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Driving skyrmions with low threshold current density in Pt/CoFeB thin
film: Magnetic skyrmions are topologically stable spin swirling particle like
entities which are appealing for next generation spintronic devices. The
expected low critical current density for the motion of skyrmions makes them
potential candidates for future energy efficient electronic devices. Several
heavy metal/ferromagnetic (HM/FM) systems have been explored in the past decade
to achieve faster skyrmion velocity at low current densities. In this context,
we have studied Pt/CoFeB/MgO heterostructures in which skyrmions have been
stabilized at room temperature (RT). It has been observed that the shape of the
skyrmions are perturbed even by the small stray field arising from low moment
magnetic tips while performing the magnetic force microscopy (MFM), indicating
presence of low pinning landscape in the samples. This hypothesis is indeed
confirmed by the low threshold current density to drive the skyrmions in our
sample, at velocities of few 10m/s. | cond-mat_mtrl-sci |
Interfacial contribution to the dielectric response in semiconducting
LaBiMn4/3Co2/3O6: Impedance measurements have been performed on a sintered polycrystalline
sample of the perovskite LaBiMn4/3Co2/3O6. Colossal dielectric permittivity
often is measured in this class of semiconducting materials as a result of
extrinsic factors. Our results show that a large offset in the capacitance,
measured on a series of samples with different thickness, is due to the
interfacial polarization. This contribution then can be removed from the data,
creating a general procedure for dielectric measurements in semiconducting
samples. | cond-mat_mtrl-sci |
Stochastic Continuum Models for High--Entropy Alloys with Short-range
Order: High entropy alloys (HEAs) are a class of novel materials that exhibit superb
engineering properties. It has been demonstrated by extensive experiments and
first principles/atomistic simulations that short-range order in the atomic
level randomness strongly influences the properties of HEAs. In this paper, we
derive stochastic continuum models for HEAs with short-range order from
atomistic models. A proper continuum limit is obtained such that the mean and
variance of the atomic level randomness together with the short-range order
described by a characteristic length are kept in the process from the atomistic
interaction model to the continuum equation. The obtained continuum model with
short-range order is in the form of an Ornstein--Uhlenbeck (OU) process. This
validates the continuum model based on the OU process adopted
phenomenologically by Zhang et al. [Acta Mater., 166 (2019), pp. 424--434] for
HEAs with short-range order. We derive such stochastic continuum models with
short-range order for both elasticity in HEAs without defects and HEAs with
dislocations (line defects). The obtained stochastic continuum models are based
on the energy formulations, whose variations lead to stochastic partial
differential equations. | cond-mat_mtrl-sci |
Halogenation induced transition of superconductor-to-semiconductor in
MXene-like MOene with direct band gap and long carrier lifetime: Traditional MXenes with intriguing mechanical and electronic properties,
together with the fertilities of elemental compositions and chemical
decorations have aroused much attentions. However, the semiconducting traits
with direc band gap are extremetely rare among reported MXenes. Thus,
broadening the family of MXene beyond carbides and nitrides with unique
behaviors is still an extraordinary and fascinating field. | cond-mat_mtrl-sci |
Structure and Dielectric Properties of Amorphous High-kappa Oxides:
HfO2, ZrO2 and their alloys: High-$\kappa$ metal oxides are a class of materials playing an increasingly
important role in modern device physics and technology. Here we report
theoretical investigations of the properties of structural and lattice
dielectric constants of bulk amorphous metal oxides by a combined approach of
classical molecular dynamics (MD) - for structure evolution, and quantum
mechanical first principles density function theory (DFT) - for electronic
structure analysis. Using classical MD based on the Born-Mayer-Buckingham
potential function within a melt and quench scheme, amorphous structures of
high-$\kappa$ metal oxides Hf$_{1-x}$Zr$_x$O$_2$ with different values of the
concentration $x$, are generated. The coordination numbers and the radial
distribution functions of the structures are in good agreement with the
corresponding experimental data. We then calculate the lattice dielectric
constants of the materials from quantum mechanical first principles, and the
values averaged over an ensemble of samples agree well with the available
experimental data, and are very close to the dielectric constants of their
cubic form. | cond-mat_mtrl-sci |
Tight-binding molecular-dynamics studies of defects and disorder in
covalently-bonded materials: Tight-binding (TB) molecular dynamics (MD) has emerged as a powerful method
for investigating the atomic-scale structure of materials --- in particular the
interplay between structural and electronic properties --- bridging the gap
between empirical methods which, while fast and efficient, lack
transferability, and ab initio approaches which, because of excessive
computational workload, suffer from limitations in size and run times. In this
short review article, we examine several recent applications of TBMD in the
area of defects in covalently-bonded semiconductors and the amorphous phases of
these materials. | cond-mat_mtrl-sci |
Resolving diverse oxygen transport pathways across Sr-doped lanthanum
ferrite and metal-perovskite heterostructures: Perovskite structured transition metal oxides are important technological
materials for catalysis and solid oxide fuel cell applications. Their
functionality often depends on oxygen diffusivity and mobility through complex
oxide heterostructures, which can be significantly impacted by structural and
chemical modifications, such as doping. Further, when utilized within
electrochemical cells, interfacial reactions with other components (e.g. Ni-
and Cr-based alloy electrodes and interconnects) can influence the perovskite's
reactivity and ion transport, leading to complex dependencies that are
difficult to control in real-world environments. Here we use isotopic tracers
and atom probe tomography to directly visualize oxygen diffusion and transport
pathways across perovskite and metal-perovskite heterostructures, i.e. (Ni-Cr
coated) Sr-doped lanthanum ferrite (LSFO). Annealing in 18O2(g) results in
elemental and isotopic redistributions through oxygen exchange (OE) in the LSFO
while Ni-Cr undergoes oxidation via multiple mechanisms and transport pathways.
Complementary density functional theory (DFT) calculations at experimental
conditions provide rationale for OE reaction mechanisms and reveal a complex
interplay of different thermodynamic and kinetic drivers. Our results shed
light on the fundamental coupling of defects and oxygen transport in an
important class of catalytic materials. | cond-mat_mtrl-sci |
Development of local plasticity around voids during tensile deformation: Voids can limit the life of engineering components. This motivates us to
understand local plasticity around voids in a nickel base superalloy combining
experiments and simulations. Single crystal samples were deformed in tension
with in-situ high angular resolution electron back scatter diffraction to probe
the heterogeneous local stress field under load; the reference stress is
informed by crystal plasticity finite element simulations. This information is
used to understand the activation of plastic deformation around the void. Our
investigation indicates that while the resolved shear stress would indicate
slip activity on multiple slip systems, slip is reduced to specific systems due
to image forces and forest hardening. This study rationalizes the observed
development of plastic deformation around the void, aiding in our understanding
of component failure and engineering design. | cond-mat_mtrl-sci |
Magnetism in Graphene Induced by Single-Atom Defects: We study from first principles the magnetism in graphene induced by single
carbon atom defects. For two types of defects considered in our study, the
hydrogen chemisorption defect and the vacancy defect, the itinerant magnetism
due to the defect-induced extended states has been observed. Calculated
magnetic moments are equal to 1 $\mu_B$ per hydrogen chemisorption defect and
1.12$-$1.53 $\mu_B$ per vacancy defect depending on the defect concentration.
The coupling between the magnetic moments is either ferromagnetic or
antiferromagnetic, depending on whether the defects correspond to the same or
to different hexagonal sublattices of the graphene lattice, respectively. The
relevance of itinerant magnetism in graphene to the high-$T_C$ magnetic
ordering is discussed. | cond-mat_mtrl-sci |
Reflectometry with registration of secondary radiation at total neutron
reflection: Neutron reflectometry is a method for measuring of the spatial dependence
(profile) of the potential interaction between neutron and medium. At interface
of media the interaction potential is the sum of the elements potentials. For
definition of potentials of separate elements (isotopes) a secondary radiation
is recorded. Recording channels of secondary radiation are created on
spectrometer REMUR at pulsed reactor IBR-2 in Dubna (Russia). The results for
testing of the channels are reported and perspectives of reflectometry with
registration of secondary radiation are discussed | cond-mat_mtrl-sci |
Pair vs many-body potentials: influence on elastic and plastic behavior
in nanoindentation: Molecular-dynamics simulation can give atomistic information on the processes
occurring in nanoindentation experiments. In particular, the nucleation of
dislocation loops, their growth, interaction and motion can be studied. We
investigate how realistic the interatomic potentials underlying the simulations
have to be in order to describe these complex processes. Specifically we
investigate nanoindentation into a Cu single crystal. We compare simulations
based on a realistic many-body interaction potential of the
embedded-atom-method type with two simple pair potentials, a Lennard-Jones and
a Morse potential. We find that qualitatively many aspects of nanoindentation
are fairly well reproduced by the simple pair potentials: elastic regime,
critical stress and indentation depth for yielding, dependence on the crystal
orientation, and even the level of the hardness. The quantitative deficits of
the pair potential predictions can be traced back (i) to the fact that the pair
potentials are unable in principle to model the elastic anisotropy of cubic
crystals; (ii) as the major drawback of pair potentials we identify the gross
underestimation of the stable stacking fault energy. As a consequence these
potentials predict the formation of too large dislocation loops, the too rapid
expansion of partials, too little cross slip and in consequence a severe
overestimation of work hardening. | cond-mat_mtrl-sci |
Nonpolar p-GaN/n-Si heterojunction diode characteristics: A comparison
between ensemble and single nanowire devices: The electrical and photodiode characteristics of ensemble and single p-GaN
nanowire and n-Si heterojunction devices were studied. Ideality factor of the
single nanowire p-GaN/n-Si device was found to be about three times lower
compared to that of the ensemble nanowire device. Apart from the deep-level
traps in p-GaN nanowires, defect states due to inhomogeneity in Mg dopants in
the ensemble nanowire device are attributed to the origin of high ideality
factor. Photovoltaic mode of ensemble nanowire device showed an improvement in
the fill-factors up to 60 percent over the single nanowire device with
fill-factors up to 30 percent. Reponsivity of the single nanowire device in
photoconducting mode was found to be enhanced by five orders, at 470 nm. The
enhanced photoresponse of the single nanowire device also confirms the
photoconduction due to defect states in p-GaN nanowires. | cond-mat_mtrl-sci |
A novel high-current, high-resolution, low-kinetic-energy electron
source for inverse photoemission spectroscopy: A high-current electron source for inverse photoemission spectroscopy (IPES)
is described. The source comprises a thermal cathode electron emission system,
an electrostatic deflector-monochromator, and a lens system for variable
kinetic energy (1.6 - 20 eV) at the target. When scaled to the energy
resolution, the electron current is an order of magnitude higher than that of
previously described electron sources developed in the context of electron
energy loss spectroscopy. Surprisingly, the experimentally measured energy
resolution turned out to be significantly better than calculated by standard
programs, which include the electron-electron repulsion in the continuum
approximation. The achieved currents are also significantly higher than
predicted. We attribute this "inverse Boersch-effect" to a mechanism of
velocity selection in the forward direction by binary electron-electron
collisions. | cond-mat_mtrl-sci |
On the competition for ultimately stiff and strong architected materials: Advances in manufacturing techniques may now realize virtually any imaginable
microstructures, paving the way for architected materials with properties
beyond those found in nature. This has lead to a quest for closing gaps in
property-space by carefully designed metamaterials. Development of mechanical
metamaterials has gone from open truss lattice structures to closed plate
lattice structures with stiffness close to theoretical bounds. However, the
quest for optimally stiff and strong materials is complex. Plate lattice
structures have higher stiffness and (yield) strength but are prone to buckling
at low volume fractions. Hence here, truss lattice structures may still be
optimal. To make things more complicated, hollow trusses or structural
hierarchy bring closed-walled microstructures back in the competition. Based on
analytical and numerical studies of common microstructures from the literature,
we provide higher order interpolation schemes for their effective stiffness and
(buckling) strength. Furthermore, we provide a case study based on
multi-property Ashby charts for weight-optimal porous beams under bending, that
demonstrates the intricate interplay between structure and microarchitecture
that plays the key role in the design of ultimate load carrying structures. The
provided interpolation schemes may also be used to account for microstructural
yield and buckling in multiscale design optimization schemes. | cond-mat_mtrl-sci |
Unipolar and bipolar fatigue in antiferroelectric lead zirconate thin
films and evidences for switching-induced charge injection inducing fatigue: For the first time, we show that unipolar fatigue does occur in
antiferroelectric capacitors, confirming the predictions of a previous work
[Appl. Phys. Lett., 94, 072901 (2009)]. We also show that unipolar fatigue in
antiferroelectrics is less severe than bipolar fatigue if the driving field is
of the same magnitude. This phenomenon has been attributed to the
switching-induced charge injection, the main cause for polarization fatigue in
ferroelectric and antiferroelectric materials. Other evidences for polarization
fatigue caused by the switching-induced charge injection from the nearby
electrode rather than the charge injection during stable/quasi-stable leakage
current stage are also discussed. | cond-mat_mtrl-sci |
The effect of exclusion on nonlinear reaction diffusion system in
inhomogeneous media: We study a minimal model to understand the formation of clusters on surfaces
in the presence of surface defects. We consider reaction diffusion model in
which atoms undergoes reactions at the defect centers to form clusters. Volume
exclusion between particles is introduced through a drift term in the reaction
diffusion equation that arises due the repulsive force field produced by the
clustering atoms. The volume exclusion terms can be derived from master
equation with a concentration dependent hopping rate. Perturbative analysis is
performed for both cross-exclusion and self-exclusion one dimensional system.
For two dimension numerical analysis is performed. We have found that the
clusterization process slows down due to exclusion. As a result the size of the
clusters reduces. In this model reaction scheme has algebraic nonlinearity and
plausible mechanism is also given. | cond-mat_mtrl-sci |
Small-Angle X-ray and neutron scattering from diamond single crystals: Results of Small-Angle Scattering study of diamonds with various types of
point and extended defects and different degrees of annealing are presented. It
is shown that thermal annealing and/or mechanical deformation cause formation
of nanosized planar and threedimensional defects giving rise to Small-Angle
Scattering. The defects are often facetted by crystallographic planes 111, 100,
110, 311, 211 common for diamond. The scattering defects likely consist of
clusters of intrinsic and impurity-related defects; boundaries of mechanical
twins also contribute to the SAS signal. There is no clear correlation between
concentration of nitrogen impurity and intensity of the scattering. | cond-mat_mtrl-sci |
Internal stresses analysis on welded joint in Grade 91 steel under creep
test: synchrotron DRX tests and modelling: The analysis and understanding of creep damage of Grade 91 steel welded
joints is an important topic in the energy industry. Creep tests on welded
joints were carried out at 600$^{\circ}$C, 100MPa and then interrupted at 0%,
10%, 30%, 50%, 80% of the expected life and after failure. Creep damage is
characterised by cavity bands located exclusively in the core of the sample in
the InterCritical Heat Affected Zone (ICHAZ). These samples were tested using
\textit{in situ} synchrotron XRD along the welded joint under creep conditions
for the different creep life time. The experimental results show a significant
strain evolution and creep damage characteristic on the welded joint, with a
local maximum at the Heat Affected Zone (HAZ). Following this, a finite element
creep strain analysis was performed for comparison with the experimental
results. | cond-mat_mtrl-sci |
Observation of Large Unidirectional Rashba Magnetoresistance in Ge(111): Relating magnetotransport properties to specific spin textures at surfaces or
interfaces is an intense field of research nowadays. Here, we investigate the
variation of the electrical resistance of Ge(111) grown epitaxially on
semi-insulating Si(111) under the application of an external magnetic field. We
find a magnetoresistance term which is linear in current density j and magnetic
field B, hence odd in j and B, corresponding to a unidirectional
magnetoresistance. At 15 K, for I = 10 $\mu$A (or j = 0.33 A/m) and B = 1 T, it
represents 0.5 % of the zero field resistance, a much higher value compared to
previous reports on unidirectional magnetoresistance. We ascribe the origin of
this magnetoresistance to the interplay between the externally applied magnetic
field and the current-induced pseudo-magnetic field in the spin-splitted
subsurface states of Ge(111). This unidirectional magnetoresistance is
independent of the current direction with respect to the Ge crystal axes. It
progressively vanishes, either using a negative gate voltage due to carrier
activation into the bulk (without spin-splitted bands), or by increasing the
temperature due to the Rashba energy splitting of the subsurface states lower
than $\sim$58 k$_B$. The highly developed technologies on semiconductor
platforms would allow the rapid optimization of devices based on this
phenomenon. | cond-mat_mtrl-sci |
Identifying vacancy complexes in compound semiconductors with positron
annihilation spectroscopy: a case study of InN: We present a comprehensive study of vacancy and vacancy-impurity complexes in
InN combining positron annihilation spectroscopy and ab-initio calculations.
Positron densities and annihilation characteristics of common vacancy-type
defects are calculated using density functional theory and the feasibility of
their experimental detection and distinction with positron annihilation methods
is discussed. The computational results are compared to positron lifetime and
conventional as well as coincidence Doppler broadening measurements of several
representative InN samples. The particular dominant vacancy-type positron traps
are identified and their characteristic positron lifetimes, Doppler ratio
curves and lineshape parameters determined. We find that In vacancies and their
complexes with N vacancies or impurities act as efficient positron traps,
inducing distinct changes in the annihilation parameters compared to the InN
lattice. Neutral or positively charged N vacancies and pure N vacancy complexes
on the other hand do not trap positrons. The predominantly introduced positron
trap in irradiated InN is identified as the isolated In vacancy, while in
as-grown InN layers In vacancies do not occur isolated but complexed with one
or more N vacancies. The number of N vacancies per In vacancy in these
complexes is found to increase from the near surface region towards the
layer-substrate interface. | cond-mat_mtrl-sci |
Direct measurements of the magnetocaloric effect in ribbon samples of
Heusler alloys Ni - Mn - M (M = In, Sn): Direct measurements of the magnetocaloric effect in samples of rapidly
quenched ribbons of Mn50Ni40In10 and Ni50Mn37Sn13 Heusler alloys, with
potential applications in magnetic refrigeration technology, are carried out.
The measurements were made by a precise method based on the measurement of the
oscillation amplitude of the temperature in the sample while is subjected to a
modulated magnetic field. In the studied compositions both direct and inverse
magnetocaloric effects associated with magnetic (paramagnet - ferromagnet -
antiferromagnet) and structural (austenite - martensite) phase transitions are
found. Additional inverse magnetocaloric effects of small value are observed
around the ferromagnetic transitions. | cond-mat_mtrl-sci |
Energy gap opening in submonolayer lithium on graphene: Local density
functional and tight-binding calculations: The adsorption of an alkali-metal submonolayer on graphene occupying every
third hexagon of the honeycomb lattice in a commensurate
$(\sqrt{3}\times\sqrt{3})R30^\circ$ arrangement induces an energy gap in the
spectrum of graphene. To exemplify this type of band gap, we present \textit{ab
initio} density functional theory calculations of the electronic band structure
of C$_6$Li. An examination of the lattice geometry of the compound system shows
the possibility that the nearest-neighbor hopping amplitudes have alternating
values constructed in a Kekul\'e-type structure. The band structure of the
textured tight-binding model is calculated and shown to reproduce the expected
band gap as well as other characteristic degeneracy removals in the spectrum of
graphene induced by lithium adsorption. More generally we also deduce the
possibility of energy gap opening in periodic metal on graphene compounds
C$_x$M if $x$ is a multiple of 3. | cond-mat_mtrl-sci |
Ionization potentials in the limit of large atomic number: By extrapolating the energies of non-relativistic atoms and their ions with
up to 3000 electrons within Kohn-Sham density functional theory, we find that
the ionization potential remains finite and increases across a row, even as
$Z\rightarrow\infty$. The local density approximation becomes chemically
accurate (and possibly exact) in some cases. Extended Thomas-Fermi theory
matches the shell-average of both the ionization potential and density change.
Exact results are given in the limit of weak electron-electron repulsion. | cond-mat_mtrl-sci |
Atomistic simulations of ductile failure in a b.c.c. high entropy alloy: Ductile failure is studied in a bcc HfNbTaZr High Entropy Alloy (HEA) with a
pre-existing void. Using molecular dynamics simulations of uniaxial tensile
tests, we explore the effect of void radius on the elastic modulus and yield
stress. The elastic modulus scales with porosity as in closed-cell foams. The
critical stress for dislocation nucleation as a function of the void radius is
very well described by a model designed after pure bcc metals, taking into
account a larger core radius for the HEA. Twinning takes place as a
complementary deformation mechanism, and some detwinning occurs at large
strain. No solid-solid phase transitions are identified. The concurrent effects
of element size mismatch and plasticity lead to significant lattice disorder.
By comparing our HEA results to pure tantalum simulations, we show that the
critical stress for dislocation nucleation and the resulting dislocation
densities are much lower than for pure Ta, as expected from lower energy
barriers due to chemical complexity | cond-mat_mtrl-sci |
Color centers in NaCl by hybrid functionals: We present in this work the electronic structure and transition energies
(both thermodynamic and optical) of Cl vacancies in NaCl by hybrid density
functionals. The underestimated transition energies by the semi-local
functional inherited from the band gap problem are recovered by the PBE0 hybrid
functional through the non-local exact exchange, whose amount is adjusted to
reproduce the experimental band gap. The hybrid functional also gives a better
account of the lattice relaxation for the defect systems arising from the
reduced self-interaction. On the other hand, the quantitative agreement with
experimental vertical transition energy cannot be achieved with hybrid
functionals due to the inaccurate descriptions of the ionization energies of
the localized defect and the positions of the band edges. | cond-mat_mtrl-sci |
Time-dependent phase quantification and local structure analysis of
hydroxide-activated slag via X-ray total scattering and molecular modeling: Here, an approach to quantify the amorphous-to-disordered/crystalline
transformation occurring in NaOH-activated ground granulated blast-furnace slag
(GGBS) is outlined that combines atomistic modeling with in situ pair
distribution function (PDF) analysis. Firstly, by using force-field molecular
dynamics (MD) simulations, a detailed structural representation is generated
for the amorphous GGBS that is in agreement with experimental X-ray scattering
data. Use of this structural representation along with literature-derived
structures for the reaction products allows for real space X-ray PDF refinement
of the alkaline activation of GGBS, resulting in the quantification of all
phases and the degree of reaction (DOR) as a function of reaction time. All
phases and the DOR are seen to approximately follow a logarithmic-type
time-dependent behavior up to 5 months, while at early age (up to 11 hours) the
DOR is accurately captured by a modified pseudo-single step first-order
reaction model. Lastly, the evolution of DOR is found to agree with several
other complementary in situ data containing quantitative reaction information,
including isothermal conduction calorimetry, Fourier transform infrared
spectroscopy, and quasi-elastic neutron scattering. | cond-mat_mtrl-sci |
Anomalous Strength Characteristics of Tilt Grain Boundaries in Graphene: Using molecular dynamics simulations and first principles calculations, we
have studied the structure and mechanical strength of tilt grain boundaries in
graphene sheets that arise during CVD growth of graphene on metal substrates.
Surprisingly, we find that for tilt boundaries in the vicinity of both the
zig-zag and arm-chair orientations, large angle boundaries with a higher
density of 5-7 defect pairs are stronger than the low-angle boundaries which
are comprised of fewer defects per unit length. Interestingly, the trends in
our results cannot be explained by a continuum Griffith-type fracture mechanics
criterion, which predicts the opposite trend due to that fact that it does not
account for the critical bonds that are responsible for the failure mechanism.
We have identified the highly-strained bonds in the 7-member rings that lead to
the failure of the sheets, and we have found that large angle boundaries are
able to better accommodate the strained 7-rings. Our results provide guidelines
for designing growth methods to obtain grain boundary structures that can have
strengths close to that of pristine graphene. | cond-mat_mtrl-sci |
Ytterbium divalency and lattice disorder in near-zero thermal expansion
YbGaGe: While near-zero thermal expansion (NZTE) in YbGaGe is sensitive to
stoichiometry and defect concentration, the NZTE mechanism remains elusive. We
present x-ray absorption spectra that show unequivocally that Yb is nearly
divalent in YbGaGe and the valence does not change with temperature or with
nominally 1% B or 5% C impurities, ruling out a valence-fluctuation mechanism.
Moreover, substantial changes occur in the local structure around Yb with B and
C inclusion. Together with inelastic neutron scattering measurements, these
data indicate a strong tendency for the lattice to disorder, providing a
possible explanation for NZTE in YbGaGe. | cond-mat_mtrl-sci |
Probing quantum geometry through optical conductivity and magnetic
circular dichroism: Probing ground-state quantum geometry and topology through optical response
is not only of fundamental interest, but it can also offer several practical
advantages. Here, using first-principles calculations on antiferromagnetic
topological insulator MnBi$_2$Te$_4$ thin films, we demonstrate how the
generalized optical weight arising from the absorptive part of the optical
conductivity can be used to probe the ground state quantum geometry and
topology. We show that three septuple layers MnBi$_2$Te$_4$ exhibit an enhanced
almost perfect magnetic circular dichroism for a narrow photon energy window in
the infrared region. We calculate the quantum weight in a few septuple layers
MnBi$_2$Te$_4$ and show that it far exceeds the lower bound provided by the
Chern number. Our results suggest that the well-known optical methods are
powerful tools for probing the ground state quantum geometry and topology. | cond-mat_mtrl-sci |
Design of efficient vdW thermionic heterostructures from first
principles: This work is the first step towards understanding thermionic transport
properties of graphene/phosphorene/graphene van der Waals heterostructures in
contact with gold electrodes by using density functional theory based first
principles calculations combined with real space Green's function formalism. We
show that for monolayer phosphorene in the heterostructure, quantum tunneling
dominates the transport. By adding more phosphorene layers, one can switch from
tunneling dominated transport to thermionic dominated transport, resulting in
transporting more heat per charge carrier, thus, enhancing the cooling
coefficient of performance. The thermionic coefficient of performance for the
proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13,
which is significant for nanoscale devices. | cond-mat_mtrl-sci |
Diffusion-controlled growth and microstructural evolution of aluminide
coatings: The diffusion-controlled growth and microstructural evolution at the
interface of aluminide coatings and different substrates such as Ni-base
superalloys and steel are reviewed. Quantitative diffusion analysis indicates
that the diffusion rates of components in the beta-NiAl phase increases with
the addition of Pt. This directly reflects on the growth rate of the
interdiffusion zone. The thickness and formation of precipitates between the
bond coat and the superalloys increase significantly with the Pt addition.
Mainly Fe2Al5 phase grows during hot dip aluminization of steel along with few
other phases with very thin layer. Chemical vapor deposition process is being
established for a better control of the composition of the Fe-aluminide coating
on steel. | cond-mat_mtrl-sci |
Diffuse Neutron Scattering Study of a Disordered Complex Perovskite
Pb(Zn1/3Nb2/3)O3 Crystal: Diffuse scattering around the (110) reciprocal lattice point has been
investigated by elastic neutron scattering in the paraelectric and the relaxor
phases of the disordered complex perovskite crystal-Pb(Zn1/3Nb2/3)O3(PZN). The
appearance of a diffuse intensity peak indicates the formation of polar
nanoregions at temperature T*, approximately 40K above Tc=413K. The analysis of
this diffuse scattering indicates that these regions are in the shape of
ellipsoids, more extended in the <111> direction than in the <001> direction.
The quantitative analysis provides an estimate of the correlation length, \xi,
or size of the regions and shows that \xi <111>~1.2\xi < 001>, consistent with
the primary or dominant displacement of Pb leading to the low temperature
rhombohedral phase. Both the appearance of the polar regions at T*and the
structural transition at Tc are marked by kinks in the \xi < 111> curve but not
in the \xi < 001> one, also indicating that the primary changes take place in a
<111> direction at both temperatures. | cond-mat_mtrl-sci |
Perfect Spin-filtering and Giant Magnetoresistance with Fe-terminated
Graphene Nanoribbon: Spin-dependent electronic transport properties of Fe-terminated zig-zag
graphene nanoribbons (zGNR) have been studied using first-principles transport
simulations. The spin configuration of proposed zGNR junction can be controlled
with external magnetic field, and the tunneling junction show MR>1000 at small
bias and is a perfect spin-filter by applying uniform external magnetic filed
at small bias. | cond-mat_mtrl-sci |
Noncovalent functionalization of carbon nanotubes and graphene with
tetraphenylporphyrins: Stability and optical properties from ab-initio
calculations: The stability, electronic and optical properties of single-walled carbon
nanotubes (CNTs) and graphene noncovalently functionalized with free-base
tetraphenylporphyrin (TPP) molecules is addressed by density functional theory
calculations, including corrections to dispersive interactions. We study the
TPP physisorption on 42 CNT species, particularly those with chiral indices
($n$,$m$), where $5 \leq n \leq 12$ and $0\leq m\leq n$. Our results show a
quite strong $\pi$-$\pi$ interaction between TPP and the CNT surface, with
binding energies ranging from 1.1 to 1.8 eV, where higher energies can be
associated with increasing CNT diameters. We also find that the TPP optical
absorptions would not be affected by the CNT diameter or chirality. Results for
the TPP physisorption on graphene show a remarkable stability with binding
energy of 3.2 eV, inducing a small redshift on the $\pi$-stacked TPP absorption
bands. The strong graphene-TPP interaction also induces a charge transfer from
TPP to graphene, indicating a $n$-type doping mechanism without compromising
the graphene structure. | cond-mat_mtrl-sci |
Scanning Tunneling Microscopy of Defect States in the Semiconductor
Bi$_2$Se$_3$: Scanning tunneling spectroscopy images of Bi$_2$Se$_3$ doped with excess Bi
reveal electronic defect states with a striking shape resembling clover leaves.
With a simple tight-binding model we show that the geometry of the defect
states in Bi$_2$Se$_3$ can be directly related to the position of the
originating impurities. Only the Bi defects at the Se sites five atomic layers
below the surface are experimentally observed. We show that this effect can be
explained by the interplay of defect and surface electronic structure. | cond-mat_mtrl-sci |
Nonequilibrium molecular dynamics simulation of rapid directional
solidification: We present the results of non-equilibrium molecular dynamics simulations for
the growth of a solid binary alloy from its liquid phase. The regime of high
pulling velocities, $V$, for which there is a progressive transition from
solute segregation to solute trapping, is considered. In the segregation
regime, we recover the exponential form of the concentration profile within the
liquid phase. Solute trapping is shown to settle in progressively as $V$ is
increased and our results are in good agreement with the theoretical
predictions of Aziz [J. Appl. Phys. {\bf 53}, 1158 (1981)]. In addition, the
fluid advection velocity is shown to remain directly proportional to $V$, even
at the highest velocities considered here ($V\simeq10$ms$^{-1}$). | cond-mat_mtrl-sci |
A closer look at how symmetry constraints and the spin-orbit coupling
shape the electronic structure of Bi(111): Relativistic density-functional-theory calculations of Bi(111) thin films are
performed to revisit their band structure and that of macroscopic samples. The
band structure of a our 39-bilayer film ($\sim$~15~nm) shows that (1)
$\sim$9-nm films are enough to describe that of Bi(111), (2) The two split
surface-state metallic branches along the $\overline{\Gamma M}$ direction do
not overlap with the bulk band at the zone boundary but lie within the
A7-distortion-induced conduction-valence band gap, and (3) Neither the
existence of the metallic surface states nor their observed splitting is
related to inversion \emph{asymmetry}. Thus, the spin texture observed in such
states is not caused by the lifting of the Kramers degeneracy and their
splitting is not of the Rashba-type. We instead propose that (1) the large
splitting of the metallic branches is a $m_j=\pm1/2$-$m_j=\pm3/2$ splitting and
(2) the spin texture observed for the metallic branches may only occur because
the almost unaltered strong covalent bonds retained by Bi(111) surface atoms
cannot afford magnetic polarization. We emphasize that degeneracy at the
$M$-point of the SBZ of Bi(111) -- implied by the translational symmetry of the
surface -- is satisfied irrespectively of the presence of inversion symmetry
centers. We show that the magnetic-moment discontinuity at $M$ does not exist,
which also explains why the measured spin-polarization of the metallic branches
vanishes near $M$. We induce the Rashba effect on the band structure of Bi(111)
via different structural/electronic perturbations to reveal the actual lifting
of the Kramers degeneracy and find that the magnitude of the perturbation
imposed on a film correlates with the magnitude of the splitting and the
localization of the Rashba-split states. | cond-mat_mtrl-sci |
SAMPLE: Surface structure search enabled by coarse graining and
statistical learning: In this publication we introduce SAMPLE, a structure search approach for
commensurate organic monolayers on inorganic substrates. Such monolayers often
show rich polymorphism with diverse molecular arrangements in differently
shaped unit cells. Determining the different commensurate polymorphs from first
principles poses a major challenge due to the large number of possible
molecular arrangements. To meet this challenge, SAMPLE employs coarse-grained
modeling in combination with Bayesian linear regression to efficiently map the
minima of the potential energy surface. In addition, it uses ab initio
thermodynamics to generate phase diagrams. Using the example of naphthalene on
Cu(111), we comprehensively explain the SAMPLE approach and demonstrate its
capabilities by comparing the predicted with the experimentally observed
polymorphs. | cond-mat_mtrl-sci |
Large Longitudinal Magnetoelectric Coupling in NiFe2O4-BaTiO3 Laminates: In contrast to the Pb-based magnetoelectric laminates (MELs), we find in the
BaTiO3 and NiFe2O4 laminates (number of layers n = 5-25) that the longitudinal
magnetoelectric (ME) voltage coefficient Alpha E33 becomes much larger than the
transverse one due to preferential alignment of magnetic moments along the
NiFe2O4 plane. Moreover, upon decreasing each layer thickness down to 15 um, we
realize enhanced Alpha E33 up to 18 mV/ (cm Oe) and systematic increase of the
ME sensitivity in proportion to n to achieve the largest in the Pb-free MELs
(400*10^-6V/Oe), thereby providing pathways for tailoring ME coupling in
mass-produced, environment friendly laminates. | cond-mat_mtrl-sci |
Comments on frequency dependent ac conductivity in polymeric materials
at low frequency regime: The AC conductivity response in a broad frequency range of disordered
materials is of great interest not only for technological applications, but
also from a theoretical point of view. The Jonscher power exponent value, and
its temperature dependence, is a very important parameter in dielectric data
analysis as well as the physical interpretation of conduction mechanisms in
disordered materials. In some cases the power exponent of AC conductivity has
been reported to be greater than 1 at the low frequency regime. This fact seems
to contradict the universal dynamic response. The present work focuses on the
analysis of dielectric spectroscopy measurements in polymeric materials, below
100 MHz. The apparent power exponent n gets values in the range (0,1) and is
directly related to the characteristics of mobile charges at shorter time
scales, in the case of the occurrence of DC conduction and the slowest
polarization mechanism that is due to the charge motions within sort length
scales, in log(epsilon'')-log(frequenvy) plot. The emergence of apparent n
values in the range [1,2], for a relatively narrow frequency range, may be
attributed to an additional molecular dipolar relaxation contribution at higher
frequencies, in log(epsilon'')-log(frequency) plot. The appearance of apparent
n values in the range (1,2], can be assigned to the existence of a well defined
minimum between DC conductivity contribution and a molecular dipolar dispersion
or between two well separated dielectric loss mechanisms, in
log(epsilon'')-log(frequency) plots, above the crossover frequency. In these
latter cases, the apparent power exponent n is merely related to the
Havriliak-Negami equation shape parameters of the higher frequencies molecular
dipolar relaxations. | cond-mat_mtrl-sci |
Helium-Implantation-Induced Lattice Strains and Defects in Tungsten
probed by X-ray Micro-diffraction: Tungsten is the main candidate material for plasma-facing armour components
in future fusion reactors. Bombardment with energetic fusion neutrons causes
collision cascade damage and defect formation. Interaction of defects with
helium, produced by transmutation and injected from the plasma, modifies defect
retention and behaviour. Here we investigate the residual lattice strains
caused by different doses of helium-ion-implantation into tungsten and
tungsten-rhenium alloys. Energy and depth-resolved synchrotron X-ray
micro-diffraction uniquely permits the measurement of lattice strain with
sub-micron 3D spatial resolution and ~10-4 strain sensitivity. Increase of
helium dose from 300 appm to 3000 appm increases volumetric strain by only ~2.4
times, indicating that defect retention per injected helium atom is ~3 times
higher at low helium doses. This suggests that defect retention is not a simple
function of implanted helium dose, but strongly depends on material composition
and presence of impurities. Conversely, analysis of W-1wt% Re alloy samples and
of different crystal orientations shows that both the presence of rhenium, and
crystal orientation, have comparatively small effect on defect retention. These
insights are key for the design of armour components in future reactors where
it will be essential to account for irradiation-induced dimensional change when
predicting component lifetime and performance. | cond-mat_mtrl-sci |
Revealing process and material parameter effects on densification via
phase-field studies: Sintering is an important processing step in both ceramics and metals
processing. The microstructure resulting from this process determines many
materials properties of interest. Hence the accurate prediction of the
microstructure, depending on processing and materials parameters, is of great
importance. The phase-field method offers a way of predicting this
microstructural evolution on a mesoscopic scale. The present paper employs this
method to investigate concurrent densification and grain growth and the
influence of stress on densification. Furthermore, the method is applied to
simulate the entire freeze-casting process chain for the first time ever by
simulating the freezing and sintering processes separately and passing the
frozen microstructure to the present sintering model. | cond-mat_mtrl-sci |
Substrate Effect on Excitonic Shift and Radiative Lifetime of
Two-Dimensional Materials: Substrates have strong effects on optoelectronic properties of
two-dimensional (2D) materials, which have emerged as promising platforms for
exotic physical phenomena and outstanding applications. To reliably interpret
experimental results and predict such effects at 2D interfaces, theoretical
methods accurately describing electron correlation and electron-hole
interaction such as first-principles many-body perturbation theory are
necessary. In our previous work [Phys. Rev. B 102, 205113(2020)], we developed
the reciprocal-space linear interpolation method that can take into account the
effects of substrate screening for arbitrarily lattice-mismatched interfaces at
the GW level of approximation. In this work, we apply this method to examine
the substrate effect on excitonic excitation and recombination of 2D materials
by solving the Bethe-Salpeter equation. We predict the nonrigid shift of 1s and
2s excitonic peaks due to substrate screening, in excellent agreements with
experiments. We then reveal its underlying physical mechanism through 2D
hydrogen model and the linear relation between quasiparticle gaps and exciton
binding energies when varying the substrate screening. At the end, we calculate
the exciton radiative lifetime of monolayer hexagonal boron nitride with
various substrates at zero and room temperature, as well as the one of WS2
where we obtain good agreement with experimental lifetime. Our work answers
important questions of substrate effects on excitonic properties of 2D
interfaces. | cond-mat_mtrl-sci |
Sol-gel synthesis and multiferroic properties of pyrochlore-free
Pb(Fe0.5Nb0.5)O3 thin films: Lead iron niobate (PbFe0.5Nb0.5O3 - PFN) thin films were synthesized by a
modified sol-gel route, which offers the advantage of a rapid, simple and
non-toxic reaction method. Polycrystalline perovskite-structured PFN thin films
without pyrochlore phases were obtained on Pt/Ti/SiO2/Si substrates after
sintering by rapid thermal annealing at 650 {\deg}C. TEM and AFM images
confirmed the excellent quality of the sintered film, while EDS spectroscopy
revealed the presence of oxygen vacancies near the film/substrate interface.
Electric measurements show good dielectric properties and ferroelectric
behavior, characterized by typical C-V curves and well-defined P-E
ferroelectric loops at 1 kHz, with remanent polarization values of ~12 uC/cm2.
The polarization, however, increases with decreasing frequency, indicating the
presence of leakage currents. I-V measurements show a significant increase in
DC-conduction at relatively low fields (around 100 kV/cm). The films display
ferromagnetic behavior at room temperature, with magnetic remanence around 30
emu/cm3 and a coercive field of 1 kOe. These values are significantly higher
than those obtained for PFN powders fabricated by the same sol-gel route, as
well as the magnetization values reported in the literature for epitaxial
films. | cond-mat_mtrl-sci |
Mesoscopic tunneling in strontium titanate: Spatial correlation between atoms can generate a depletion in the energy
dispersion of acoustic phonons. Two well known examples are rotons in
superfluid helium and the Kohn anomaly in metals. Here we report on the
observation of a large softening of the transverse acoustic mode in quantum
paraelectric SrTiO$_3$ by means of inelastic neutron scattering. In contrast to
other known cases, this softening occurs at a tiny wave vector implying spatial
correlation extending over a distance as long as 40 lattice parameters. We
attribute this to the formation of mesoscopic fluctuating domains due to the
coupling between local strain and quantum ferroelectric fluctuations. Thus, a
hallmark of the ground state of insulating SrTiO$_3$ is the emergence of
hybridized optical-acoustic phonons. Mesoscopic fluctuating domains play a role
in quantum tunneling, which impedes the emergence of a finite macroscopic
polarisation. | cond-mat_mtrl-sci |
Numerical solution of the relativistic single-site scattering problem
for the Coulomb and the Mathieu potential: For a reliable fully-relativistic Korringa-Kohn-Rostoker Green function
method, an accurate solution of the underlying single-site scattering problem
is necessary. We present an extensive discussion on numerical solutions of the
related differential equations by means of standard methods for a direct
solution and by means of integral equations. Our implementation is tested and
exemplarily demonstrated for a spherically symmetric treatment of a Coulomb
potential and for a Mathieu potential to cover the full-potential
implementation. For the Coulomb potential we include an analytic discussion of
the asymptotic behaviour of irregular scattering solutions close to the origin
($r\ll1$). | cond-mat_mtrl-sci |
Domain-knowledge-aided machine learning method for properties prediction
of soft magnetic metallic glasses: A machine learning (ML) method aided by domain knowledge was proposed to
predict saturated magnetization (Bs) and critical diameter (Dmax) of soft
magnetic metallic glass (MGs). Two datasets were established based on published
experimental works about soft magnetic MGs. A general feature space was
proposed and proved to be adaptive for ML model training for different
prediction tasks. It was found that the predictive performance of ML models was
better than traditional physical knowledge-based estimation methods. In
addition, domain knowledge aided feature selection can greatly reduce the
number of features without significantly reducing the prediction accuracy.
Finally, binary classification of the critical size of soft magnetic metallic
glass was studied. | cond-mat_mtrl-sci |
A surrogate model for studying random field energy release rates in 2D
brittle fractures: This article proposes a weighted-variational model as an approximated
surrogate model to lessen numerical complexity and lower the execution times of
brittle fracture simulations. Consequently, Monte Carlo studies of brittle
fractures become possible when energy release rates are modelled as a random
field. In the weighed-variational model, we propose applying a Gaussian random
field with a Mat\'ern covariance function to simulate a non-homogeneous energy
release rate ($G_c$) of a material. Numerical solutions to the
weighed-variational model, along with the more standard but computationally
demanding hybrid phase-field models, are obtained using the FEniCS open-source
software. The results have indicated that the weighted-variational model is a
competitive surrogate model of the hybrid phase-field method to mimic brittle
fractures in real structures. This method reduces execution times by 90\%. We
conducted a similar study and compared our results with an actual brittle
fracture laboratory experiment. We present an example where a Monte Carlo study
is carried out, modeling $G_c$ as a Gaussian Process, obtaining a distribution
of possible fractures, and load-displacement curves. | cond-mat_mtrl-sci |
Spin Polarized and Valley Helical Edge Modes in Graphene Nanoribbons: Inspired by recent progress in fabricating precisely zigzag-edged graphene
nanoribbons and the observation of edge magnetism, we find that spin polarized
edge modes with well-defined valley index can exist in a bulk energy gap opened
by a staggered sublattice potential such as that provided by a hexagonal
Boron-Nitride substrate. Our result is obtained by both tight-binding model and
first principles calculations. These edge modes are helical with respect to the
valley degree of freedom, and are robust against scattering, as long as the
disorder potential is smooth over atomic scale, resulting from the protection
of the large momentum separation of the valleys. | cond-mat_mtrl-sci |
Energy storage properties of ferroelectric nanocomposites: An atomistic effective Hamiltonian technique is used to investigate the
finite-temperature energy storage properties of a ferroelectric nanocomposite
consisting of an array of BaTiO$_{3}$ nanowires embedded in a SrTiO$_{3}$
matrix, for electric field applied along the long axis of the nanowires. We
find that the energy density \textit{versus} temperature curve adopts a
nonlinear, mostly temperature-independent response when the system exhibits
phases possessing an out-of-plane polarization and vortices while the energy
density more linearly increases with temperature when the nanocomposite either
only possesses vortices (and thus no spontaneous polarization) or is in a
paraelectric and paratoroidic phase for its equilibrium state. Ultrahigh energy
density up to $\simeq$140 J/cm$^{3}$ and an ideal 100% efficiency are also
predicted in this nanocomposite. A phenomenological model, involving a coupling
between polarization and toroidal moment, is further proposed to interpret
these energy density results. | cond-mat_mtrl-sci |
Violation of the `Zero-Force Theorem' in the time-dependent
Krieger-Li-Iafrate approximation: We demonstrate that the time-dependent Krieger-Li-Iafrate approximation in
combination with the exchange-only functional violates the `Zero-Force
Theorem'. By analyzing the time-dependent dipole moment of Na5 and Na9+, we
furthermore show that this can lead to an unphysical self-excitation of the
system depending on the system properties and the excitation strength.
Analytical aspects, especially the connection between the `Zero-Force Theorem'
and the `Generalized-Translation Invariance' of the potential, are discussed. | cond-mat_mtrl-sci |
The Interplay Between Imprint, Wake-Up Like Effects and Domains in
Ferroelectric AlScN: This paper investigates wake-up and imprint in ferroelectric AlScN films. The
study employs a series of I-V and P-E measurements with varying electric field
amplitudes and voltage cycles as well as structural investigation via Scanning
Electron Microscopy to understand the origin and underlying principle of
wake-up and imprint as well as their relation. It is shown that the material
can be considered wake-up free, however inherent imprint and imprint shift in
combination with minor loops result in a wake-up like effect. We introduce a
proposition to explain the influence of initial switching cycles on domains,
their stabilization and corresponding changes in imprint. Unipolar fields and
temperature investigations are used to explore the reversibility of imprint and
ways to program it, while partial switching is applied to investigate domain
propagation and support the aforementioned approach. It is concluded, that
after an energetically more demanding domain nucleation, domain wall motion can
switch the majority of polarization in Al1-xScxN. As a consequence, the
presence of initial domains reduces the coercive field in respect to unipolar
films. | cond-mat_mtrl-sci |
Tunneling magnetoresistance in (La,Pr,Ca)MnO3 nanobridges: The manganite (La,Pr,Ca)MnO3 is well known for its micrometer scale phase
separation into coexisting ferromagnetic metallic and antiferromagnetic
insulating (AFI) regions. Fabricating bridges with widths smaller than the
phase separation length scale has allowed us to probe the magnetic properties
of individual phase separated regions. We observe tunneling magnetoresistance
across naturally occurring AFI tunnel barriers separating adjacent
ferromagnetic regions spanning the width of the bridges. Further, near the
Curie temperature, a magnetic field induced metal-to-insulator transition among
a discrete number of regions within the narrow bridges gives rise to abrupt and
colossal low-field magnetoresistance steps at well defined switching fields. | cond-mat_mtrl-sci |
Evidence of orbit-selective electronic kagome lattice with planar
flat-band in correlated paramagnetic YCr6Ge6: Electronic properties of kagome lattice have drawn great attention recently.
In associate with flat-band induced by destructive interference and Dirac
cone-type dispersion, abundant exotic phenomena have been theoretically
discussed. The material realization of electronic kagome lattice is a crucial
step towards comprehending kagome physics and achieving novel quantum phases.
Here, combining angle-resolved photoemission spectroscopy, transport
measurements and first-principle calculations, we expose a planar flat-band in
paramagnetic YCr6Ge6 as a typical signature of electronic kagome lattice. We
unearth that the planar flat-band arises from the d_(z^2 ) electrons with
intra-kagome-plane hopping forbidden by destructive interference. On the other
hand, the destructive interference and flatness of the d_(x^2-y^2 ) and d_xy
bands are decomposed possibly due to additional in-plane hopping terms, but the
Dirac cone-type dispersion is reserved near chemical potential. We explicitly
unveil that orbital character plays an essential role to realize electronic
kagome lattice in bulk materials with transition metal kagome layers.
Paramagnetic YCr6Ge6 provides an opportunity to comprehend intrinsic properties
of electronic kagome lattice as well as its interplays with spin orbit coupling
and electronic correlation of Cr-3d electrons, and be free from complications
induced by strong local moment of ions in kagome planes. | cond-mat_mtrl-sci |
Novel time-saving first-principles calculation method for
electron-transport properties: We present a time-saving simulator within the framework of the density
functional theory to calculate the transport properties of electrons through
nanostructures suspended between semi-infinite electrodes. By introducing the
Fourier transform and preconditioning conjugate-gradient algorithms into the
simulator, a highly efficient performance can be achieved in determining
scattering wave functions and electron-transport properties of nanostructures
suspended between semi-infinite jellium electrodes. To demonstrate the
performance of the present algorithms, we study the conductance of metallic
nanowires and the origin of the oscillatory behavior in the conductance of an
Ir nanowire. It is confirmed that the $s$-$d_{z^2}$ channel of the Ir nanowire
exhibits the transmission oscillation with a period of two-atom length, which
is also dominant in the experimentally obtained conductance trace. | cond-mat_mtrl-sci |
Flexibility of Fluorinated Graphene-Based Materials: The resistivity of different films and structures containing fluorinated
graphene (FG) flakes and chemical vapor deposition (CVD) grown graphene of
various fluorination degrees under tensile and compressive strains due to
bending deformations was studied. Graphene and multilayer graphene films grown
by means of the CVD method were transferred onto the flexible substrate by
laminating and were subjected to fluorination. They demonstrated a weak
fluorination degree (F/C lower 20%). Compressive strains led to a strong
(one-two orders of magnitude) decrease in the resistivity in both cases, which
was most likely connected with the formation of additional conductive paths
through fluorinated graphene. Tensile strain up to 3% caused by the bending of
both types of CVD-grown FG led to a constant value of the resistivity or to an
irreversible increase in the resistivity under repeated strain cycles. In the
case of the structures with the FG thin film printed on polyvinyl alcohol, a
stable bipolar resistive switching was observed up to 6.5% of the tensile
strain (bending radius was 2 mm). The excellent performance of the crossbar
memristor structures under tensile strain shows that the FG films and
structures created from suspension are especially promising for flexible
electronics. | cond-mat_mtrl-sci |
Synthetic control over polymorph formation in the d-band semiconductor
system FeS$_2$: Pyrite, also known as fool's gold is the thermodynamic stable polymorph of
FeS$_2$. It is widely considered as a promising d-band semiconductor for
various applications due to its intriguing physical properties. Marcasite is
the other naturally occurring polymorph of FeS$_2$. Measurements on natural
crystals have shown that it has similarly promising electronic, mechanical, and
optical properties as pyrite. However, it has been only scarcely investigated
so far, because the laboratory-based synthesis of phase-pure samples or
high-quality marcasite single crystal has been a challenge until now. Here, we
report the targeted phase formation via hydrothermal synthesis of marcasite and
pyrite. The formation condition and phase purity of the FeS$_2$ polymorphs are
systematically studied in the form of a comprehensive synthesis map. We,
furthermore, report on a detailed analysis of marcasite single crystal growth
by a space-separated hydrothermal synthesis. We observe that single phase
product of marcasite forms only on the surface under the involvement of H$_2$S
and sulphur vapor. The availability of high-quality crystals of marcasite
allows us to measure the fundamental physical properties, including an allowed
direct optical bandgap of 0.76 eV, temperature independent diamagnetism, an
electronic transport gap of 0.11 eV, and a room-temperature carrier
concentration of 4.14 $\times$ 10$^{18}$ cm$^{-3}$. X-ray absorption/emission
spectroscopy are employed to measure the band gap of the two FeS$_2$ phases. We
find marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87
eV. Our results indicate that marcasite -- that is now synthetically available
in a straightforward fashion -- is as equally promising as pyrite as candidate
for various semiconductor applications based on earth abundant elements. | cond-mat_mtrl-sci |
Physical properties of KMgBi single crystals: KMgBi single crystals are grown by using the Bi flux successfully. KMgBi
shows semiconducting behavior with a metal-semiconductor transition at high
temperature region and a resistivity plateau at low temperature region,
suggesting KMgBi could be a topological insulator with a very small band gap.
Moreover, KMgBi exhibits multiband feature with strong temperature dependence
of carrier concentrations and mobilities. | cond-mat_mtrl-sci |
A computational study of the configurational and vibrational
contributions to the thermodynamics of substitutional alloys: the Ni3Al case: We have developed a methodology to study the thermodynamics of order-disorder
transformations in n-component substitutional alloys that combines
nonequilibrium methods, which can efficiently compute free energies, with Monte
Carlo simulations, in which configurational and vibrational degrees of freedom
are simultaneously considered on an equal footing basis. Furthermore, by
appropriately constraining the system, we were able to compute the
contributions to the vibrational entropy due to bond proportion, atomic size
mismatch, and bulk volume effects. We have applied this methodology to
calculate configurational and vibrational contributions to the entropy of the
Ni3Al alloy as functions of temperature. We found that the bond proportion
effect reduces the vibrational entropy at the order-disorder transition, while
the size mismatch and the bond proportion effects combined do not change the
vibrational entropy at the transition. We also found that the volume increase
at the order-disorder transition causes a vibrational entropy increase of 0.08
kB/atom, which is significant when compared to the configurational entropy
increase of 0.27 kB/atom. Our calculations indicate that the inclusion of
vibrations reduces in about 30 percent the order-disorder transition
temperature determined solely considering the configurational degrees of
freedom. | cond-mat_mtrl-sci |
Magneto-transport characteristics of La1.4Ca1.6Mn2O7 thin film deposited
by spray pyrolysis: Polycrystalline thin films of double layer manganite La_1.4Ca_1.6Mn_2O_7
(DLCMO) have been deposited by nebulized spray pyrolysis on single crystal
LaAlO_3 substrates. These single phase films having grain size in the range
70-100 nm exhibit ferromagnetic transition at T_C ~ 107K. The short range
ferromagnetic ordering due to in plane spin coherence is evidenced to occur at
a higher temperature around 225 K. Insulator/semiconductor to metal transition
occurs at a lower temperature T_P ~ 55K. The transport mechanism above T_C is
of Mott`s variable range hopping type. Below T_C the current-voltage
characteristics show non-linear behaviour that becomes stronger with decreasing
temperature. At low temperatures below T_CA ~ 30K a magnetically frustrated
spin canted state is observed. The DLCMO films exhibit resonable low field
magnetoresistance and at 77K the magnetoresistance ratio is ~ 5% at 0.6 kOe and
\~ 13% at 3 kOe. | cond-mat_mtrl-sci |
Defect-induced $4p$-magnetism in layered platinum diselenide: Platinum diselenide (PtSe$_{2}$) is a recently-discovered extrinsic magnet,
with its magnetism attributed to the presence of Pt-vacancies. The host
material to these defects itself displays interesting structural and electronic
properties, some of which stem from an unusually strong interaction between its
layers. To date, it is not clear how the unique intrinsic properties of
PtSe$_2$ will affect its induced magnetism. In this theoretical work, we show
that the defect-induced magnetism in PtSe$_{2}$ thin films is highly sensitive
to: (i) the layer-thickness (ii) defect density, and (iii) substrate choice.
These different factors dramatically modify all magnetic properties, including
the magnitude of local moments, strength of the coupling, and even nature of
the coupling between the moments. We further show that the strong inter-layer
interactions are key to understanding these effects. A better understanding of
the various influences on magnetism, can enable controllable tuning of the
magnetic properties in Pt-based dichalcogenides, which can be used to design
novel devices for magnetoelectric and magneto-optic applications. | cond-mat_mtrl-sci |
Extension of the standard Heisenberg Hamiltonian to multispin exchange
interactions: An extension of the Heisenberg Hamiltonian is discussed, that allows to go
beyond the standard bilinear spin Hamiltonian taking into account various
contributions due to multispin interactions having both chiral and non-chiral
character. The parameters of the extended Hamiltonian are calculated from first
principles within the framework of the multiple scattering Green function
formalism giving access to an explicit representation of these parameters in
real space. The discussions are focused on the chiral interactions, i.e.\
biquadratic and three-spin Dzyaloshinskii-Moriya like vector interactions
$\vec{\cal{D}}_{ijij}$ (BDMI) and $\vec{\cal{D}}_{ijkj}$ (TDMI), respectively,
as well as the three-spin chiral interaction (TCI) $J_{ijk}$. Although all
parameters are driven by spin-orbit coupling (SOC), some differences in their
properties are demonstrated by calculations for real materials. In particular
it is shown that the three-spin chiral interactions $J_{ijk}$ may be topology
as well as SOC induced, while the TDMI is associated only with the SOC. As the
magnitude of the chiral interactions can be quite sizable, they can lead to a
stabilization of a noncollinear magnetic texture in some materials that is
absent when these interactions are neglected. | cond-mat_mtrl-sci |
Signatures of nonlinear magnetoelectricity in second harmonic spectra of
SU(2) symmetry broken quantum many-body systems: Quantum mechanical perturbative expressions for second order dynamical
magnetoelectric (ME) susceptibilities have been derived and calculated for a
small molecular system using the Hubbard Hamiltonian with SU(2) symmetry
breaking in the form of spin-orbit coupling (SOC) or spin-phonon coupling.
These susceptibilities will have signatures in second harmonic generation
spectra. We show that SU(2) symmetry breaking is the key to generate these
susceptibilities. We have calculated these ME coefficients by solving the
Hamiltonian for low lying excited states using Lanczos method. Varying the
Hubbard term along with SOC strength, we find spin and charge and both
spin-charge dominated spectra of dynamical ME coefficients. We have shown that
intensities of the peaks in the spectra are highest when the magnitudes of
Hubbard term and SOC coupling term are in similar range. | cond-mat_mtrl-sci |
Ultra fast bit addressing in a magnetic memory matrix with crossed wire
write line geometry: An ultra fast bit addressing scheme for magnetic random access memories
(MRAM) in a crossed wire geometry is proposed. In the addressing scheme a word
of cells is programmed simultaneously by sub nanosecond field pulses making use
of the magnetization precession of the free layer. Single spin simulations of
the free layer dynamics show that the pulse parameters for programming an
arbitrary word of the array can be chosen such that the magnetization of the
cells to be written performs either a half or a full precessional turn during
application of the programming pulse depending on the initial and final
magnetization orientation of the addressed cells. Such bit addressing scheme
leads to a suppression of the magnetization ringing in all cells of the memory
array thereby allowing ultra high MRAM write clock rates above 1 GHz. | cond-mat_mtrl-sci |
A first-principles calculation of electron-phonon interactions for the
$\text{C}_2\text{C}_\text{N}$ and $\text{V}_\text{N}\text{N}_\text{B}$
defects in hexagonal boron nitride: Quantum emitters in two-dimensional hexagonal boron nitride (h-BN) have
generated significant interest due to observations of ultra-bright emission
made at room temperature. The expectation that solid-state emitters exhibit
broad zero-phonon lines at elevated temperatures has been put in question by
recent observations of Fourier transform (FT) limited photons emitted from h-BN
flakes at room temperature. The mechanism responsible for the narrow lines has
been suggested to be a mechanical decoupling from in-plane phonons due to an
out-of-plane distortion of the emitter's orbitals. All decoupled emitters
produce photons that are directed in-plane, suggesting that the dipoles are
oriented perpendicular to the h-BN plane. Motivated by the promise of an
efficient and scalable source of indistinguishable photons that can operate at
room temperature, we have developed an approach using density functional theory
(DFT) to determine the electron-phonon coupling for defects that have in- and
out-of-plane transition dipole moments. Our DFT calculations reveal that the
$\text{C}_2 \text{C}_\text{N}$ defect has an in-plane transition dipole moment,
and that of the $\text{V}_\text{N} \text{N}_\text{B}$ defect is perpendicular
to the plane. We exploit the two-dimensional framework recently implemented in
\texttt{QUANTUM ESPRESSO} to determine both the phonon density of states and
the electron-phonon matrix elements associated with the h-BN defective
structures. We find no indication that an out-of-plane transition dipole is
sufficient to obtain FT-limited photons at room temperature. Our work also
provides direction to future DFT software developments and adds to the growing
list of calculations relevant to researchers in the field of solid-state
quantum information processing. | cond-mat_mtrl-sci |
First-principles study of the structural energetics of PdTi and PtTi: The structural energetics of PdTi and PtTi have been studied using
first-principles density-functional theory with pseudopotentials and a
plane-wave basis. We predict that in both materials, the experimentally
reported orthorhombic $B19$ phase will undergo a low-temperature phase
transition to a monoclinic $B19'$ ground state. Within a soft-mode framework,
we relate the $B19$ structure to the cubic $B2$ structure, observed at high
temperature, and the $B19'$ structure to $B19$ via phonon modes strongly
coupled to strain. In contrast to NiTi, the $B19$ structure is extremely close
to hcp. We draw on the analogy to the bcc-hcp transition to suggest likely
transition mechanisms in the present case. | cond-mat_mtrl-sci |
Structural and magnetic properties of half-heusler alloys NiCrZ (Z = Si,
P, Ge, As, Te): First principle study: We present a first principle study of new class of high-$T_c$ half-heusler
ferromagnets NiCrZ (Z = Si, P, Ge, As, Te). The structure and magnetic
properties are investigated through the calculation of the electronic
structure, equilibrium lattice constant, magnetic exchange interaction $J_{ij}$
and Curie temperature $T_c$. The role of $sp$-elements and the influence of
lattice expansion/compression are also studied. In alloys having 20 valence
electrons, a pseudo-gap of the majority band can be formed at Fermi level.
Otherwise, the half-metallicity and ferromagnetism at temperatures much higher
than room temperature are found to be stable in a wide range of lattice
expansion. Based on these results, NiCrZ can be expected to be promising
materials for spintronics. | cond-mat_mtrl-sci |
An Analytical Model to Quantify the Local Lattice Distortion of
Refractory High Entropy Alloys: Local lattice distortion (LLD) of high entropy alloys (HEAs) especially
refractory HEAs, which is different from one lattice site to another,
determines the mechanical properties of HEAs such as yield strength and
radiation resistance, and is crucial to modulating catalytic activity of HEAs
via the atomic strain. In particular, this site-to-site LLD is strongly coupled
with the short-range order (SRO) of HEAs. Therefore it is essential to reveal
the physical picture of LLD. However, the random distribution of
multi-principal constituents of HEAs prohibits the understanding of LLD,
including the determinants of LLD and their coupling rules. Herein, we build
the first analytical model to realize the site-to-site prediction of LLD in
refractory HEAs, by using the neighbor number ratio of central atoms, the
central-atom radii, the standard deviation of constituent radii and the
constituent number, which demonstrates that LLD surprisingly exhibits a similar
mechanism as the relaxation of metal surfaces. The involved parameters depend
only on the radii of constituents and are readily accessible. Moreover, our
scheme determines not only LLD but also the average lattice distortion, which
enables us to predict the phase stability and yield strength of HEAs. These
results build a novel physical picture of LLD, in particular the quantitative
relationship between LLD and SRO, which lay a solid foundation for the further
target-oriented design of HEAs. | cond-mat_mtrl-sci |
Comprehensive search for buckled honeycomb binary compounds based on
noble metals (Cu, Ag, and Au): Honeycomb structure has been frequently observed in two-dimensional (2D)
materials. CuAu in the buckled honeycomb (BHC) structure has been synthesized
recently, which is the first case of 2D intermetallic compounds. Here, the
dynamical stability of 2D $AX$ in the BHC structure, where $A=$ Cu, Ag, and Au
and $X$ is a metallic element in the periodic table, is systematically studied
by calculating phonon dispersions from first-principles. Among 135 $AX$, more
than 50 $AX$ are identified to be dynamically stable. In addition, (i) a
relationship between the dynamical stability and the formation energy, (ii) a
correlation of dynamical stability between different constituents $A$, (iii) a
trend of lattice parameters, and (iv) electronic and magnetic properties are
discussed. Furthermore, a stable phase of B11-type AuZr is predicted based on
both the result (ii) and the stability relationship between 2D and
three-dimensional structures. The present findings stimulate future studies
exploring physics and chemistry of 2D intermetallic compounds. | cond-mat_mtrl-sci |
Electric Field-Dependent Charge-Carrier Velocity in Semiconducting
Carbon Nanotubes: Charge transport in semiconducting single-walled nanotubes (SWNTs) with
Schottky-barrier contacts has been studied at high bias. We observe nearly
symmetric ambipolar transport with electron and hole currents significantly
exceeding 25 micron-ampere, the reported current limit in metallic SWNTs due to
optical phonon emission. Four simple models for the field-dependent velocity
(ballistic, current saturation, velocity saturation, and constant mobility) are
studied in the unipolar regime; the high-bias behavior is best explained by a
velocity saturation model with a saturation velocity of 2 x 10^7 cm/s. | cond-mat_mtrl-sci |
Fermi Surface Deformation near Charge-Ordering Transition: We study the deformation of a Fermi surface (FS) near charge-ordering (CO)
transition. By applying a fluctuation-exchange approximation to the
two-dimensional extended Hubbard model, we show that the FS is largely modified
by strong charge fluctuations when the wave number of the CO pattern does not
match the nesting vector of the FS in a noninteracting system. We also discuss
the enhanced anisotropy in quasiparticle properties in the resultant metallic
state. | cond-mat_mtrl-sci |
Microscopic Property of Amorphous Semiconductor Metal Oxide
InGaZnO$_{4}$ and Role of O-deficiency: We investigated the microscopic and electronic structures amorphous oxide
semiconductors InGaZnO$_{4}$ (a-IGZO) and the role of O-deficiency through the
first-principle calculations. The structure of the amorphous oxide is
complicated by the admixture of many different kinds of substructures, however
it is surprisingly found that the band tail states, which are well-known to be
present in the amorphous semiconductors, are few generated for the conduction
band minimum (CBM). The electronic structure around CBM is little affected by
the disorder and also by the O-deficiency. Free electron carriers can be
generated without a creation of donor-level in the O-deficient amorphous oxide. | cond-mat_mtrl-sci |
Thermal expansion coefficient and lattice anharmonicity of cubic boron
arsenide: Recent measurements of an unusual high thermal conductivity of around 1000 W
m-1 K-1 at room temperature in cubic boron arsenide (BAs) confirm predictions
from theory and suggest potential applications of this semiconductor compound
for thermal management applications. Knowledge of the thermal expansion
coefficient and Gr\"uneisen parameter of a material contributes both to the
fundamental understanding of its lattice anharmonicity and to assessing its
utility as a thermal-management material. However, previous theoretical
calculations of the thermal expansion coefficient and Gr\"uneisen parameter of
BAs yield inconsistent results. Here we report the linear thermal expansion
coefficient of BAs obtained from the X-ray diffraction measurements from 300 K
to 773 K. The measurement results are in good agreement with our ab initio
calculations that account for atomic interactions up to fifth nearest
neighbours. With the measured thermal expansion coefficient and specific heat,
a Gr\"uneisen parameter of BAs of 0.84 +/- 0.09 is obtained at 300 K, in
excellent agreement with the value of 0.82 calculated from first principles and
much lower than prior theoretical results. Our results confirm that BAs
exhibits a better thermal expansion coefficient match with commonly used
semiconductors than other high-thermal conductivity materials such as diamond
and cubic boron nitride. | cond-mat_mtrl-sci |
Metal-semiconductor (semimetal) superlattices on a graphite sheet with
vacancies: It has been found that periodically closely spaced vacancies on a graphite
sheet cause a significant rearrange-ment of its electronic spectrum: metallic
waveguides with a high density of states near the Fermi level are formed along
the vacancy lines. In the direction perpendicular to these lines, the spectrum
exhibits a semimetal or semiconductor character with a gap where a vacancy
miniband is degenerated into impurity levels. | cond-mat_mtrl-sci |
Theory of electron-plasmon coupling in semiconductors: The ability to manipulate plasmons is driving new developments in
electronics, optics, sensing, energy, and medicine. Despite the massive
momentum of experimental research in this direction, a predictive
quantum-mechanical framework for describing electron-plasmon interactions in
real materials is still missing. Here, starting from a many-body Green's
function approach, we develop an ab initio approach for investigating
electron-plasmon coupling in solids. As a first demonstration of this
methodology, we show that electron-plasmon scattering is the primary mechanism
for the cooling of hot carriers in doped silicon, it is key to explain measured
electron mobilities at high doping, and it leads to a quantum zero-point
renormalization of the band gap in agreement with experiment. | cond-mat_mtrl-sci |
Mesoscopic and Microscopic Phase Segregation in Manganese Perovskites: Mesoscopic (500-2000 Angstrom) and microscopic (5-20 Angstrom) phase
segregation with temperature and magnetic field was studied in the model
manganite Pr0.7Ca0.3MnO3 by high-resolution neutron diffraction and inelastic
neutron scattering. Intra-granular strain-driven mesoscopic segregation between
two insulating phases, one of which is charge ordered (CO), sets in below the
CO temperature in zero field. The CO phase orders antiferromagnetically, while
the other insulating phase shows spin-glass behavior. After field-induced
metallization, the CO phase coexists with a ferromagnetic metallic phase. | cond-mat_mtrl-sci |
Ab initio calculations of the structural, electronic and elastic
properties of the MZN2 (M=Be, Mg; Z=C, Si) chalcopyrite semiconductors: Four ternary semiconductors with the chalcopyrite structure (BeCN2, BeSiN2,
MgCN2, and MgSiN2) were studied using the first principles methods. The
structural, electronic, optical and elastic properties were calculated. All
these materials were found to be the indirect band gap semiconductors, with the
calculated band gaps in the range from 3.46 eV to 3.88 eV. Comparison of the
degree of covalency/ionicity of the chemical bonds in these compounds was
performed. Anisotropy of the optical properties of these tetragonal crystals
was demonstrated by calculating the real and imaginary parts of the dielectric
function {\epsilon}. Anisotropy of the elastic properties of these materials
was analyzed by plotting the three-dimensional dependences of the Young moduli
and their two-dimensional cross-sections. It was also shown (at least,
qualitatively) that there exists a correlation between the optical and elastic
anisotropy: the most optically anisotropic MgSiN2 is also most elastically
anisotropic material in the considered group. High hardness (bulk moduli up to
300 GPa) together with large band gaps may lead to new potential applications
of these compounds. | cond-mat_mtrl-sci |
Grains and grain boundaries in highly crystalline monolayer molybdenum
disulfide: Recent progress in large-area synthesis of monolayer molybdenum disulfide, a
new two-dimensional direct-bandgap semiconductor, is paving the way for
applications in atomically thin electronics. Little is known, however, about
the microstructure of this material. Here we have refined chemical vapor
deposition synthesis to grow highly crystalline islands of monolayer molybdenum
disulfide up to 120 um in size with optical and electrical properties
comparable or superior to exfoliated samples. Using transmission electron
microscopy, we correlate lattice orientation, edge morphology, and
crystallinity with island shape to demonstrate that triangular islands are
single crystals. The crystals merge to form faceted tilt and mirror boundaries
that are stitched together by lines of 8- and 4- membered rings. Density
functional theory reveals localized mid-gap states arising from these 8-4
defects. We find that mirror boundaries cause strong photoluminescence
quenching while tilt boundaries cause strong enhancement. In contrast, the
boundaries only slightly increase the measured in-plane electrical
conductivity. | cond-mat_mtrl-sci |
A quantitative theory of current-induced step bunching on Si(111): We use a one-dimensional step model to study quantitatively the growth of
step bunches on Si(111) surfaces induced by a direct heating current.
Parameters in the model are fixed from experimental measurements near 900 deg C
under the assumption that there is local mass transport through surface
diffusion and that step motion is limited by the attachment rate of adatoms to
step edges. The direct heating current is treated as an external driving force
acting on each adatom. Numerical calculations show both qualitative and
quantitative agreement with experiment. A force in the step down direction will
destabilize the uniform step train towards step bunching. The average size of
the step bunches grows with electromigration time as t^beta, with beta = 0.5,
in agreement with experiment and with an analytical treatment of the steady
states. The model is extended to include the effect of direct hopping of
adatoms between different terraces. Monte-Carlo simulations of a solid-on-solid
model, using physically motivated assumptions about the dynamics of surface
diffusion and attachment at step edges, are carried out to study two
dimensional features that are left out of the present step model and to test
its validity. These simulations give much better agreement with experiment than
previous work. We find a new step bending instability when the driving force is
along the step edge direction. This instability causes the formation of step
bunches and antisteps that is similar to that observed in experiment. | cond-mat_mtrl-sci |
Second-harmonic phonon spectroscopy of $α$-quartz: We demonstrate midinfrared second-harmonic generation as a highly sensitive
phonon spectroscopy technique that we exemplify using $\alpha$-quartz (SiO$_2$)
as a model system. A midinfrared free-electron laser provides direct access to
optical phonon resonances ranging from $350\ \mathrm{cm}^{-1}$ to $1400\
\mathrm{cm}^{-1}$. While the extremely wide tunability and high peak fields of
an free-electron laser promote nonlinear spectroscopic studies---complemented
by simultaneous linear reflectivity measurements---azimuthal scans reveal
crystallographic symmetry information of the sample. Additionally,
temperature-dependent measurements show how damping rates increase, phonon
modes shift spectrally and in certain cases disappear completely when
approaching $T_c=846\ \mathrm{K}$ where quartz undergoes a structural phase
transition from trigonal $\alpha$-quartz to hexagonal $\beta$-quartz,
demonstrating the technique's potential for studies of phase transitions. | cond-mat_mtrl-sci |
Insight into Two-Dimensional Borophene: Five-Center Bond and
Phonon-Mediated Superconductivity: We report a previously unknown monolayer borophene allotrope and we call it
super-B with a flat structure based on the ab initio calculations. It has good
thermal, dynamical, and mechanical stability compared with many other typical
borophenes. We find that super-B has a fascinating chemical bond environment
consisting of standard sp, sp2 hybridizations, and delocalized five-center
three-electron $\pi$ bond, called $\pi$(5c-3e). This particular electronic
structure plays a pivotal role in stabilizing the super-B chemically. By extra
doping, super-B can be transformed into a Dirac material from pristine metal.
Like graphene, it can also sustain tensile strain smaller than 24%, indicating
superior flexibility. Moreover, due to the small atomic mass and large density
of states at the Fermi level, super-B has the highest critical temperature Tc
of 25.3 K in single-element superconductors at ambient conditions. We attribute
this high Tc of super-B to the giant anharmonicity of two linear acoustic
phonon branches and an unusually low optic phonon mode. These predictions
provide new insight into the chemical nature of low dimensional boron
nanostructures and highlight the potential applications of designing flexible
devices and high Tc superconductors. | cond-mat_mtrl-sci |
Density-functional approach to the band gaps of finite and periodic
two-dimensional systems: We present an approach based on density-functional theory for the calculation
of fundamental gaps of both finite and periodic two-dimensional (2D) electronic
systems. The computational cost of our approach is comparable to that of total
energy calculations performed via standard semi-local forms. We achieve this by
replacing the 2D local density approximation with a more sophisticated -- yet
computationally simple -- orbital-dependent modeling of the exchange potential
within the procedure by Guandalini et al. [Phys. Rev. B 99, 125140 (2019)]. We
showcase promising results for semiconductor 2D quantum dots and artificial
graphene systems, where the band structure can be tuned through, e.g., Kekul\'e
distortion. | cond-mat_mtrl-sci |
Mechanical behavior of high-entropy alloys: A review: High-entropy alloys (HEAs) are materials that consist of equimolar or
near-equimolar multiple principal components but tend to form single phases,
which is a new research topic in the field of metallurgy, have attracted
extensive attention in the past decade. The HEAs families contain the
face-centered-cubic (fcc), body-centered-cubic (bcc), and
hexagonal-close-packed (hcp)-structured HEAs. On one hand, mechanical
properties, e.g. hardness, strength, ductility, fatigue, and elastic moduli,
are essential for practical applications of HEAs. Scientists have explored in
this direction since the advent of HEAs. On the other hand, the pursuit of high
strength and good plasticity is the critical research issue of materials.
Hence, strengthening of HEAs is a crucial issue. Recently, many articles are
focusing on the strengthening strategies of HEAs[1-14]. In this chapter, we
reviewed the recent work on the room-temperature elastic properties and
mechanical behavior of HEAs, including the mechanisms behind the plastic
deformation of HEAs at both low and high temperatures. Furthermore, the present
work examined the strengthening strategies of HEAs, e.g. strain hardening,
grain-boundary strengthening, solid-solution strengthening, and particle
strengthening. The fatigue, creep, and fracture properties were briefly
introduced. Lastly, the future scientific issues and challenges of HEAs were
discussed. | cond-mat_mtrl-sci |
Topological Nature of the Phonon Hall Effect: We provide a topological understanding on phonon Hall effect in dielectrics
with Raman spinphonon coupling. A general expression for phonon Hall
conductivity is obtained in terms of the Berry curvature of band structures. We
find a nonmonotonic behavior of phonon Hall conductivity as a function of
magnetic field. Moreover, we observe a phase transition in phonon Hall effect,
which corresponds to the sudden change of band topology, characterized by the
altering of integer Chern numbers. This can be explained by touching and
splitting of phonon bands. | cond-mat_mtrl-sci |
Optical properties of Mn4+ ions in GaN:Mn codoped with Mg acceptors: The optical properties of Mn-Mg codoped epitaxial GaN were studied. Addition
of Mg acceptors quenches the weak manganese-related photoluminescence (PL) band
at 1.3 eV in GaN:Mn and a series of sharp PL peaks are observed at 1 eV in
codoped epilayers. The change in PL spectra indicates that Mg addition
stabilizes the Mn4+ charge state by decreasing the Fermi level. The 1 eV PL
peaks are tentatively attributed to intra center transitions involving Mn4+
ions. Spin allowed 3d-shell 4T2-4T1 transitions and their phonon replicas are
involved. The relative intensities of the sharp peaks are strongly dependent on
the excitation wavelength, indicating the optically active Mn4+ centers
involved in the separate peaks are different. The temperature dependence of the
PL spectrum suggests the presence of at least three distinct Mn4+ complex
centers. | cond-mat_mtrl-sci |
Atoms, dimers, and nanoparticles from orbital-free density-potential
functional theory: Density-potential functional theory (DPFT) is an alternative formulation of
orbital-free density functional theory that may be suitable for modeling the
electronic structure of large systems. To date, DPFT has been applied mainly to
quantum gases in one- and two dimensional settings. In this work, we study the
performance of DPFT when applied to real-life systems: atoms, dimers, and
nanoparticles. We build on systematic Suzuki-Trotter factorizations of the
quantum-mechanical propagator and on the Wigner function formalism,
respectively, to derive nonlocal as well as semilocal functional approximations
in complete analogy to their well-established lower-dimensional versions --
without resorting to system-specific approximations or ad-hoc measures of any
kind. The cost for computing the associated semiclassical ground-state
single-particle density scales (quasi-)linearly with particle number. We
illustrate that the developed density formulae become relatively more accurate
for larger particle numbers, can be improved systematically, are quite
universally applicable, and, hence, may offer alternatives to existing
orbital-free methods for mesoscopic quantum systems. | cond-mat_mtrl-sci |
Initial stages of the graphite-SiC(0001) interface formation studied by
photoelectron spectroscopy: Graphitization of the 6H-SiC(0001) surface as a function of annealing
temperature has been studied by ARPES, high resolution XPS, and LEED. For the
initial stage of graphitization - the 6root3 reconstructed surface - we observe
sigma-bands characteristic of graphitic sp2-bonded carbon. The pi-bands are
modified by the interaction with the substrate. C1s core level spectra indicate
that this layer consists of two inequivalent types of carbon atoms. The next
layer of graphite (graphene) formed on top of the 6root3 surface at
TA=1250-1300 degree C has an unperturbed electronic structure. The annealing at
higher temperatures results in the formation of a multilayer graphite film. It
is shown that the atomic arrangement of the interface between graphite and the
SiC(0001) surface is practically identical to that of the 6root3 reconstructed
layer. | cond-mat_mtrl-sci |
Dirac energy spectrum and inverted band gap in metamorphic InAsSb/InSb
superlattices: A Dirac-type energy spectrum was demonstrated in gapless ultra-short-period
metamorphic InAsSb/InSb superlattices by angle-resolved photoemission
spectroscopy (ARPES_ measurements. The Fermi velocity value 7.4x10^5 m/s in a
gapless superlattice with a period of 6.2nm is in a good agreement with the
results of magneto-absorption experiments. An "inverted" bandgap opens in the
center of the Brillouin zone at higher temperatures and in the SL with a larger
period. The ARPES data indicate the presence of a surface electron accumulation
layer | cond-mat_mtrl-sci |
Photoemission study of the skutterudite compounds
Co(Sb$_{1-x}$Te$_{x}$)$_3$ and RhSb$_3$: We have studied the electronic structure of the skutterudite compounds
Co(Sb$_{1-x}$Te$_{x}$)$_3$ (x= 0, 0.02, 0.04) by photoemission spectroscopy.
Valence-band spectra revealed that Sb 5p states are dominant near the Fermi
level and are hybridized with Co 3d states just below it. The spectra of {\it
p}-type CoSb$_3$ are well reproduced by the band-structure calculation, which
suggests that the effect of electron correlations is not strong in CoSb$_3$.
When Te is substituted for Sb and n-type carriers are doped into CoSb$_3$, the
spectra are shifted to higher binding energies as predicted by the rigid-band
model. From this shift and the free-electron model for the conduction and
valence bands, we have estimated the band gap of CoSb$_3$ to be 0.03-0.04 eV,
which is consistent with the result of transport measurements. Photoemission
spectra of RhSb$_3$ have also been measured and revealed similarities to and
differences from those of CoSb$_3$. | cond-mat_mtrl-sci |
Pressure induced crossing of the core-levels in 5d metals: Pressure induced interaction between core electrons, the core level crossing
(CLC) transition has been observed in hcp Os at P~400 GPa [L. Dubrovinksy, et
al., Nature 525, 226-229 (2015)]. In this work, we carry out a systematic study
of the influence of pressure on the electronic structure in all metals of the
5d series (Hf,Ta,W,Re,Os,Ir,Pt,Au) using first-principles electronic structure
calculations. We have found that CLC is a general effect for this series of
metals. While in Pt it occurs at ~1500 GPa, at a pressure substantially higher
than in Os, in Ir it occurs already at 80 GPa. Moreover, we predict that in Re
the CLC transition may appear at ambient pressure. We analyze the shifts of the
CLC transition pressure across the series within the Thomas-Fermi model, and
show that the effect has many common features to the atomic collapse in the
rare-earth elements. | cond-mat_mtrl-sci |
Ultrafast terahertz field control of the emergent magnetic and
electronic interactions at oxide interfaces: Ultrafast electric-field control of emergent electronic and magnetic states
at oxide interfaces offers exciting prospects for the development of new
generations of energy-efficient devices. Here, we demonstrate that the
electronic structure and emergent ferromagnetic interfacial state in epitaxial
LaNiO3/CaMnO3 superlattices can be effectively controlled using intense
single-cycle THz electric-field pulses. We employ a combination of
polarization-dependent X-ray absorption spectroscopy with magnetic circular
dichroism and X-ray resonant magnetic reflectivity to measure a detailed
magneto-optical profile and thickness of the ferromagnetic interfacial layer.
Then, we use time-resolved and temperature-dependent magneto-optical Kerr
effect, along with transient optical reflectivity and transmissivity
measurements, to disentangle multiple correlated electronic and magnetic
processes driven by ultrafast high-field (~1 MV/cm) THz pulses. These processes
include an initial sub-picosecond electronic response, consistent with
non-equilibrium Joule heating; a rapid (~270 fs) demagnetization of the
ferromagnetic interfacial layer, driven by THz-field-induced nonequilibrium
spin-polarized currents; and subsequent multi-picosecond dynamics, possibly
indicative of a change in the magnetic state of the superlattice due to the
transfer of spin angular momentum to the lattice. Our findings shed light on
the intricate interplay of electronic and magnetic phenomena in this strongly
correlated material system, suggesting a promising avenue for efficient control
of two-dimensional ferromagnetic states at oxide interfaces using ultrafast
electric-field pulses. | cond-mat_mtrl-sci |
High-performance Computation of Kubo Formula with Vectorization of
Batched Linear Algebra Operation: We have proposed a method to accelerate the computation of Kubo formula
optimized to vector processors. The key concept is parallel evaluation of
multiple integration points, enabled by batched linear algebra operations.
Through benchmark comparisons between the vector-based NEC SX-Aurora TSUBASA
and the scalar-based Xeon machines in node performance, we verified that the
vectorized implementation was speeded up to approximately 2.2 times faster than
the baseline. We have also shown that the performance improvement due to
padding, indicating that avoiding the memory-bank conflict is critically
important in this type of task. | cond-mat_mtrl-sci |
Experimental demonstration of a generalized Fourier's Law for
non-diffusive thermal transport: Phonon heat conduction over length scales comparable to their mean free paths
is a topic of considerable interest for basic science and thermal management
technologies. Although the failure of Fourier's law beyond the diffusive regime
is well understood, debate exists over the proper physical description of
thermal transport in the ballistic to diffusive crossover. Here, we derive a
generalized Fourier's law that links the heat flux and temperature fields,
valid from ballistic to diffusive regimes and for general geometries, using the
Peierls-Boltzmann transport equation within the relaxation time approximation.
This generalized Fourier's law predicts that thermal conductivity not only
becomes nonlocal at length scales smaller than phonon mean free paths, but also
requires the inclusion of an inhomogeneous nonlocal source term that has been
previously neglected. We demonstrate the validity of this generalized Fourier's
law through direct comparison with time-domain thermoreflectance (TDTR)
measurements in the nondiffusive regime without adjustable parameters.
Furthermore, we show that interpreting experimental data without this
generalized Fourier's law leads to inaccurate measurement of thermal transport
properties. | cond-mat_mtrl-sci |
High-temperature oxidation and nitridation of substoichiometric
zirconium carbide in isothermal air: The influence of nitrogen on the oxidation behavior of hot-pressed zirconium
carbide was investigated using a flow-tube furnace at temperatures ranging from
1000 to 1600 {\deg}C. Mass gain, oxide formation characteristics, and oxide
transitions were evaluated at various experimental conditions. Differences in
oxidation behavior across the range of temperatures investigated show both
kinetic and microstructural dependence with implications pointing to this
materials efficacy in ultra-high temperature applications. Results suggest that
at temperatures above 1400 {\deg}C, although oxidation mechanisms remain
dominant, nitridation and reduction mechanisms may be appreciable enough to
require consideration. Supporting discussions regarding polymorphism and
microstructural influences are outlined. | cond-mat_mtrl-sci |
Electric field-controlled reversible order-disorder switching of a metal
tip surface: While it is well established that elevated temperatures can induce surface
roughening of metal surfaces, the effect of a high electric field on the atomic
structure at ambient temperature has not been investigated in detail. Here we
show with atomic resolution using in situ transmission electron microscopy how
intense electric fields induce reversible switching between perfect crystalline
and disordered phases of gold surfaces at room temperature. Ab initio molecular
dynamics simulations reveal that the mechanism behind the structural change can
be attributed to a vanishing energy cost in forming surface defects in high
electric fields. Our results demonstrate how surface processes can be directly
controlled at the atomic scale by an externally applied electric field, which
promotes an effective decoupling of the topmost surface layers from the
underlying bulk. This opens up opportunities for development of active
nanodevices in e.g. nanophotonics and field-effect transistor technology as
well as fundamental research in materials characterization and of yet
unexplored dynamically-controlled low-dimensional phases of matter. | cond-mat_mtrl-sci |
Efficient thermo-spin conversion in van der Waals ferromagnet FeGaTe: Recent discovery of 2D van der Waals (vdW) magnetic materials has spurred
progress in developing advanced spintronic devices. A central challenge lies in
enhancing the spin-conversion efficiency for building spin-logic or spin-memory
devices. We systematically investigated the anomalous Hall effect and anomalous
Nernst effect in above-room-temperature van der Waals ferromagnet FeGaTe with
perpendicular anisotropy, uncovering significant spin-conversion effects. The
anomalous Hall effect demonstrated an efficient electric spin-charge
conversion, with a notable spin Hall angle of 6 $\%$ - 10.38 $\%$. The
temperature-dependent behavior of the anomalous Nernst voltage primarily
results from the thermo-spin conversion. Uniquely, we have experimentally
achieved thermo-spin polarization values of over 690 $\%$ at room temperature
and extremely large of 4690 $\%$ at about 93 K. This study illuminates the
potential of vdW ferromagnets in advancing efficient spin conversion devices. | cond-mat_mtrl-sci |
Nanostructured Immunosensors. Application to the detection of
Progesterone: A novel nanostructured electrochemical immunsensor for the determination of
progesterone is reported. The approach combines the properties of gold
nanoparticles with the use of a graphite-Teflon composite electrode matrix,
into which gold nanoparticles are incorporated by simple physical inclusion.
The antibody anti-progesterone was directly attached to the electrode surface.
The immunosensor functioning is based on competitive assay between progesterone
and alkaline phosphatase-labelled progesterone. Monitoring of the affinity
reaction was accomplished by the electrochemical oxidation of 1-naphtol.
Modification of the graphite -Teflon electrode matrix with gold nanoparticles
improves substantially the electrooxidation response of 1-naphtol. Using a
detection potential of +0.3V, a detection limit for progesterone of 0.84 ng
ml-1 was obtained. Analysis of seven milk samples spiked at a 3.5 ng ml-1
progesterone concentration level yielded a mean recovery of 101+6%. Detection
of the antigen-antibody reaction with a graphite - Teflon - colloidal - gold -
Tyrosinase electrode, using phenylphosphate as alkaline phosphatase substrate
to generate phenol, which is subsequently reduced at -0.1 V at the composite
electrode, produced a high improvement in the sensitivity for progesterone
detection | cond-mat_mtrl-sci |
Pitfalls and solutions for perovskite transparent conductors: Transparent conductors-nearly an oxymoron-are in pressing demand, as
ultra-thin-film technologies become ubiquitous commodities. As current
solutions rely on non-abundant elements, perovskites such as SrVO3 and SrNbO3
have been suggested as next generation transparent conductors. Our ab-initio
calculations and analytical insights show, however, that reducing the plasma
frequency below the visible spectrum by strong electronic correlations-a
recently proposed strategy-unavoidably comes at a price: an enhanced scattering
and thus a substantial optical absorption above the plasma edge. As a way out
of this dilemma we identify several perovskite transparent conductors, relying
on hole doping, somewhat larger bandwidths and separations to other bands. | cond-mat_mtrl-sci |
An Accurate and Transferable Machine-Learning Interatomic Potential for
Silicon: The development of modern ab initio methods has rapidly increased our
understanding of physics, chemistry and materials science. Unfortunately,
intensive ab initio calculations are intractable for large and complex systems.
On the other hand, empirical force fields are less accurate with poor
transferability even though they are efficient to handle large and complex
systems. The recent development of machine-learning based neural-network (NN)
for local atomic environment representation of density functional theory (DFT)
has offered a promising solution to this long-standing challenge. Si is one of
the most important elements in science and technology, however, an accurate and
transferable interatomic potential for Si is still lacking. Here, we develop a
generalized NN potential for Si, which correctly predicts the Si(111)-(7x7)
ground-state surface reconstruction for the first time and accurately
reproduces the DFT results in a wide range of complex Si structures. We
envision similar developments will be made for a wide range of materials
systems in the near future. | cond-mat_mtrl-sci |
Three-dimensional coordinates of individual atoms in materials revealed
by electron tomography: Crystallography, the primary method for determining the three-dimensional
(3D) atomic positions in crystals, has been fundamental to the development of
many fields of science. However, the atomic positions obtained from
crystallography represent a global average of many unit cells in a crystal.
Here, we report, for the first time, the determination of the 3D coordinates of
thousands of individual atoms and a point defect in a material by electron
tomography with a precision of ~19 picometers, where the crystallinity of the
material is not assumed. From the coordinates of these individual atoms, we
measure the atomic displacement field and the full strain tensor with a 3D
resolution of ~1nm^3 and a precision of ~10^-3, which are further verified by
density functional theory calculations and molecular dynamics simulations. The
ability to precisely localize the 3D coordinates of individual atoms in
materials without assuming crystallinity is expected to find important
applications in materials science, nanoscience, physics and chemistry. | cond-mat_mtrl-sci |
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