature $T>0$, in contrast with
the prediction of mean field theory. The discontinuity in the orientation angle
creates a jump in the magnon energy gap, and it is the reason why, for $T>0$,
the energy does not go to zero at the reorientation field. Above the Curie
temperature $T_C$, the magnon energy gap $E_0(H)$ vanishes for H=0 both in the
easy and in the hard case. As $H$ is increased, the gap is found to increase
almost linearly with $H$, but with different slopes depending on the field
orientation. In particular, the slope is smaller when $H$ is along the hard
axis. Such a magnetic anisotropy of the spin-wave energies is shown to persist
well above $T_C$ ($T \approx 1.2 T_C$). | cond-mat_mtrl-sci |
Adsorption and Vibrational Spectroscopy of CO on the Surface of MgO from
Periodic Local Coupled-Cluster Theory: The adsorption of CO on the surface of MgO has long been a model problem in
surface chemistry. Here, we report periodic Gaussian-based calculations for
this problem using second-order perturbation theory (MP2) and coupled-cluster
theory with single and double excitations (CCSD) and perturbative triple
excitations [CCSD(T)], with the latter two performed using a recently developed
extension of the local natural orbital approximation to problems with periodic
boundary conditions. The low cost of periodic local correlation calculations
allows us to calculate the full CCSD(T) binding curve of CO approaching the
surface of MgO (and thus the adsorption energy) and the two-dimensional
potential energy surface (PES) as a function of the distance from the surface
and the CO stretching coordinate. From the PES, we obtain the fundamental
vibrational frequency of CO on MgO, whose shift from the gas phase value is a
common experimental probe of surface adsorption. We find that CCSD(T) correctly
predicts a positive frequency shift upon adsorption of
$+14.7~\textrm{cm}^{-1}$, in excellent agreement with the experimental shift of
$+14.3~\textrm{cm}^{-1}$. We use our CCSD(T) results to assess the accuracy of
MP2, CCSD, and several density functional theory (DFT) approximations,
including exchange correlation functionals and dispersion corrections. We find
that MP2 and CCSD yield reasonable binding energies and frequency shifts,
whereas many DFT calculations overestimate the magnitude of the adsorption
energy by $5$ -- $15$~kJ/mol and predict a negative frequency shift of about
$-20~\textrm{cm}^{-1}$, which we attribute to self-interaction-induced
delocalization errors that are mildly ameliorated with hybrid functionals. Our
findings highlight the accuracy and computational efficiency of the periodic
local correlation for the simulation of surface chemistry with accurate
wavefunction methods. | cond-mat_mtrl-sci |
First-principles approach to dielectric response of graded spherical
particles: We have studied the effective response of composites of spherical particles
each having a dielectric profile which varies along the radius of the
particles. We developed a first-principles approach to compute the dipole
moment of the individual spherical particle and hence the effective dielectric
response of a dilute suspension. The approach has been applied to two model
dielectric profiles, for which exact solutions are available. Moreover, we used
the exact results to validate the results from the differential effective
dipole approximation, recently developed to treat graded spherical particles of
an arbitrary dielectric profile. Excellent agreement between the two approaches
were obtained. While the focus of this work has been on dielectric responses,
the approach is equally applicable to analogous systems such as the
conductivity and elastic problems. | cond-mat_mtrl-sci |
Large-Area Two-Dimensional Layered MoTe$_2$ by Physical Vapor Deposition
and Solid-Phase Crystallization in a Tellurium-Free Atmosphere: Molybdenum ditelluride (MoTe$_2$) has attracted considerable interest for
nanoelectronic, optoelectronic, spintronic, and valleytronic applications
because of its modest band gap, high field-effect mobility, large
spin-orbit-coupling splitting, and tunable 1T'/2H phases. However, synthesizing
large-area, high-quality MoTe$_2$ remains challenging. The complicated design
of gas-phase reactant transport and reaction for chemical vapor deposition or
tellurization is nontrivial because of the weak bonding energy between Mo and
Te. Here, we report a new method for depositing MoTe$_2$ that entails using
physical vapor deposition followed by a post-annealing process in a Te-free
atmosphere. Both Mo and Te were physically deposited onto the substrate by
sputtering a MoTe$_2$ target. A composite SiO$_2$ capping layer was designed to
prevent Te sublimation during the post-annealing process. The post-annealing
process facilitated 1T'-to-2H phase transition and solid-phase crystallization,
leading to the formation of high-crystallinity few-layer 2H-MoTe$_2$ with a
field-effect mobility of ~10 cm$^2$/(V-s), the highest among all nonexfoliated
2H-MoTe$_2$ currently reported. Furthermore, 2H-MoS$_2$ and Td-WTe$_2$ can be
deposited using similar methods. Requiring no transfer or chemical reaction of
metal and chalcogen reactants in the gas phase, the proposed method is
potentially a general yet simple approach for depositing a wide variety of
large-area, high-quality, two-dimensional layered structures. | cond-mat_mtrl-sci |
Electronic Structures of N-doped Graphene with Native Point Defects: Nitrogen doping in graphene has important implications in graphene-based
devices and catalysts. We have performed the density functional theory
calculations to study the electronic structures of N-doped graphene with
vacancies and Stone-Wales defect. Our results show that monovacancies in
graphene act as hole dopants and that two substitutional N dopants are needed
to compensate for the hole introduced by a monovacancy. On the other hand,
divacancy does not produce any free carriers. Interestingly, a single N dopant
at divacancy acts as an acceptor rather than a donor. The interference between
native point defect and N dopant strongly modifies the role of N doping
regarding the free carrier production in the bulk pi bands. For some of the
defects and N dopant-defect complexes, localized defect pi states are partially
occupied. Discussion on the possibility of spin polarization in such cases is
given. We also present qualitative arguments on the electronic structures based
on the local bond picture. We have analyzed the 1s-related x-ray photoemission
and adsorption spectroscopy spectra of N dopants at vacancies and Stone-Wales
defect in connection with the experimental ones. We also discuss characteristic
scanning tunneling microscope (STM) images originating from the electronic and
structural modifications by the N dopant-defect complexes. STM imaging for
small negative bias voltage will provide important information about possible
active sites for oxygen reduction reaction. | cond-mat_mtrl-sci |
Multi-Fields Modulation of Physical Properties of Oxide Thin Films: Oxide thin films exhibit versatile physical properties such as magnetism,
ferroelectricity, piezoelectricity, metal-insulator transition (MIT),
multiferroicity, colossal magnetoresistivity, switchable resistivity, etc. More
importantly, the exhibited multifunctionality could be tuned by various
external fields, which has enabled demonstration of novel electronic devices.
In this article, recent studies of the multi-fields modulation of physical
properties in oxide thin films have been reviewed. Some of the key issues and
prospects about this field are also addressed. | cond-mat_mtrl-sci |
Communication: Hole localization in Al-doped quartz SiO2 within ab
initio hybrid-functional DFT: We investigate the long-standing problem of the hole localization at the Al
impurity in quartz SiO$_2$, using a relatively recent DFT hybrid-functional
method in which the exchange fraction is obtained \emph{ab initio}, based on an
analogy with the static many-body COHSEX approximation to the electron
self-energy. As the amount of the admixed exact exchange in hybrid functionals
has been shown to be determinant for properly capturing the hole localization,
this problem constitutes a prototypical benchmark for the accuracy of the
method, allowing one to assess to what extent self-interaction effects are
avoided. We obtain good results in terms of description of the charge
localization and structural distortion around the Al center, improving with
respect to the more popular B3LYP hybrid-functional approach. We also discuss
the accuracy of computed hyperfine parameters, by comparison with previous
calculations based on other self-interaction-free methods, as well as
experimental values. We discuss and rationalize the limitations of our approach
in computing defect-related excitation energies in low-dielectric-constant
insulators. | cond-mat_mtrl-sci |
Tunable 2D Electron- and 2D Hole States Observed at Fe/SrTiO$_3$
Interfaces: Oxide electronics provide the key concepts and materials for enhancing
silicon-based semiconductor technologies with novel functionalities. However, a
basic but key property of semiconductor devices still needs to be unveiled in
its oxidic counterparts: the ability to set or even switch between two types of
carriers - either negatively (n) charged electrons or positively (p) charged
holes. Here, we provide direct evidence for individually emerging n- or p-type
2D band dispersions in STO-based heterostructures using resonant photoelectron
spectroscopy. The key to tuning the carrier character is the oxidation state of
an adjacent Fe-based interface layer: For Fe and FeO, hole bands emerge in the
empty band gap region of STO due to hybridization of Ti and Fe-derived states
across the interface, while for Fe$_3$O$_4$ overlayers, an 2D electron system
is formed. Unexpected oxygen vacancy characteristics arise for the hole-type
interfaces, which as of yet had been exclusively assigned to the emergence of
2DESs. In general, this finding opens up the possibility to straightforwardly
switch the type of conductivity at STO interfaces by the oxidation state of a
redox overlayer. This will extend the spectrum of phenomena in oxide
electronics, including the realization of combined n/p-type all-oxide
transistors or logic gates. | cond-mat_mtrl-sci |
Cubic-scaling algorithm and self-consistent field for the random-phase
approximation with second-order screened exchange: The random-phase approximation with second-order screened exchange
(RPA+SOSEX) is a model of electron correlation energy with two caveats: its
accuracy depends on an arbitrary choice of mean field, and it scales as
$\mathcal{O}(n^5)$ operations and $\mathcal{O}(n^3)$ memory for $n$ electrons.
We derive a new algorithm that reduces its scaling to $\mathcal{O}(n^3)$
operations and $\mathcal{O}(n^2)$ memory using controlled approximations and a
new self-consistent field that approximates Brueckner coupled-cluster doubles
(BCCD) theory with RPA+SOSEX, referred to as Brueckner RPA (BRPA) theory. The
algorithm comparably reduces the scaling of second-order
M$\mathrm{{\o}}$ller-Plesset (MP2) perturbation theory with smaller cost
prefactors than RPA+SOSEX. Within a semiempirical model, we study H$_2$
dissociation to test accuracy and H$_n$ rings to verify scaling. | cond-mat_mtrl-sci |
Two Dimensional Ferromagnetic Semiconductor: Monolayer CrGeS$_3$: Recently, two-dimensional ferromagnetic semiconductors have been an important
class of materials for many potential applications in spintronic devices. Based
on density functional theory, we systematically explore the magnetic and
electronic properties of CrGeS$_3$ with the monolayer structures. The
comparison of total energy between different magnetic states ensures the
ferromagnetic ground state of monolayer CrGeS$_3$. It is also shown that
ferromagnetic and semiconducting properties are exhibited in monolayer
CrGeS$_3$ with the magnetic moment of 3 $\mu_{B}$ for each Cr atom, donated
mainly by the intense $dp$$\sigma$-hybridization of Cr $e_g$-S $p$. There are
the bandgap of 0.70 eV of spin-up state in the monolayer structure when 0.77 eV
in spin-down state. The global gap is 0.34 eV (2.21 eV by using HSE06
functional), which originates from bonding $dp\sigma$ hybridized states of Cr
$e_g$-S $p$ and unoccupied Cr $t_{2g}$-Ge $p$ hybridization. Besides, we
estimate that the monolayer CrGeS$_3$ possesses the Curie temperature of 161 K
by mean-field theory. | cond-mat_mtrl-sci |
A new method for measuring excess carrier lifetime in bulk silicon:
Photoexcited muon spin spectroscopy: We have measured the optically injected excess carrier lifetime in silicon
using photoexcited muon spin spectroscopy. Positive muons implanted deep in a
wafer can interact with the excess carriers and directly probe the bulk carrier
lifetime whilst minimizing the effect from surface recombination. The method is
based on the relaxation rate of muon spin asymmetry, which depends on the
excess carrier concentration. The underlying microscopic mechanism has been
understood by simulating the four-state muonium model in Si under illumination.
We apply the technique to different injection levels and temperatures, and
demonstrate its ability for injection- and temperature-dependent lifetime
spectroscopy. | cond-mat_mtrl-sci |
Temperature-dependent stability of polytypes and stacking faults in SiC:
reconciling theory and experiments: The relative stability of SiC polytypes, changing with temperature, has been
considered a paradox for about thirty years, due to discrepancies between
theory and experiments. Based on ab-initio calculations including van der Waals
corrections, a temperature-dependent polytypic diagram consistent with the
experimental observations is obtained. Results are easily interpreted based on
the influence of the hexagonality on both cohesive energy and entropy.
Temperature-dependent stability of stacking faults is also analyzed and found
to be in agreement with experimental evidences. Our results suggest that lower
temperatures during SiC crystal deposition are advantageous in order to reduce
ubiquitous stacking faults in SiC-based power devices. | cond-mat_mtrl-sci |
Critical Ruptures: The fracture of materials is a catastrophic phenomenon of considerable
technological and scientific importance. Here, we analysed experiments designed
for industrial applications in order to test the concept that, in heterogeneous
materials such as fiber composites, rocks, concrete under compression and
materials with large distributed residual stresses, rupture is a genuine
critical point, i.e. the culmination of a self-organization of damage and
cracking characterized by power law signatures. Specifically, we analyse the
acoustic emissions recorded during the pressurisation of spherical tanks of
kevlar or carbon fibers pre-impregnated in a resin matrix wrapped up around a
thin metallic liner (steel or titanium) fabricated and instrumented by
A\'erospatiale-Matra Inc. These experiments are performed as part of a routine
industrial procedure which tests the quality of the tanks prior to shipment and
varies in nature. We find that the seven acoustic emission recordings of seven
pressure tanks which was brought to rupture exhibit clear acceleration in
agreement with a power law ``divergence'' expected from the critical point
theory. In addition, we find strong evidence of log-periodic corrections that
quantify the intermittent succession of accelerating bursts and quiescent
phases of the acoustic emissions on the approach to rupture. An improved model
accounting for the cross-over from the non-critical to the critical region
close to the rupture point exhibits interesting predictive potential. | cond-mat_mtrl-sci |
Putting Density Functional Theory to the Test in
Machine-Learning-Accelerated Materials Discovery: Accelerated discovery with machine learning (ML) has begun to provide the
advances in efficiency needed to overcome the combinatorial challenge of
computational materials design. Nevertheless, ML-accelerated discovery both
inherits the biases of training data derived from density functional theory
(DFT) and leads to many attempted calculations that are doomed to fail. Many
compelling functional materials and catalytic processes involve strained
chemical bonds, open-shell radicals and diradicals, or metal-organic bonds to
open-shell transition-metal centers. Although promising targets, these
materials present unique challenges for electronic structure methods and
combinatorial challenges for their discovery. In this Perspective, we describe
the advances needed in accuracy, efficiency, and approach beyond what is
typical in conventional DFT-based ML workflows. These challenges have begun to
be addressed through ML models trained to predict the results of multiple
methods or the differences between them, enabling quantitative sensitivity
analysis. For DFT to be trusted for a given data point in a high-throughput
screen, it must pass a series of tests. ML models that predict the likelihood
of calculation success and detect the presence of strong correlation will
enable rapid diagnoses and adaptation strategies. These "decision engines"
represent the first steps toward autonomous workflows that avoid the need for
expert determination of the robustness of DFT-based materials discoveries. | cond-mat_mtrl-sci |
Novel supercell compounds of layered Bi-Rh-O with $p$-type metallic
conduction materialized as a thin film form: Layered oxides have been intensively studied due to their high degree of
freedom in designing various electromagnetic properties and functionalities.
While Bi-based layered supercell (LSC) compounds
[Bi$_n$O$_{n+\delta}$]-[$M$O$_2$] ($M$ = Mn, Mn/Al, Mn/Fe, or Mn/Ni; $n=2, 3$)
are a group of prospective candidates, all of the reported compounds are
insulators. Here, we report on the synthesis of two novel metallic LSC
compounds [Bi$_{n}$O$_{n+\delta}$]-[RhO$_2$] ($n=2, 3$) by pulsed laser
deposition and subsequent annealing. With tuning the thickness of the
sublattice from Bi$_2$O$_{2+\delta}$ to Bi$_3$O$_{3+\delta}$, a
dimensionality-dependent electrical transport is revealed from a conventional
metallic transport in $n=2$ to a localized transport in $n=3$. Our successful
growth will be an important step for further exploring novel layered oxide
compounds. | cond-mat_mtrl-sci |
First-principles study of excitonic effects in Raman intensities: The ab initio prediction of Raman intensities for bulk solids usually relies
on the hypothesis that the frequency of the incident laser light is much
smaller than the band gap. However, when the photon frequency is a sizeable
fraction of the energy gap, or higher, resonance effects appear. In the case of
silicon, when excitonic effects are neglected, the response of the solid to
light increases by nearly three orders of magnitude in the range of frequencies
between the static limit and the gap. When excitonic effects are taken into
account, an additional tenfold increase in the intensity is observed. We
include these effects using a finite-difference scheme applied on the
dielectric function obtained by solving the Bethe-Salpeter equation. Our
results for the Raman susceptibility of silicon show stronger agreement with
experimental data compared with previous theoretical studies. For the sampling
of the Brillouin zone, a double-grid technique is proposed, resulting in a
significant reduction in computational effort. | cond-mat_mtrl-sci |
Exciton-Exciton transitions involving strongly bound excitons: an ab
initio approach: In pump-probe spectroscopy, two laser pulses are employed to garner dynamical
information from the sample of interest. The pump initiates the optical process
by exciting a portion of the sample from the electronic ground state to an
accessible electronic excited state, an exciton. Thereafter, the probe
interacts with the already excited sample. The change in the absorbance after
pump provides information on transitions between the excited states and their
dynamics. In this work we study these exciton-exciton transitions by means of
an ab initio real time propagation scheme based on dynamical Berry phase
formulation. The results are then analyzed taking advantage of a Fermi-golden
rule approach formulated in the excitonic basis-set and in terms of the
symmetries of the excitonic states. Using bulk LiF and 2D hBN as two prototype
materials, we discuss the selection rules for transitions involving strongly
bound excitons, for which the hydrogen model cannot be used. | cond-mat_mtrl-sci |
First principles investigations in the carbon silicon system of novel
tetragonal C8 (diamond) and Si8 allotropes and binary Si4C4 phase: Novel extended networks of C8, Si8 and silicon carbide Si4C4 are proposed
based on crystal chemistry rationale and optimized structures to ground state
energies and derived physical properties within the density functional theory
(DFT). The two carbon and silicon allotropes and the silicon carbide belong to
primitive tetragonal space group P-4m2 Number 115. C8 allotrope structure made
of corner sharing C4 and Si4 tetrahedra is illustrated by charge density
projections exhibiting sp3 like carbon hybridization. From careful symmetry
analysis, Symmetry analysis of C8 indicated that it is another representation
of cubic diamond, space group F-d3m Number 227. C8 is identified as ultra-hard
with a similar magnitude of Vickers hardness. The interest in C8 is to serve as
template to study Si8 and Si-C binary. Si8 allotrope is found soft with HV =13
GPa alike cubic Si, and Si4C4 is identified with HV =33 GPa close to
experimental SiC. All three new phases are mechanically (elastic constants) and
dynamically (phonons) stable, and their electronic band structures are
characteristic of insulating C8 (diamond) with large band gaps of about 5 eV,
and semi-conducting Si8 and Si4C4 with band gaps of about 1 eV. | cond-mat_mtrl-sci |
Persistent Current in Two Coupled Rings: We report the solution of the persistent current in two coupled rings in the
presence of external magnetic fluxes. We showed that the magnetic fluxes modify
the global phase of the electronic wave function for multiple connected
geometry formed by the coupled rings. We obtained an exact solution for the
persistent current and investigated the exact solution numerically. For two
large coupled rings with equal fluxes, we found that the persistent current in
the two coupled rings is in fact equal to that in a single ring. This theory
explains the experimental results observed in a line of sixteen coupled rings.
(Phys. Rev. Lett. 86, 3124 (2001).) | cond-mat_mtrl-sci |
sim-trhepd-rheed -- Open-source simulator of total-reflection
high-energy positron diffraction (TRHEPD) and reflection high-energy electron
diffraction (RHEED): The present paper reports sim-trhepd-rheed (STR), an open-source simulator of
total-reflection high-energy positron diffraction (TRHEPD) and reflection
high-energy electron diffraction (RHEED) experiments which are used for
atom-scale surface structure determination of a material. The STR simulator is
used for the analysis of experimental diffraction data by simulating the
rocking curve from a given trial surface structure by solving the partial
differential equation of the dynamical quantum diffraction theory for positron
or electron wavefunctions. Using the obtained surface structure, electronic
structure, and other physical quantities can be evaluated through
first-principles calculations. For this purpose, a utility software was also
developed in order to realize a first principles calculation with the Quantum
ESPRESSO suite. | cond-mat_mtrl-sci |
Optimal Paths for Spatially Extended Metastable Systems Driven by Noise: The least action principle is exploited as a simulation tool to find the
optimal dynamic path for spatially extended systems driven by a small noise.
Applications are presented for thermally activated switching of a
spatially-extended bistable system as well as the switching dynamics of
magnetic thin films. The issue of nucleation versus propagation is discussed
and the scaling for the number of nucleation events as a function of the
terminal time and other material parameters is computed. | cond-mat_mtrl-sci |
WannSymm: A symmetry analysis code for Wannier orbitals: We derived explicit expressions of symmetry operators on Wannier basis, and
implemented these operators in WannSymm software. Based on this implementation,
WannSymm can i) symmetrize the real-space Hamiltonian output from Wannier90
code, ii) generate symmetry operators of the little group at a specific
k-point, and iii) perform symmetry analysis for Wannier band structure. In
general, symmetrized Hamiltonians yield improved results compared with the
original ones when they are employed for nodal structure searching, surface
Green's function calculations, and other model calculations. | cond-mat_mtrl-sci |
Icosahedral quasicrystal enhanced nucleation in commercially pure Ni
processed by selective laser melting: This work provides unambiguous evidence for the occurrence of icosahedral
quasicrystal (iQC) enhanced nucleation during selective laser melting of gas
atomized commercially-pure Ni powders. This solidification mechanism, which has
only been recently reported in a few alloys and has to date never been observed
in pure metals, consists on the solidification of grains of the primary phase
on the facets of iQCs formed due to the presence of icosahedral short range
order in the liquid. The occurrence of iQC enhanced nucleation has been
inferred from the observation in the SLM processed pure Ni samples of an excess
fraction of partially incoherent twin boundaries and of clusters of twinned
grain pairs sharing common <110> five-fold symmetry axes. This work further
evidences that additive manufacturing methods may constitute an invaluable tool
for investigating the fundamentals of solidification and for the design of
unprecedented grain boundary networks. | cond-mat_mtrl-sci |
Time Constants of Spin-Dependent Recombination Processes: We present experiments to systematically study the time constants of
spin-dependent recombination processes in semiconductors using pulsed
electrically detected magnetic resonance (EDMR). The combination of
time-programmed optical excitation and pulsed spin manipulation allows us to
directly measure the recombination time constants of electrons via localized
spin pairs and the time constant of spin pair formation as a function of the
optical excitation intensity. Using electron nuclear double resonance, we show
that the time constant of spin pair formation is determined by an electron
capture process. Based on these time constants we devise a set of rate
equations to calculate the current transient after a resonant microwave pulse
and compare the results with experimental data. Finally, we critically discuss
the effects of different boxcar integration time intervals typically used to
analyze pulsed EDMR experiments on the determination of the time constants. The
experiments are performed on phosphorus-doped silicon, where EDMR via spin
pairs formed by phosphorus donors and Si/SiO2 interface dangling bond defects
is detected. | cond-mat_mtrl-sci |
Incorporation of Mn in Al$_{x}$Ga$_{1-x}$N probed by x-ray absorption
and emission spectroscopy, high-resolution microscopy, x-ray diffraction and
first-principles calculations: Synchrotron radiation x-ray absorption and emission spectroscopy techniques,
complemented by high-resolution transmission electron microscopy methods and
density functional theory calculations are employed to investigate the effect
of Mn in Al$_{x}$Ga$_{1-x}$N:Mn samples with an Al content up to 100%. The
atomic and electronic structure of Mn is established together with its local
environment and valence state. A dilute alloy without precipitation is obtained
for Al$_{x}$Ga$_{1-x}$N:Mn with Al concentrations up to 82%, and the surfactant
role of Mn in the epitaxial process is confirmed. | cond-mat_mtrl-sci |
Depolarizing-Field Effect in Strained Nanoscale Ferroelectric Capacitors
and Tunnel Junctions: The influence of depolarizing field on the magnitude and stability of a
uniform polarization in ferroelectric capacitors and tunnel junctions is
studied using a nonlinear thermodynamic theory. It is predicted that, in
heterostructures involving strained epitaxial films and metal electrodes, the
homogeneous polarization state may remain stable against transformations into
the paraelectric phase and into polydomain states down to the nanoscale. This
result supports the possibility of depolarizing-field-related resistive
switching in ferroelectric tunnel junctions with dissimilar electrodes. The
resistance on/off ratio in such junctions is shown to be governed by the
difference between the reciprocal capacitances of screening space charges in
the electrodes. | cond-mat_mtrl-sci |
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