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Reaction rate approach to dipolar relaxation in alkali halides:
Adiabaticity versus classical, activated-tunneling, and quantal dipoles: This paper is aimed at presenting a simple vibronic model for describing the
dipolar reorientation in crystals by means of reaction rate theory. The
Hamiltonian of an isolated dipole is simplified so as to render the problem
solvable. Depending on the crossover splitting the dipoles may reorientate
adiabatically with a high electron-transfer expectancy or exhibit low
reorientation rates due to low expectancy. An important quantity to distinguish
between adiabatic dipoles behaving classically and ones reorientating by means
of quantum-mechanical tunneling is Christov's characteristic temperature which
is found to relate to the barrier height and crossover splitting. ITC data on
impurity-vacancy dipoles in Eu-doped alkali halides are reanalyzed. | cond-mat_mtrl-sci |
Prediction of three-fold fermions in a nearly-ideal Dirac semimetal
BaAgAs: Materials with triply-degenerate nodal points in their low-energy electronic
spectrum produce crystalline-symmetry-enforced three-fold fermions, which
conceptually lie between the two-fold Weyl and four-fold Dirac fermions. Here
we show how a silver-based Dirac semimetal BaAgAs realizes three-fold fermions
through our first-principles calculations combined with a low-energy effective
$\mathbf{k.p}$ model Hamiltonian analysis. BaAgAs is shown to harbor
triply-degenerate nodal points, which lie on its $C_{3}$ rotation axis, and are
protected by the $C_{6v}$($C_2\otimes C_{3v}$) point-group symmetry in the
absence of spin-orbit coupling (SOC) effects. When the SOC is turned on, BaAgAs
transitions into a nearly-ideal Dirac semimetal state with a pair of Dirac
nodes lying on the $C_{3}$ rotation axis. We show that breaking inversion
symmetry in the BaAgAs$_{1-x}$P$_x$ alloy yields a clean and tunable three-fold
fermion semimetal. Systematic relaxation of other symmetries in BaAgAs
generates a series of other topological phases. BaAgAs materials thus provide
an ideal platform for exploring tunable topological properties associated with
a variety of different fermionic excitations. | cond-mat_mtrl-sci |
Low-field microwave-free sensors using dipolar spin relaxation of
quartet spin states in silicon carbide: Paramagnetic defects and nuclear spins are the major sources of magnetic
field-dependent spin relaxation in point defect quantum bits. The detection of
related optical signals has led to the development of advanced relaxometry
applications with high spatial resolution. The nearly degenerate quartet ground
state of the silicon vacancy qubit in silicon carbide (SiC) is of special
interest in this respect, as it gives rise to relaxation rate extrema at
vanishing magnetic field values and emits in the first near-infra-red
transmission window of biological tissues, providing an opportunity for
developing novel sensing applications for medicine and biology. However, the
relaxation dynamics of the silicon vacancy center in SiC have not yet been
fully explored. In this paper, we present results from a comprehensive
theoretical investigation of the dipolar spin relaxation of the quartet spin
states in various local spin environments. We discuss the underlying physics
and quantify the magnetic field and spin bath dependent relaxation time $T_1$.
Using these findings we demonstrate that the silicon vacancy qubit in SiC can
implement microwave-free low magnetic field quantum sensors of great potential. | cond-mat_mtrl-sci |
Field-free spin-orbit torque switching through domain wall motion: Deterministic current-induced spin-orbit torque (SOT) switching of
magnetization in a heavy transition metal/ferromagnetic metal/oxide magnetic
heterostructure with the ferromagnetic layer being perpendicularly-magnetized
typically requires an externally-applied in-plane field to break the switching
symmetry. We show that by inserting an in-plane magnetized ferromagnetic layer
CoFeB underneath the conventional W/CoFeB/MgO SOT heterostructure,
deterministic SOT switching of the perpendicularly-magnetized top CoFeB layer
can be realized without the need of in-plane bias field. Kerr imaging study
further unveils that the observed switching is mainly dominated by domain
nucleation and domain wall motion, which might limit the potentiality of using
this type of multilayer stack design for nanoscale SOT-MRAM application.
Comparison of the experimental switching behavior with micromagnetic
simulations reveals that the deterministic switching in our devices cannot be
explained by the stray field contribution of the in-plane magnetized layer, and
the roughness-caused N\'eel coupling effect might play a more important role in
achieving the observed field-free deterministic switching. | cond-mat_mtrl-sci |
Tuning valleys and wave functions of van der Waals heterostructures by
varying the number of layers: A first-principles study: In van der Waals heterostructures of two-dimensional transition-metal
dichalcogenides (2D TMDCs) electron and hole states are spatially localized in
different layers forming long-lived interlayer excitons. Here, we have
investigated, from first principles, the influence of additional electron or
hole layers on the electronic properties of a MoS2/WSe2 heterobilayer (HBL),
which is a direct band gap material. Additional layers modify the interlayer
hybridization, mostly affecting the quasiparticle energy and real-space extend
of hole states at the G and electron states at the Q valleys. For a sufficient
number of additional layers, the band edges move from K to Q or G,
respectively. Adding electron layers to the HBL leads to more delocalized Q
states, while G states do not extend much beyond the HBL, even when more hole
layers are added. These results suggest a simple and yet powerful way to tune
band edges and the real-space extend of the electron and hole wave function in
TMDC heterostructures, strongly affecting the lifetime and dynamics of
interlayer excitons. | cond-mat_mtrl-sci |
Computational analysis of short-range interactions between an edge
dislocation and an array of equally-spaced identical shearable or
non-shearable precipitates: The interaction between dislocations and precipitates plays an important role
in the mechanical behavior of alloys. To provide more insight into the physics
of this interaction, this research analyzes short-range interactions of an edge
dislocation with an array of equally-spaced identical precipitates. We use a
modified dislocation dynamics approach accounting for penetrable and
impenetrable precipitates. This research quantifies the effects of precipitate
resistance on the geometry of the dislocation-precipitation interaction and the
local distribution of plastic strain near a precipitate. The results show that
a precipitate with a higher resistance causes an increase in the maximum value
of dislocation curvature during the bypass. In addition, a higher level of
precipitate resistance leads to a lower level of plastic deformation. Moreover,
we observed a high plastic strain gradient at the interface of non-shearable
precipitates. | cond-mat_mtrl-sci |
Cooperatively Modulating Magnetic Anisotropy and Colossal
Magnetoresistance via Atomic-Scale Buffer Layers in Highly Strained
La0.7Sr0.3MnO3 Films: Simultaneous control of magnetic anisotropy and magnetoresistance, especially
with atomic scale precision, remains a pivotal challenge for realizing advanced
spintronic functionalities. Here we demonstrate cooperative continuous control
over both magnetoresistance and magnetic anisotropy in highly strained
La0.7Sr0.3MnO3 (LSMO) thin films. By inserting varying perovskite buffer
layers, compressively strained LSMO films transition from a ferromagnetic
insulator with out-of-plane magnetic anisotropy to a metallic state with
in-plane anisotropy. Atomic-scale buffer layer insertion enables remarkably
acute, precise control to sharply modulate this magnetic phase transformation.
A gigantic 10,000% modulation of the colossal magnetoresistance (CMR) and an
exceptionally sharp transition from out-of-plane to in-plane magnetic
anisotropy are attained in just a few contiguous layers. These atomic-scale
correlations among electronic, magnetic, and structural order parameters yield
flexible multifunctional control promising for next-generation oxide
spintronics. | cond-mat_mtrl-sci |
Impact of Cr doping on the structure, optical and magnetic properties of
nanocrystalline ZnO particles: The role of Cr incorporation into the ZnO were probed through investigations
into the structural, optical and magnetic properties. Zn1-xCrxO with x = 0,
0.01, 0.03 and 0.05, nanoparticles were prepared by solution combustion method.
Powder x-ray diffraction (XRD) results reveal, all the synthesized samples are
in single hexagonal wurtzite crystal structures, indicating that Cr3+ ions
substitute the Zn2+ ions without altering the structure. The crystallite size
and microstrain were calculated using the Willamson-Hall method and found to be
36 +- 2 nm for ZnO and it reduced with the increase of Cr dopant concentration
to 20 +- 2 nm for Zn0.95Cr0.05O. Transmission electron microscopy (TEM)
revealed that the particle size were 48 +- 2 nm, 29 +- 2 nm and 25 +- 2 nm for
the Zn1-xCrxO with x = 0, 0.03 and 0.05, respectively. TEM morphology indicated
particles are agglomerated in the doped samples. The band-gap decreases
slightly from 3.305 +- 0.003 eV to 3.292 +- 0.003 eV with increase of Cr
content from x = 0 to 0.05, respectively. Photoluminescence measurements
revealed the presence of defects in the samples, associated with zinc vacancies
and singly ionized oxygen vacancy. The field-dependent magnetization
measurements of ZnO and Cr-doped ZnO were carried out using a vibrating sample
magnetometer (VSM) at 300 K. All the samples exhibits ferromagnetic behavior.
This long-range ferromagnetism ordering observed in ZnO is explained based on
bound magnetic polaron (BMP) mechanism. The singly ionized oxygen vacancies
playing a crucial role in observed room temperature ferromagnetism (RTFM) in
ZnO. There is a sufficient amount of BMPs formed in Cr doped ZnO because of the
defects present in these samples. Therefore, the overlapping of BMPs results in
the RTFM. However, the antiferromagnetic coupling at a higher doping
concentration of Cr, weakens the observed RTFM. | cond-mat_mtrl-sci |
Room temperature ferromagnetic-like behavior in Mn-implanted and
post-annealed InAs layers deposited by Molecular Beam Epitaxy: We report on the magnetic and structural properties of Ar and Mn implanted
InAs epitaxial films grown on GaAs (100) by Molecular Beam Epitaxy (MBE) and
the effect of Rapid Thermal Annealing (RTA) for 30 seconds at 750C. Channeling
Particle Induced X- ray Emission (PIXE) experiments reveal that after Mn
implantation almost all Mn atoms are subsbtitutional in the In-site of the InAs
lattice, like in a diluted magnetic semiconductor (DMS). All of these samples
show diamagnetic behavior. But, after RTA treatment the Mn-InAs films exhibit
room-temperature magnetism. According to PIXE measurements the Mn atoms are no
longer substitutional. When the same set of experiments were performed with As
as implantation ion all of the layers present diamagnetism without exception.
This indicates that the appearance of room-temperature ferromagnetic-like
behavior in the Mn-InAs-RTA layer is not related to lattice disorder produce
during implantation, but to a Mn reaction produced after a short thermal
treatment. X-ray diffraction patterns (XRD) and Rutherford Back Scattering
(RBS) measurements evidence the segregation of an oxygen deficient-MnO2 phase
(nominally MnO1.94) in the Mn-InAs-RTA epitaxial layers which might be on the
origin of room temperature ferromagnetic-like response observed. | cond-mat_mtrl-sci |
Deposition and photoluminescence of zinc gallium oxide thin films with
varied stoichiometry made by reactive magnetron co-sputtering: This paper reports on the deposition and photoluminescence of amorphous and
crystalline thin films of zinc gallium oxide with Ga:Zn atomic ratio varied
between 0.3 and 5.7. The films are prepared by reactive direct current
magnetron co-sputtering from liquid/solid gallium/zinc targets onto fused
quartz substrates; the temperature of the substrate is varied from room
temperature (RT) to 800{\deg}C. The sputtering process is effectively
controlled by fixing the sputtering power of one of the targets and controlling
the power of the other target by plasma optical emission spectroscopy. The
method, in conjunction with oxygen flow adjustment, enables the production of
near-stoichiometric films at any temperature used. The composition analysis
suggests a few at.% oxygen deficiency in the films. The resulting deposition
rate is at least an order of magnitude higher compared to the commonly used
radio-frequency sputtering from a ceramic ZnO:Ga2O3 target. Deposited onto
unheated substrates, the films with Ga:Zn {\approx} 2 are X-ray amorphous.
Well-defined X-ray diffraction peaks of spinel ZnGa2O4 start to appear at a
substrate temperature of 300{\deg}C. The surface of the as-deposited films is
dense and exhibits a fine-featured structure observed in electron microscopy
images. Increasing the deposition temperature from RT to 800{\deg}C eliminates
defects and improves crystallinity, which for the films with Ga:Zn ratio close
to 2 results in an increase in the optical band gap from 4.6 eV to 5.1 eV. Room
temperature photoluminescence established the main peak at 3.1 eV (400 nm); a
similar peak in Ga2O3 is ascribed to oxygen-vacancy related transitions. A
prominent feature around 2.9 eV (428 nm) is attributed to self-activation
center of the octahedral Ga-O groups in the spinel lattice of ZnGa2O4. It was
found that photoluminescence from ZnGa2O4 depends significantly on the ratio
Ga:Zn. | cond-mat_mtrl-sci |
Inelastic electron tunneling spectroscopy of local "spin accumulation"
devices: We investigate the origin of purported "spin accumulation" signals observed
in local "three-terminal" (3T) measurements of ferromagnet/insulator/n-Si
tunnel junctions using inelastic electron tunneling spectroscopy (IETS).
Voltage bias and magnetic field dependences of the IET spectra were found to
account for the dominant contribution to 3T magnetoresistance signals, thus
indicating that it arises from inelastic tunneling through impurities and
defects at junction interfaces and within the barrier, rather than from spin
accumulation due to pure elastic tunneling into bulk Si as has been previously
assumed. | cond-mat_mtrl-sci |
Magnetic and structural properties of Co2FeAl thin films grown on Si
substrate: The correlation between magnetic and structural properties of Co_{2} FeAl
(CFA) thin films of different thickness (10 nm<d< 100 nm) grown at room
temperature on MgO-buffered Si/SiO2 substrates and annealed at
600\lyxmathsym{\textdegree}C has been studied. XRD measurements revealed an
(011) out-of-plane texture growth of the films. The deduced lattice parameter
increases with the film thickness. Moreover, pole figures showed no in-plane
preferential growth orientation. The magneto-optical Kerr effect hysteresis
loops showed the presence of a weak in-plane uniaxial anisotropy with a random
easy axis direction. The coercive field measured with an applied field along
the easy axis direction and the uniaxial anisotropy field increase linearly
with the inverse of the CFA thickness. The microstrip line ferromagnetic
resonance measurements for in-plane and perpendicular applied magnetic fields
revealed that the effective magnetization and the uniaxial in-palne anisotropy
field follow a linear variation versus the inverse CFA thickness. This allows
deriving a perpendicular surface anisotropy coefficient of -1.86 erg/cm2 | cond-mat_mtrl-sci |
Experimentally informed structure optimization of amorphous TiO2 films
grown by atomic layer deposition: Amorphous titanium dioxide TiO2 (a-TiO2) has been widely studied,
particularly as a protective coating layer on semiconductors to prevent
corrosion and promote electron-hole conduction in photoelectrochemical
reactions. The stability and longevity of a-TiO2 is strongly affected by the
thickness and structural heterogeneity, implying that understanding the
structure properties of a-TiO2 is crucial for improving the performance. This
study characterized the structural and electronic properties of a-TiO2 thin
films (~17nm) grown on Si by Atomic Layer Deposition (ALD). Fluctuation spectra
V(k) and angular correlation functions were determined with 4-dimensional
scanning transmission electron microscopy (4D-STEM), which revealed the
distinctive medium-range ordering in the a-TiO2 film. A realistic atomic model
of a-TiO2 was established guided by the medium-range ordering and the
previously reported short-range ordering of a-TiO2 film, as well as the
interatomic potential. The structure was optimized by the StructOpt code using
a genetic algorithm that simultaneously minimizes energy and maximizes match to
experimental short- and medium-range ordering. The StructOpt a-TiO2 model
presents an improved agreements with the medium-range ordering and the k-space
location of the dominant 2-fold angular correlations compared with a
traditional melt-quenched model. The electronic structure of the StructOpt
a-TiO2 model was studied by ab initio calculation and compared to the
crystalline phases and experimental results. This work uncovered the
medium-range ordering in a-TiO2 thin film and provided a realistic a-TiO2
structure model for further investigation of structure-property relationships
and materials design. In addition, the improved multi-objective optimization
package StructOpt was provided for structure determination of complex materials
guided by experiments and simulations. | cond-mat_mtrl-sci |
Simulating dark-field x-ray microscopy images with wave front
propagation techniques: Dark-Field X-ray Microscopy (DFXM) is a diffraction-based synchrotron imaging
techique capable of imaging defects in the bulk of extended crystalline
samples. We present numerical simulations of image-formation in such a
microscope using numerical integration of the dynamical Takagi-Taupin Equations
(TTE) and wave front propagation. We validate our approach by comparing
simulated images to experimental data from a near-perfect single crystal of
diamond containing a single stacking fault defect in the illuminated volume. | cond-mat_mtrl-sci |
Identification and tunable optical coherent control of transition-metal
spins in silicon carbide: Color centers in wide-bandgap semiconductors are attractive systems for
quantum technologies since they can combine long-coherent electronic spin and
bright optical properties. Several suitable centers have been identified, most
famously the nitrogen-vacancy defect in diamond. However, integration in
communication technology is hindered by the fact that their optical transitions
lie outside telecom wavelength bands. Several transition-metal impurities in
silicon carbide do emit at and near telecom wavelengths, but knowledge about
their spin and optical properties is incomplete. We present all-optical
identification and coherent control of molybdenum-impurity spins in silicon
carbide with transitions at near-infrared wavelengths. Our results identify
spin $S=1/2$ for both the electronic ground and excited state, with highly
anisotropic spin properties that we apply for implementing optical control of
ground-state spin coherence. Our results show optical lifetimes of $\sim$60 ns
and inhomogeneous spin dephasing times of $\sim$0.3 $\mu$s, establishing
relevance for quantum spin-photon interfacing. | cond-mat_mtrl-sci |
Resonant Raman of OH/OD vibrations and photoluminescence studies in
LiTaO3 thin film: Resonant Raman spectra (RRS) of O-H and O-D vibration and libration modes,
their combinations and higher harmonics have been observed in LiTaO3
polycrystalline thin films. RRS peaks are superimposed on photoluminescence
(PL) spectrum. Monochromatic light from a xenon lamp is used as excitation
source. PL spectrum shows two broad peaks, first near the band gap in UV
(4.4-4.8eV) and another in the sub band gap region (< 4.0 eV). Band gap PL
along with RRS peaks are reported for the first time. Photoluminescence
excitation spectrum (PLE) shows a peak at 4.8 eV. Peak positions and full width
at half maximum (FWHM) of RRS peaks depend upon the excitation energy.
Dispersions of the fundamental and the third harmonic of the stretching mode of
O-H with excitation energy are about 800 cm-1/eV and 2000 cm-1/eV respectively.
This dispersion is much higher than reported in any other material. | cond-mat_mtrl-sci |
Parabolic Metamaterials and Dirac Bridges: A new class of multi-scale structures, referred to as `parabolic
metamaterials' is introduced and studied in this paper. For an elastic
two-dimensional triangular lattice, we identify dynamic regimes, which
corresponds to so-called `Dirac Bridges' on the dispersion surfaces. Such
regimes lead to a highly localised and focussed unidirectional beam when the
lattice is excited. We also show that the flexural rigidities of elastic
ligaments are essential in establishing the `parabolic metamaterial' regimes. | cond-mat_mtrl-sci |
Sub 20 nm Silicon Patterning and Metal Lift-Off Using Thermal Scanning
Probe Lithography: The most direct definition of a patterning process' resolution is the
smallest half-pitch feature it is capable of transferring onto the substrate.
Here we demonstrate that thermal Scanning Probe Lithography (t-SPL) is capable
of fabricating dense line patterns in silicon and metal lift-off features at
sub 20 nm feature size. The dense silicon lines were written at a half pitch of
18.3 nm to a depth of 5 nm into a 9 nm polyphthalaldehyde thermal imaging layer
by t-SPL. For processing we used a three-layer stack comprising an evaporated
SiO2 hardmask which is just 2-3 nm thick. The hardmask is used to amplify the
pattern into a 50 nm thick polymeric transfer layer. The transfer layer
subsequently serves as an etch mask for transfer into silicon to a nominal
depth of 60 nm. The line edge roughness (3 sigma) was evaluated to be less than
3 nm both in the transfer layer and in silicon. We also demonstrate that a
similar three-layer stack can be used for metal lift-off of high resolution
patterns. A device application is demonstrated by fabricating 50 nm half pitch
dense nickel contacts to an InAs nanowire. | cond-mat_mtrl-sci |
Large magnetoresistance in intermetallic compounds R2Mn3Si5 (R = Tb, Dy
and Ho): Magnetization (M) and magnetoresistance (MR) measurements on polycrystalline
R2Mn3Si5 (R = Tb, Dy and Ho) compounds (tetragonal, space group P4/mnc) have
been carried out in the temperature range of 2 K-300 K, in various applied
fields. Both, the rare earth and the Mn, are found to carry magnetic moments in
these compounds. Mn has two sub-lattices (Mn1 and Mn2) that order magnetically
at two different temperatures. Rare earth and Mn1 moments order
ferromagnetically at TC1 whereas Mn2 is found to magnetically order at TC2 (TC1
= 89 K, 86 K, 78 K and TC2 = 18 K, 34 K, 16 K for R = Tb, Dy and Ho compounds,
respectively). Magnetoresistance measurements reveal large negative MR values
of about 50 % near TC2 at 9 T in all these compounds. This giant
magnetoresistance is attributed to the spin-dependent scattering effects,
competing exchange interactions and the layered structure of these compounds | cond-mat_mtrl-sci |
Grapheayne: a class of low-energy carbon allotropes with diverse
optoelectronic and topological properties: A series of carbon allotropes with novel optoelectronic and rich topological
properties is predicted by systematic first-principles calculations. These
fascinating carbon allotropes can be derived by inserting acetylenic linkages
(-C$\equiv$C-) into graphite, hence they are termed as grapheaynes. Grapheaynes
possess two different space groups, $P$2/$m$ or $C$2/$m$, and contain
simultaneously the $sp$, $sp^2$, and $sp^3$ chemical bonds. They have formation
energies lower than the already experimentally synthesized graphdiyne and other
theoretically predicted carbon allotropes with acetylenic linkages.
Particularly, when the width $n$ of grapheayne-$n$ exceeds 15, its cohesive
energy is lower than that of diamond, and approaches that of graphite with
increasing $n$. Remarkably, we find that some grapheaynes behave as
semiconductors with direct narrow band gaps and own the highest absorption
coefficients among all known semiconducting carbon allotropes, while some
others are topological semimetals with nodal lines. Especially, some
grapheaynes can be engineered with tunable direct band gaps in the range of
1.07-1.87 eV and have ideal properties for photovoltaic applications. Our work
not only uncovers the unique atomic arrangement and prominent properties of the
grapheayne family, but also offers a treasury that provides promising materials
for catalyst, energy storage, molecular sieves, solar cell, and electronic
devices. | cond-mat_mtrl-sci |
Growth of $α-Ga_2O_3$ on $Al_2O_3$ by conventional molecular-beam
epitaxy and metal-oxide-catalyzed epitaxy: We report the growth of $\alpha-Ga_2O_3$ on $m$-plane $Al_2O_3$ by
conventional plasma-assisted molecular-beam epitaxy (MBE) and In-mediated
metal-oxide-catalyzed epitaxy (MOCATAXY). We report a growth-rate-diagram for
$\alpha-Ga_2O_3$ (10-10), and observe (i) a growth rate increase, (ii) an
expanded growth window, and (iii) reduced out-of-lane mosaic spread when
MOCATAXY is employed for the growth of $\alpha-Ga_2O_3$. Through the use of
In-mediated catalysis, growth rates over $0.2\,\mu\text{m}\,\text{hr}^{-1}$ and
rocking curves with full width at half maxima of $\Delta\omega \approx
0.45^{\circ}$ are achieved. Faceting is observed along the $\alpha-Ga_2O_3$
film surface and is explored through scanning transmission electron microscopy. | cond-mat_mtrl-sci |
Towards colloidal spintronics through Rashba spin-orbit interaction in
lead sulphide nanosheets: Employing the spin degree of freedom of charge carriers offers the
possibility to extend the functionality of conventional electronic devices,
while colloidal chemistry can be used to synthesize inexpensive and tuneable
nanomaterials. In order to benefit from both concepts, Rashba spin-orbit
interaction has been investigated in colloidal lead sulphide nanosheets by
electrical measurements on the circular photo-galvanic effect. Lead sulphide
nanosheets possess rock salt crystal structure, which is centrosymmetric. The
symmetry can be broken by quantum confinement, asymmetric vertical interfaces
and a gate electric field leading to Rashba-type band splitting in momentum
space at the M points, which results in an unconventional selection mechanism
for the excitation of the carriers. The effect, which is supported by
simulations of the band structure using density functional theory, can be tuned
by the gate electric field and by the thickness of the sheets. Spin-related
electrical transport phenomena in colloidal materials open a promising pathway
towards future inexpensive spintronic devices. | cond-mat_mtrl-sci |
Metal-Ferroelectric-Metal heterostructures with Schottky contacts I.
Influence of the ferroelectric properties: A model for Metal-Ferroelectric-Metal structures with Schottky contacts is
proposed. The model adapts the general theories of metal-semiconductor
rectifying contacts for the particular case of metal-ferroelectric contact by
introducing: the ferroelectric polarization as a sheet of surface charge
located at a finite distance from the electrode interface; a deep trapping
level of high concentration; the static and dynamic values of the dielectric
constant. Consequences of the proposed model on relevant quantities of the
Schottky contact such as built-in voltage, charge density and depletion width,
as well as on the interpretation of the current-voltage and capacitance-voltage
characteristics are discussed in detail. | cond-mat_mtrl-sci |
Two-dimensional ferromagnetic semiconductors of rare-earth Janus
2H-GdIBr monolayer with large valley polarization: Based on a rare-earth Gd atom with 4$f$ electrons, through first-principles
calculations, we demonstrate that the Janus 2H-GdIBr monolayer exhibits an
intrinsic ferromagnetic (FM) semiconductor character with an indirect band gap
of 0.75 eV, high Curie temperature T$_{c}$ of 260 K, significant magnetic
moment of 8 $\mu_{B}$/f.u. (f.u.=formula unit), in-plane magnetic anisotropy
(IMA) and large spontaneous valley polarization of 118 meV. The MAE,
inter-atomic distance or angle, and T$_{c}$ can be efficiently modulated by
in-plane strains and charge carrier doping. Under the strain range from $-$5%
to 5% and charge carrier doping from $-$0.3e to 0.3e/f.u., the system still
remains FM ordering and the corresponding T$_{c}$ can be modulated by strains
from 233 K to 281 K and by charge carrier doping from 140 K to 245 K.
Interestingly, under various strains, the matrix elements differences
($d_{z^{2}}$, $d_{yz}$), ($d_{x^{2}-y^{2}}$, $d_{xy}$) and ($p_{x}$, $p_{y}$)
of Gd atoms dominate the MAE behaviors, which originates from the competition
between the contributions of Gd-$d$, Gd-$p$ orbitals, and $p$ orbitals of
halogen atoms based on the second-order perturbation theory. Inequivalent Dirac
valleys are not energetic degenerate due to the time-reversal symmetry breaking
in the Janus 2H-GdIBr monolayer. A considerable valley gap between the Berry
curvature at the K and K$^{\prime}$ points provides an opportunity to
selectively control the valley freedom and to manipulate the anomalous Hall
effect. External tensile (compressive) strain further increases (decreases) the
valley gap up to a maximum (minimum) value of 158 (37) meV, indicating that the
valley polarization in the Janus 2H-GdIBr monolayer is robust to the external
strains. | cond-mat_mtrl-sci |
Landau modeling of dynamical nucleation of martensite at grain
boundaries under local stress: The dynamical nucleation of martensite in polycrystals is simulated by means
of Lagrange-Rayleigh dynamics with Landau energetics, which is capable of
obtaining the local stress as a result of the interplay of the potential of
transformation and external loadings. By monitoring the patio-temporal
distribution of the strain in response to the local stress, we demonstrate that
the postcursors, high angle grain boundaries and triple junctions act as
favorable heterogeneous nucleation sites corresponding to different loading and
cooling conditions, and predict the phase diagram of the nucleation mode of
martensite. | cond-mat_mtrl-sci |
Substrate-induced structures of bismuth adsorption on graphene: a first
principle study: The geometric and electronic properties of Bi-adsorbed monolayer graphene,
enriched by the strong effect of substrate, are investigated by
first-principles calculations. The six-layered substrate, corrugated buffer
layer, and slightly deformed monolayer graphene are all simulated. Adatom
arrangements are thoroughly studied by analyzing the ground-state energies,
bismuth adsorption energies, and Bi-Bi interaction energies of different adatom
heights, inter-adatom distance, adsorption sites, and hexagonal positions. A
hexagonal array of Bi atoms is dominated by the interactions between the buffer
layer and the monolayer graphene. An increase in temperature can overcome a
$\sim 50$ meV energy barrier and induce triangular and rectangular
nanoclusters. The most stable and metastable structures agree with the scanning
tunneling microscopy measurements. The density of states exhibits a finite
value at the Fermi level, a dip at $\sim -0.2$ eV, and a peak at $\sim -0.6$
eV, as observed in the experimental measurements of the tunneling conductance. | cond-mat_mtrl-sci |
Topological skyrmions in monolayer multiferroic MoPtGe2S6: Two-dimensional (2D) multiferroic materials with coexisting ferroelectricity
and ferromagnetism have garnered substantial attention for their intriguing
physical properties and diverse promising applications in spintronics. For
example, multiferroic materials with electronically controlled broken central
symmetry provide a versatile platform for designing and manipulating
topological skyrmions and diverse spintronic applications. Here, we investigate
the complex magnetic properties of room-temerature multiferroic material
MoPtGe2S6 and its electrical control of topological skyrmions using
first-principles calculations and atomistic micromagnetic simulations. A
sizable Dzyaloshinskii-Moriya interaction (DMI) (2.1 meV) is found in the
multiferroic material MoPtGe2S6 with an electrically polarized ground state.
The magnetic skyrmions can be stabilized in monolayer MoPtGe2S6 under zero
magnetic field, and the chirality of skyrmions can be reversed with electric
field-induced flipping of electrical polarization due to the reversed chirality
of the DMI. Furthermore, an external magnetic fielc can reverse the
magnetization direction and topological charge of the skyrmions as well as tune
the size of skyrmions. These results demonstrate that the monolayer MoPtGe2S6
can enrich the 2D skyrmion community and pave the way for electronically
controlled spintronic devices. | cond-mat_mtrl-sci |
Magnetocaloric effect and its implementation in critical behavior study
of Mn4FeGe3-xSix intermetallic compounds: Magnetocaloric effect in Mn4FeGe3-xSix compounds has been studied by dc
magnetization measurements. For the parent compound Mn4FeGe3, the paramagnetic
to ferromagnetic transition temperature TC is above room temperature (320 K),
which initially remains constant for small Si substitution at the Ge site and
then decreases marginally with an increase in Si concentration. A large change
in magnetic entropy at the TC, under a magnetic field variation of 50 kOe, with
typical values of 5.9, 6.5, 5.9 and 4.4 J kg-1 K-1 for x = 0, 0.2, 0.6, and 1
samples, respectively, along with a broad operating temperature range and a
negligible hysteresis make Mn4FeGe3-xSix series a promising candidate for
magnetic refrigerant material around room temperature. Mn4FeGe3-xSix series is
found to undergo a second-order magnetic phase transition. The field dependence
of the magnetic entropy change has been brought out and implemented it to
deduce the critical exponents. The critical behavior study shows that the
magnetic interactions for x = 0 and 0.2 samples have two different behaviors
below and above TC. Below TC, it follows the mean field theory with long-range
magnetic interaction and above TC it follows the Heisenberg three-dimensional
model with short-range or local magnetic interaction. The magnetic exchange
interactions for the x = 0.6 and 1 samples follow the mean-field theory. | cond-mat_mtrl-sci |
Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the
Presence of Defects, Strain, and Charged Impurities: Few- and single-layer MoS2 host substantial densities of defects. They are
thought to influence the doping level, the crystal structure, and the binding
of electron-hole pairs. We disentangle the concomitant spectroscopic expression
of all three effects and identify to what extent they are intrinsic to the
material or extrinsic to it, i.e., related to its local environment. We do so
by using different sources of MoS2 -- a natural one and one prepared at high
pressure and high temperature -- and different substrates bringing varying
amounts of charged impurities and by separating the contributions of internal
strain and doping in Raman spectra. Photoluminescence unveils various optically
active excitonic complexes. We discover a defect-bound state having a low
binding energy of 20 meV that does not appear sensitive to strain and doping,
unlike charged excitons. Conversely, the defect does not significantly dope or
strain MoS2. Scanning tunneling microscopy and density functional theory
simulations point to substitutional atoms, presumably individual nitrogen atoms
at the sulfur site. Our work shows the way to a systematic understanding of the
effect of external and internal fields on the optical properties of
two-dimensional materials. | cond-mat_mtrl-sci |
In-plane anisotropic optical and mechanical properties of
two-dimensional MoO$_3$: Molybdenum trioxide (MoO$_3$) in-plane anisotropy has increasingly attracted
the attention of the scientific community in the last few years. Many of the
observed in-plane anisotropic properties stem from the anisotropic refractive
index and elastic constants of the material but a comprehensive analysis of
these fundamental properties is still lacking. Here we employ Raman and
micro-reflectance measurements, using polarized light, to determine the angular
dependence of the refractive index of thin MoO$_3$ flakes and we study the
directional dependence of the MoO$_3$ Young's modulus using the buckling
metrology method. We found that MoO$_3$ displays one of the largest in-plane
anisotropic mechanical properties reported for 2D materials so far. | cond-mat_mtrl-sci |
Experimental evidence of above-threshold photoemission in solids: Nonlinear photoemission from a silver single crystal is investigated by
femtosecond laser pulses in a perturbative regime. A clear observation of
above-threshold photoemission in solids is reported for the first time. The
ratio between the three-photon above-threshold and the two-photon Fermi edges
is found to be 10^{-4}. This value constitutes the only available benchmark for
theories aimed at understanding the mechanism responsible for above-threshold
photoemission in solids. | cond-mat_mtrl-sci |
$sp^{2}$/$sp^{3}$ carbon ratio in graphite oxide with different
preparation times: Graphite oxide is an amorphous insulator. Although several models have been
suggested, its structure remains controversial. To elucidate this issue, 5
samples were prepared by the Brodie process and the Staudenmaier process. The
electronic structure of graphite oxide was examined with x-ray absorption near
edge structure and the ratio of $sp^{2}$ to $sp^{3}$ bonded carbon atoms was
investigated with x-ray photoemission spectroscopy as a function of sample
preparation times. It was found that this ratio approaches 0.3 exponentially
with a characteristic time of 1.5 weeks. We believe this long characteristic
time is the reason the structure has remained unclear. | cond-mat_mtrl-sci |
Spline-based neural network interatomic potentials: blending classical
and machine learning models: While machine learning (ML) interatomic potentials (IPs) are able to achieve
accuracies nearing the level of noise inherent in the first-principles data to
which they are trained, it remains to be shown if their increased complexities
are strictly necessary for constructing high-quality IPs. In this work, we
introduce a new MLIP framework which blends the simplicity of spline-based MEAM
(s-MEAM) potentials with the flexibility of a neural network (NN) architecture.
The proposed framework, which we call the spline-based neural network potential
(s-NNP), is a simplified version of the traditional NNP that can be used to
describe complex datasets in a computationally efficient manner. We demonstrate
how this framework can be used to probe the boundary between classical and ML
IPs, highlighting the benefits of key architectural changes. Furthermore, we
show that using spline filters for encoding atomic environments results in a
readily interpreted embedding layer which can be coupled with modifications to
the NN to incorporate expected physical behaviors and improve overall
interpretability. Finally, we test the flexibility of the spline filters,
observing that they can be shared across multiple chemical systems in order to
provide a convenient reference point from which to begin performing
cross-system analyses. | cond-mat_mtrl-sci |
Intermediate range order in (Fe,Al) silicate network glasses: a neutron
diffraction and EPSR modeling investigation: The local structural environment and the spatial distribution of iron and
aluminum ions in sodosilicate glasses with composition NaFexAl1-xSi2O6 (x = 1,
0.8, 0.5 and 0) is studied by high-resolution neutron diffraction combined with
structural modeling using the Empirical Potential Structure Refinement (EPSR)
code. This work gives evidence of differences in the structural behavior of
Al3+ and Fe3+, which are both often considered to act as network formers in
charge-balanced compositions. The short-range environment and the structural
role of the two cations are not composition dependent, and hence the structure
of intermediate glasses can then be seen as a mixture of the structures of the
two end-members. All Al3+ is 4-coordinated for a distance
d[4]Al3+-O=1.76$\pm$0.01{\AA}. The high-resolution neutron data allows
deciphering between two populations of Fe. The majority of Fe3+ is
4-coordinated (d[4]Fe3+-O=1.87$\pm$0.01{\AA}) while the remaining Fe3+ and all
Fe2+ (~12% of total Fe) are 5-coordinated (d[5]Fe-O=2.01$\pm$0.01{\AA}). Both
AlO4 and FeO4 are randomly distributed and connected with the silicate network
in which they share corners with SiO4 tetrahedra, in agreement with a
network-forming role of those species. On the contrary FeO5 tends to form
clusters and to share edges with each other. 5-coordinated Fe is interpreted as
network modifier and it turns out that, even if this coordination number is
rare in crystals, it is more common in glasses in which they can have a key
role on physical properties. | cond-mat_mtrl-sci |
Current induced spin injection and surface torque in ferromagnetic
metallic junctions: Joint influence of two effects, namely, nonequilibrium spin injection by
current, and current induced surface torque, on spin-valve type ferromagnetic
metallic junctions is considered theoretically. The CPP configuration is
assumed. The consideration is based on solving a coupled set of equations of
motion for the mobile electron and lattice magnetizations. Boundary conditions
are derived from the total magnetization flux continuity condition. A
dispersion relation is derived for current dependent spin-wave fluctuations.
The fluctuations become unstable under current density exceeding some threshold
value. Joint action of the longitudinal spin injection and the torque lowers
the instability threshold. The spin injection softens spin wave frequency near
the threshold and can pin magnetization at the injecting contact. The pinning
rises under the current increasing, so that the appearance of new spin-wave
resonance lines can be observed. | cond-mat_mtrl-sci |
Structural, magnetic and transport properties of Co$_2$CrAl epitaxial
thin films: We report the physical properties of Co$_2$CrAl Heusler alloy epitaxial thin
films grown on single crystalline MgO(001) substrate using pulsed laser
deposition technique. The x-ray diffraction pattern in $\theta$-2$\theta$ mode
showed the film growth in single phase B2-type ordered cubic structure with the
presence of (002) and (004) peaks, and the film oriented along the MgO(001)
direction. The $\phi$~scan along the (220) plane confirms the four-fold
symmetry and the epitaxial growth relation found to be
Co$_2$CrAl(001)[100]$\vert$$\vert$MgO(001)[110]. The thickness of about 12~nm
is extracted through the analysis of x-ray reflectivity data. The isothermal
magnetization (M--H) curves confirm the ferromagnetic (FM) nature of the thin
film having significant hysteresis at 5 and 300~K. From the in-plane M--H
curves, the saturation magnetization values are determined to be
2.1~$\mu$$_{\rm B}$/f.u.~at 5~K and 1.6~$\mu$$_{\rm B}$/f.u. at 300~K, which
suggests the soft FM behavior in the film having the coercive field $\approx$
522~Oe at 5~K. The thermo-magnetization measurements at 500~Oe magnetic field
show the bifurcation between field-cooled and zero-field-cooled curves below
about 100~K. The normalized field-cooled magnetization curve follows the T$^2$
dependency, and the analysis reveal the Curie temperature around 335$\pm$11~K.
Moreover, the low-temperature resistivity indicates semiconducting behavior
with the temperature, and we find a negative temperature coefficient of
resistivity (5.2 $\times$ 10$^{-4}$ /K). | cond-mat_mtrl-sci |
Defect-induced states, defect-induced phase transition and excitonic
states in bent transition metal dichalcogenide (TMD) nanoribbons: density
functional vs. many body theory: Two-dimensional (2D) transition metal dichalcogenide (TMD) materials have
versatile electronic and optical properties. TMD nanoribbons show interesting
properties due to reduced dimensionality, quantum confinement, and edge states.
Tang et al. showed that the edge bands evolved with bending can tune the
optical properties for various widths of TMD nanoribbons. Defects are commonly
present in 2D TMD materials, and can dramatically change the material
properties. In this following work, we investigate the interaction between the
edge and the defect states in WS2 nanoribbons with line defects under different
bending conditions, using density functional theory (DFT). We reveal
interesting semiconducting-to-metallic phase transitions, suggesting potential
applications in nano-electronics or molecular electronics. We also calculate
the optical absorption of the nanoribbons with different defect positions with
the many-body GW-BSE (Bethe-Salpeter equation) approach, revealing a tunable
optical spectrum and diverse exciton states in the defected TMD nanoribbons. | cond-mat_mtrl-sci |
Model for domain wall avalanches in ferromagnetic thin films: The Barkhausen jumps or avalanches in magnetic domain-walls motion between
succesive pinned configurations, due the competition among magnetic external
driving force and substrum quenched disorder, appear in bulk materials and thin
films. We introduce a model based in rules for the domain wall evolution of
ferromagnetic media with exchange or short-range interactions, that include
disorder and driving force effects. We simulate in 2-dimensions with Monte
Carlo dynamics, calculate numerically distributions of sizes and durations of
the jumps and find power-law critical behavior. The avalanche-size exponent is
in excellent agreement with experimental results for thin films and is close to
predictions of the other models, such as like random-field and random-bond
disorder, or functional renormalization group. The model allows us to review
current issues in the study of avalanches motion of the magnetic domain walls
in thin films with ferromagnetic interactions and opens a new approach to
describe these materials with dipolar or long-range interactions. | cond-mat_mtrl-sci |
Gate dependent Raman spectroscopy of graphene on hexagonal boron nitride: Raman spectroscopy, a fast and nondestructive imaging method, can be used to
monitor the doping level in graphene devices. We fabricated chemical vapor
deposition (CVD) grown graphene on atomically flat hexagonal boron nitride
(hBN) flakes and SiO$_2$ substrates. We compared their Raman response as a
function of charge carrier density using an ion gel as a top gate. The G peak
position, 2D peak position, 2D peak width and the ratio of the 2D peak area to
the G peak area show a dependence on carrier density that differs for hBN
compared to SiO$_2$. Histograms of two-dimensional mapping are used to compare
the fluctuations in the Raman peak properties between the two substrates. The
hBN substrate has been found to produce fewer fluctuations at the same charge
density owing to its atomically flat surface and reduced charged impurities. | cond-mat_mtrl-sci |
The Removal of Single Layers from Multi-Layer Graphene by Low Energy
Electron Stimulation: The removal of single atomic layers from multi-layer graphene using a He
plasma is reported. By applying sample biases of -60 and +60 V during He plasma
exposure, layer removal is found to be due to electrons instead of He ions or
neutrals in the plasma. The rate of layer removal depends on exposure time,
sample bias and pre-annealing treatments. Optical contrast microscopy and
atomic force microscopy studies show that the removal of C atoms occurs
approximately one layer at a time across the entire multi-layer sample with no
observable production of large pits or reduction in lateral dimensions. Layer
removal is proposed to arise from the electron-stimulated dissociation of C
atoms from the basal plane. This process differs from plasma techniques that
use reactive species to etch multi-layer graphene. | cond-mat_mtrl-sci |
Mn-rich MnSb2Te4: A topological insulator with magnetic gap closing at
high Curie temperatures of 45-50 K: Ferromagnetic topological insulators exhibit the quantum anomalous Hall
effect that might be used for high precision metrology and edge channel
spintronics. In conjunction with superconductors, they could host chiral
Majorana zero modes which are among the contenders for the realization of
topological qubits. Recently, it was discovered that the stable 2+ state of Mn
enables the formation of intrinsic magnetic topological insulators with A1B2C4
stoichiometry. However, the first representative, MnBi2Te4, is
antiferromagnetic with 25 K N\'eel temperature and strongly n-doped. Here, we
show that p-type MnSb2Te4, previously considered topologically trivial, is a
ferromagnetic topological insulator in the case of a few percent of Mn excess.
It shows (i) a ferromagnetic hysteresis with record high Curie temperature of
45-50 K, (ii) out-of-plane magnetic anisotropy and (iii) a two-dimensional
Dirac cone with the Dirac point close to the Fermi level which features (iv)
out-of-plane spin polarization as revealed by photoelectron spectroscopy and
(v) a magnetically induced band gap that closes at the Curie temperature as
demonstrated by scanning tunneling spectroscopy. Moreover, it displays (vi) a
critical exponent of magnetization beta~1, indicating the vicinity of a quantum
critical point. Ab initio band structure calculations reveal that the slight
excess of Mn that substitutionally replaces Sb atoms provides the ferromagnetic
interlayer coupling. Remaining deviations from the ferromagnetic order, likely
related to this substitution, open the inverted bulk band gap and render
MnSb2Te4 a robust topological insulator and new benchmark for magnetic
topological insulators. | cond-mat_mtrl-sci |
Reconstructing random heterogeneous media through differentiable
optimization: Microstructure reconstruction is a key enabler of process-structure-property
linkages, a central topic in materials engineering. Revisiting classical
optimization-based reconstruction techniques,they are recognized as a powerful
framework to reconstruct random heterogeneous media, especially due to their
generality and controllability. The stochasticity of the available approaches
is, however, identified as a performance bottleneck. In this work,
reconstruction is approached as a differentiable optimization problem, where
the error of a generic prescribed descriptor is minimized under consideration
of its derivative. As an exemplary descriptor, a suitable differentiable
version of spatial correlations is formulated, along with a multigrid scheme to
ensure scalability. The applicability of differentiable optimization realized
through this descriptor is demonstrated using a wide variety of heterogeneous
media, achieving exact statistical equivalence with errors as low as 0 in a
short time. We conclude that, while still in an early stage of development,
this approach has the potential to significantly alleviate the computational
effort currently associated with reconstructing general random heterogeneous
media. | cond-mat_mtrl-sci |
Gapless surface Dirac cone in antiferromagnetic topological insulator
MnBi$_2$Te$_4$: The recent discovered antiferromagnetic topological insulators in Mn-Bi-Te
family with intrinsic magnetic ordering have rapidly drawn broad interest since
its cleaved surface state is believed to be gapped, hosting the unprecedented
axion states with half-integer quantum Hall effect. Here, however, we show
unambiguously by using high-resolution angle-resolved photoemission
spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi$_2$Te$_4$
exists between the bulk band gap. Such unexpected surface state remains
unchanged across the bulk N\'eel temperature, and is even robust against severe
surface degradation, indicating additional topological protection. Through
symmetry analysis and $\textit{ab}$-$\textit{initio}$ calculations we consider
different types of surface reconstruction of the magnetic moments as possible
origins giving rise to such linear dispersion. Our results reveal that the
intrinsic magnetic topological insulator hosts a rich platform to realize
various topological phases such as topological crystalline insulator and
time-reversal-preserved topological insulator, by tuning the magnetic
configurations. | cond-mat_mtrl-sci |
Rational design principles for giant spin Hall effect in 5d-transition
metal oxides: Spin Hall effect (SHE), a mechanism by which materials convert a
\textit{charge} current into a \textit{spin} current, invokes interesting
physics and promises to empower transformative, energy-efficient memory
technology. However, fundamental questions remain about the essential factors
that determine SHE. Here we solve this open problem, presenting a comprehensive
theory of five \textit{foundational factors} that control the value of
intrinsic SHE in transition metal oxides. Arising from our key insight
regarding the inherently geometric nature of SHE, we demonstrate that two of
these factors are crystal field strength and structural distortions. Moreover,
we discover that a new class of materials (anti-perovskites) promises to
demonstrate \textit{giant} SHE, that is an order of magnitude larger than that
reported for any oxide. We derive three other factors that control SHE and
demonstrate the nuanced role of electron correlations. Our findings bring
deeper insight into the physics driving SHE, and could help enhance, as well
as, externally control SHE values. | cond-mat_mtrl-sci |
Structural and electronic phase evolution of Tin dioxide: We investigate the effect of controlled annealing on the structural and
electronic phase evolution of Tin dioxide from Tin (II) oxyhydroxide prepared
by simple precipitation method. Thermogravimetric analysis suggests a complex
weight loss-gain process involved, passing through an intermediate phase of tin
oxide nanoparticles. The probable structural and electronic phase evolution is
discussed using detailed X-ray diffraction and X-ray photoelectron spectroscopy
investigations. | cond-mat_mtrl-sci |
Improving empirical interatomic potentials for predicting thermophysical
properties by using an irreducible derivatives approach: The case of thorium
dioxide: The accuracy of physical property predictions using classical molecular
dynamics simulations is determined by the quality of the empirical interatomic
potentials (EIPs). We introduce a training approach for EIPs, based on direct
comparisons of the second- and third-order interatomic force constants (IFCs)
between EIP and density functional theory (DFT) calculations. This work's
unique aspect is the utilization of irreducible derivatives (IDs) of the total
energy, which leverage on the symmetry of the crystalline structure and provide
a minimal representation of the IFCs. Our approach is tailored toward accurate
predictions of thermal conductivity, thus requiring knowledge of both harmonic
and anharmonic IFCs by matching second- and third-order displacement
derivatives, whereas second-order strain derivatives are needed for determining
the elastic constants. We demonstrate this approach as an efficient and robust
manner in which to train EIPs for predicting phonons and related properties, by
optimizing parameters of an embedded-atom method potential for ThO$_2$, which
is used as a model system for fluorite oxides. Our ID-trained EIP provides
thermophysical properties in great agreement with DFT, and outperforms
previously widely utilized EIP for ThO$_2$ in phonon dispersion and thermal
conductivity calculations. It also provides reasonable estimates of thermal
expansion and the formation energies of simple defects. | cond-mat_mtrl-sci |
Phonons in Twisted Transition Metal Dichalcogenide Bilayers
("Twistnonics"): Ultra-soft Phasons, and a transition from Superlubric to
Pinned Phase: The tunability of the interlayer coupling by twisting one layer with respect
to another layer of two-dimensional materials provides a unique way to
manipulate the phonons and related properties. We refer to this engineering of
phononic properties as "Twistnonics". We study the effects of twisting on
low-frequency shear (SM) and layer breathing (LBM) modes in transition metal
dichalcogenide (TMD) bilayer using atomistic classical simulations. We show
that these low-frequency modes are extremely sensitive to twist and can be used
to infer the twist angle. We find unique "ultra-soft" phason modes (frequency
$\lesssim 1\ \mathrm{cm^{-1}}$, comparable to acoustic modes) for any non-zero
twist, corresponding to an \textit{effective} translation of the moir{\'e}
lattice by relative displacement of the constituent layers in a non-trivial
way. Unlike the acoustic modes, the velocity of the phason modes is quite
sensitive to twist angle. As twist angle decreases, ($\theta \lesssim
3^{\circ},\ \gtrsim 57^{\circ}$) the ultra-soft modes represent the acoustic
modes of the "emergent" soft moir{\'e} scale lattice. Also, new high-frequency
SMs appear, identical to those in stable bilayer TMD ($\theta =
0\degree/60\degree$), due to the overwhelming growth of stable stacking regions
in relaxed twisted structures. Furthermore, we find remarkably different
structural relaxation as $\theta \to 0^{\circ}$, $\to 60^{\circ}$ due to
sub-lattice symmetry breaking. Our study reveals the possibility of an
intriguing $\theta$ dependent superlubric to pinning behavior and of the
existence of ultra-soft modes in \textit{all} two-dimensional (2D) materials. | cond-mat_mtrl-sci |
Magnetic and Electrical Properties of Ordered 112-type Perovskite
LnBaCoMnO5+δ(Ln = Nd, Eu): Investigation of the oxygen-deficient 112-type ordered oxides of the type
LnBaCoMnO5+\delta (Ln = Nd, Eu) evidences certain unusual magnetic behavior at
low temperatures, compared to the LnBaCo2O5+\delta cobaltites. One observes
that the substitution of manganese for cobalt suppresses the ferromagnetic
state and induces strong antiferromagnetic interactions. Importantly,
NdBaCoMnO5.9 depicts a clear paramagnetic to antiferromagnetic type transition
around 220 K, whereas for EuBaCoMnO5.7 one observes an unusual magnetic
behavior below 177 K which consists of ferromagnetic regions embedded in an
antiferromagnetic matrix. The existence of two sorts of crystallographic sites
for Co/Mn and their mixed valence states favor the ferromagnetic interaction
whereas antiferromagnetism originates from the Co3+-O-Co3+ and Mn4+-O-Mn4+
interactions. Unlike the parent compounds, the present Mn-substituted phases do
not exhibit prominent magnetoresistance effects in the temperature range
75-400K. | cond-mat_mtrl-sci |
Swift heavy ion irradiation of GaSb: from ion tracks to nano-porous
networks: Ion track formation, amorphisation, and the formation of porosity in
crystalline GaSb induced by 185 MeV $^{197}$Au swift heavy ion irradiation is
investigated as a function of fluence and irradiation angle relative to the
surface normal. RBS/C and SAXS reveal an ion track radius between 3 nm and 5
nm. The observed pore morphology and saturation swelling of GaSb films shows a
strong irradiation angle dependence. Raman spectroscopy and scanning electron
microscopy show that the ion tracks act as a source of strain in the material
leading to macroscopic plastic flow at high fluences and off normal
irradiation. The results are consistent with the ion hammering model for
glasses. Furthermore, wide angle X-ray scattering reveals the formation of nano
crystallites inside otherwise amorphous GaSb after the onset of porosity. | cond-mat_mtrl-sci |
Extrinsic n-type semiconductor transition in ZrSe2 with the metallic
character through hafnium substitution: Two dimensional layered materials exhibit versatile electronic properties in
their different phases. The intrinsic electronic properties of these materials
can be modulated through doping or intercalation. In this study, we
investigated the electronic properties of Hf doped ZrSe2 single crystals using
angle-resolved photoemission spectroscopy (ARPES) combined with first
principles density functional theory (DFT) calculations. It is observed that
the valence band maxima of ZrSe2, located below the Fermi level, undergo a
significant change with the introduction of Hf substitution. Hf can introduce
extra charges into the conduction band, rather than making a mixed structure of
HfSe2 and ZrSe2 band structure, which can cross the Fermi level. Compared to
the semiconducting band structure of ZrSe2, we observed that the conduction
band crosses the Fermi level at the high symmetry M point in Hf-doped ZrSe2.
This suggests an increase of electron type carriers around the Fermi level,
resulting in an extrinsic charge carrier density in the conduction band, which
can form a metallic behaviour. It can be noticed that the Hf cations can create
disorder in the form of excess atoms of Zr, which yields more carriers in the
conduction band in the shape of smeared bands. The tails of the smeared band
occupied the d-orbitals extended into the Fermi level and left the d band
below. Similarly, the electrical resistance measurements further confirm the
metallic-like character of Hf doped ZrSe2 compared to the semiconductor ZrSe2,
indicating increased carriers. This metallic like behavior is suggested to be
predisposed by the extrinsic electrons induced by the substitutional disorder.
This study further demonstrates the possibility of band gap engineering through
heavy metal doping in 2D materials. | cond-mat_mtrl-sci |
DFT Modelling of Explicit Solid-Solid Interfaces in Batteries: Methods
and Challenges: Density Functional Theory (DFT) calculations of electrode material properties
in high energy density storage devices like lithium batteries have been
standard practice for decades. In contrast, DFT modelling of explicit
interfaces in batteries arguably lacks universally adopted methodology and
needs further conceptual development. In this paper, we focus on solid-solid
interfaces, which are ubiquitous not just in all-solid state batteries;
liquid-electrolyte-based batteries often rely on thin, solid passivating films
on electrode surfaces to function. We use metal anode calculations to
illustrate that explicit interface models are critical for elucidating contact
potentials, electric fields at interfaces, and kinetic stability with respect
to parasitic reactions. The examples emphasize three key challenges: (1) the
"dirty" nature of most battery electrode surfaces; (2) voltage calibration and
control; and (3) the fact that interfacial structures are governed by kinetics,
not thermodynamics. To meet these challenges, developing new computational
techniques and importing insights from other electrochemical disciplines will
be beneficial. | cond-mat_mtrl-sci |
Proper usage of Scherrer's and Guinier's formulas in X-ray analysis of
size distribution in systems of monocrystalline CeO2 nanoparticles: Small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) techniques
are widely used as analytical tools in the optimization and control of
nanomaterial synthesis processes. In crystalline nanoparticle systems with size
distribution, the discrepant size values determined by using SAXS and XRD still
lacks a well-established description in quantitative terms. To address
fundamental questions, the isolated effect of size distribution is investigated
by SAXS and XRD simulation in polydisperse systems of virtual nanoparticles. It
quantitatively answered a few questions, among which the most accessible and
reliable size values and what they stand for regarding the size distribution
parameters. When a finite size distribution is introduced, the two techniques
produce differing results even in perfectly crystalline nanoparticles. Once
understood, the deviation in resulting size values can, in principle, resolve
two parameters size distributions of crystalline nanoparticles. To demonstrate
data analysis procedures in light of this understanding, XRD and SAXS
experiments were carried out on a series of powder samples of cubic ceria
nanoparticles. Besides changes in the size distribution related to the
synthesis parameters, proper comparison of XRD and SAXS results revealed
particle-particle interaction effects underneath the SAXS intensity curves. It
paves the way for accurate and reliable methodologies to assess size, size
dispersion, and degree of crystallinity in synthesized nanoparticles. | cond-mat_mtrl-sci |
Statistical equilibrium measures in micromagnetics: We derive an equilibrium statistical theory for the macroscopic description
of a ferromagnetic material at positive finite temperatures. Our formulation
describes the most-probable equilibrium macrostates that yield a coherent
deterministic large-scale picture varying at the size of the domain, as well as
it captures the effect of random spin fluctuations caused by the thermal noise.
We discuss connections of the proposed formulation to the Landau-Lifschitz
theory and to the studies of domain formation based on Monte Carlo lattice
simulations. | cond-mat_mtrl-sci |
Size-dependence of non-empirically tuned DFT starting points for
$G_0W_0$ applied to $π$-conjugated molecular chains: $G_0W_0$ calculations for predicting vertical ionization potentials (IPs) and
electron affinities of molecules and clusters are known to show a significant
dependence on the density functional theory (DFT) starting point. A number of
non-empirical procedures to find an optimal starting point have been proposed,
typically based on tuning the amount of HF exchange in the underlying hybrid
functional specifically for the system at hand. For the case of
$\pi$-conjugated molecular chains, these approaches lead to a significantly
different amount of HF exchange for different oligomer sizes. In this study, we
analyze if and how strongly this size dependence affects the ability of
non-empirical tuning approaches to predict accurate IPs for $\pi$-conjugated
molecular chains of increasing chain length. To this end, we employ three
different non-empirical tuning procedures for the $G_0W_0$ starting point to
calculate the IP of polyene oligomers up to 22 repeat units and compare the
results to highly accurate coupled-cluster calculations. We find that, despite
its size dependence, using an IP-tuned hybrid functional as a starting point
for $G_0W_0$ yields excellent agreement with the reference data for all chain
lengths. | cond-mat_mtrl-sci |
Ab Initio Linear and Pump-Probe Spectroscopy of Excitons in Molecular
Crystals: Linear and non-linear spectroscopies are powerful tools used to investigate
the energetics and dynamics of electronic excited states of both molecules and
crystals. While highly accurate \emph{ab initio} calculations of molecular
spectra can be performed relatively routinely, extending these calculations to
periodic systems is challenging. Here, we present calculations of the linear
absorption spectrum and pump-probe two-photon photoemission spectra of the
naphthalene crystal using equation-of-motion coupled-cluster theory with single
and double excitations (EOM-CCSD). Molecular acene crystals are of interest due
to the low-energy multi-exciton singlet states they exhibit, which have been
studied extensively as intermediates involved in singlet fission. Our linear
absorption spectrum is in good agreement with experiment, predicting a first
exciton absorption peak at 4.4 eV, and our two-photon photoemission spectra
capture the behavior of multi-exciton states, whose double-excitation character
cannot be captured by current methods. The simulated pump-probe spectra provide
support for existing interpretations of two-photon photoemission in
closely-related acene crystals such as tetracene and pentacene. | cond-mat_mtrl-sci |
Room Temperature Electrocaloric Effect in Layered Ferroelectric CuInP2S6
for Solid State Refrigeration: A material with reversible temperature change capability under an external
electric field, known as the electrocaloric effect (ECE), has long been
considered as a promising solid-state cooling solution. However, electrocaloric
(EC) performance of EC materials generally is not sufficiently high for real
cooling applications. As a result, exploring EC materials with high performance
is of great interest and importance. Here, we report on the ECE of
ferroelectric materials with van der Waals layered structure (CuInP2S6 or CIPS
in this work in particular). Over 60% polarization charge change is observed
within a temperature change of only 10 K at Curie temperature. Large adiabatic
temperature change (|{\Delta}T|) of 3.3 K, isothermal entropy change
(|{\Delta}S|) of 5.8 J kg-1 K-1 at |{\Delta}E|=142.0 kV cm-1 at 315 K (above
and near room temperature) are achieved, with a large EC strength
(|{\Delta}T|/|{\Delta}E|) of 29.5 mK cm kV-1. The ECE of CIPS is also
investigated theoretically by numerical simulation and a further EC performance
projection is provided. | cond-mat_mtrl-sci |
Metastable precursors during the oxidation of the Ru(0001) surface: Using density-functional theory, we predict that the oxidation of the
Ru(0001) surface proceeds via the accumulation of sub-surface oxygen in
two-dimensional islands between the first and second substrate layer. This
leads locally to a decoupling of an O-Ru-O trilayer from the underlying metal.
Continued oxidation results in the formation and stacking of more of these
trilayers, which unfold into the RuO_2(110) rutile structure once a critical
film thickness is exceeded. Along this oxidation pathway, we identify various
metastable configurations. These are found to be rather close in energy,
indicating a likely lively dynamics between them at elevated temperatures,
which will affect the surface chemical and mechanical properties of the
material. | cond-mat_mtrl-sci |
Ultrafast dynamic conductivity and scattering rate saturation of
photoexcited charge carriers in silicon investigated with a midinfrared
continuum probe: We employ ultra-broadband terahertz-midinfrared probe pulses to characterize
the optical response of photoinduced charge-carrier plasmas in high-resistivity
silicon in a reflection geometry, over a wide range of excitation densities
(10^{15}-10^{19} cm^{-3}) at room temperature. In contrast to conventional
terahertz spectroscopy studies, this enables one to directly cover the
frequency range encompassing the resultant plasma frequencies. The intensity
reflection spectra of the thermalized plasma, measured using sum-frequency
(up-conversion) detection of the probe pulses, can be modeled well by a
standard Drude model with a density-dependent momentum scattering time of
approx. 200 fs at low densities, reaching approx. 20 fs for densities of
approx. 10^{19} cm^{-3}, where the increase of the scattering rate saturates.
This behavior can be reproduced well with theoretical results based on the
generalized Drude approach for the electron-hole scattering rate, where the
saturation occurs due to phase-space restrictions as the plasma becomes
degenerate. We also study the initial sub-picosecond temporal development of
the Drude response, and discuss the observed rise in the scattering time in
terms of initial charge-carrier relaxation, as well as the optical response of
the photoexcited sample as predicted by finite-difference time-domain
simulations. | cond-mat_mtrl-sci |
Rashba-like spin splitting along three momentum directions in trigonal
layered PtBi2: Spin-orbit coupling (SOC) has gained much attention for its rich physical
phenomena and highly promising applications in spintronic devices. The
Rashba-type SOC in systems with inversion symmetry breaking is particularly
attractive for spintronics applications since it allows for flexible
manipulation of spin current by external electric fields. Here, we report the
discovery of a giant anisotropic Rashba-like spin splitting along three
momentum directions (3D Rashba-like spin splitting) with a helical spin
polarization around the M points in the Brillouin zone of trigonal layered
PtBi2. Due to its inversion asymmetry and reduced symmetry at the M point,
Rashba-type as well as Dresselhaus-type SOC cooperatively yield a 3D spin
splitting with alpha~ 4.36 eVA in PtBi2. The experimental realization of 3D
Rashba-like spin splitting not only has fundamental interests but also paves
the way to the future exploration of a new class of material with unprecedented
functionalities for spintronics applications. | cond-mat_mtrl-sci |
Chemical and nuclear catalysis driven by localized anharmonic vibrations: In many-body nonlinear systems with sufficient anharmonicity, a special kind
of lattice vibrations, namely, Localized Anharmonic Vibrations (LAV) can be
excited either thermally or by external triggering, in which the amplitude of
atomic oscillations greatly exceeds that of harmonic oscillations (phonons)
that determine the system temperature. Coherency and persistence of LAV may
have drastic effect on chemical and nuclear reaction rates due to time-periodic
modulation of reaction sites. One example is a strong acceleration of chemical
reaction rates driven by thermally-activated "jumps" over the reaction barrier
due to the time-periodic modulation of the barrier height in the LAV vicinity.
At sufficiently low temperatures, the reaction rate is controlled by quantum
tunneling through the barrier rather than by classical jumping over it. A giant
increase of sub-barrier transparency was demonstrated for a parabolic potential
well with the time-periodic eigenfrequency, when the modulation frequency
exceeds the eigenfrequency by a factor of ~2 (parametric regime). Such regime
can be realized for a hydrogen or deuterium atom in metal hydrides/deuterides,
such as NiH or PdD, in the vicinity of LAV. We present an analytical solution
of the Schrodinger equation for a nonstationary harmonic oscillator, analyze
the parametric regime in details and discuss its applications to the tunnel
effect and to D-D fusion in PdD lattice. We obtain simple analytical
expressions for the increase of amplitude and energy of zero-point oscillations
(ZPO) induced by the parametric modulation. Based on that, we demonstrate a
drastic increase of the D-D fusion rate with in-creasing number of modulation
periods evaluated in the framework of Schwinger model, which takes into account
suppression of the Coulomb barrier due to lattice vibrations. | cond-mat_mtrl-sci |
Metallic atomic wires on a patterned dihydrogeneted Si(001): Electronic structure calculations for atomic wire of metals like Al, Ga and
In are performed for a patterned dihydrogeneted Si(001):1 $\times$ 1 in search
of structures with metallic behavior. The dihydrogeneted Si(001) is patterned
by depassivating hygrozen atoms only from one row of Si atoms along the
[1$\bar{1}$0] direction. Various structures of adsorbed metals and their
electronic properties are examined. It is found that Al and Ga atomic wire
structures with metallic property are strongly unstable towards the formation
of buckled metal dimers leading to semiconducting behavior. Indium atomic wire,
however, displays only marginal preference towards the formation of symmetric
dimers staying close to the metallic limit. The reasons behind the lack of
metallic atomic wires are explored. In addition, a direction is proposed for
the realization of metallic wires on the dihydrogeneted Si(001). | cond-mat_mtrl-sci |
Enhanced thermoelectricity at the ultra-thin film limit: At the ultra-thin film limit, quantum confinement strongly improves
thermoelectric figure of merit in materials such as Sb$_2$Te$_3$ and
Bi$_2$Te$_3$. These high quality films have only been realized using well
controlled techniques such as molecular beam epitaxy. We report a two fold
increase in the Seebeck coefficient for both p-type Sb$_2$Te$_3$ and n-type
Bi$_2$Te$_3$ using thermal co-evaporation, an affordable approach. At the thick
film limit greater than 100 nm, their Seebeck coefficients are around 100 $\mu
V/K$, similar to results obtained in other work. When the films are thinner
than 50 nm, the Seebeck coefficient increases to about 500 $\mu V/K$. With a
total Seebeck coefficient $\sim$ 1 mV/K and an estimate ZT $\sim$ 2, this pair
of materials is the first step to a practical micro-cooler at room temperature. | cond-mat_mtrl-sci |
Noncontact friction: Role of phonon damping and its nonuniversality: While obtaining theoretical predictions for dissipation during sliding motion
is a difficult task, one regime that allows for analytical results is the
so-called noncontact regime, where a probe is weakly interacting with the
surface over which it moves. Studying this regime for a model crystal, we
extend previously obtained analytical results and confirm them quantitatively
via particle based computer simulations. Accessing the subtle regime of weak
coupling in simulations is possible via use of Green-Kubo relations. The
analysis allows to extract and compare the two paradigmatic mechanisms that
have been found to lead to dissipation: phonon radiation, prevailing even in a
purely elastic solid, and phonon damping, e.g., caused by viscous motion of
crystal atoms. While phonon radiation is dominant at large probe-surface
distances, phonon damping dominates at small distances. Phonon radiation is
furthermore a pairwise additive phenomenon so that the dissipation due to
interaction with different parts (areas) of the surface adds up. This additive
scaling results from a general one-to-one mapping between the mean
probe-surface force and the friction due to phonon radiation, irrespective of
the nature of the underlying pair interaction. In contrast, phonon damping is
strongly nonadditive, and no such general relation exists. We show that for
certain cases, the dissipation can even {\it decrease} with increasing surface
area the probe interacts with. The above properties, which are rooted in the
spatial correlations of surface fluctuations, are expected to have important
consequences when interpreting experimental measurements, as well as scaling
with system size. | cond-mat_mtrl-sci |
Enhanced Static Approximation to the Electron Self-Energy Operator for
Efficient Calculation of Quasiparticle Energies: An enhanced static approximation for the electron self energy operator is
proposed for efficient calculation of quasiparticle energies. Analysis of the
static COHSEX approximation originally proposed by Hedin shows that most of the
error derives from the short wavelength contributions of the assumed adiabatic
accumulation of the Coulomb-hole. A wavevector dependent correction factor can
be incorporated as the basis for a new static approximation. This factor can be
approximated by a single scaling function, determined from the homogeneous
electron gas model. The local field effect in real materials is captured by a
simple ansatz based on symmetry consideration. As inherited from the COHSEX
approximation, the new approximation presents a Hermitian self-energy operator
and the summation over empty states is eliminated from the evaluation of the
self energy operator. Tests were conducted comparing the new approximation to
GW calculations for diverse materials ranging from crystals and nanotubes. The
accuracy for the minimum gap is about 10% or better. Like in the COHSEX
approximation, the occupied bandwidth is overestimated. | cond-mat_mtrl-sci |
The universal emergence of self-affine roughness from deformation: Most natural and man-made surfaces appear to be rough on many length scales.
There is presently no unifying theory of the origin of roughness or the
self-affine nature of surface topography. One likely contributor to the
formation of roughness is deformation, which underlies many processes that
shape surfaces such as machining, fracture, and wear. Using molecular dynamics,
we simulate the bi-axial compression of single-crystal Au, the high-entropy
alloy Ni$_{36.67}$Co$_{30}$Fe$_{16.67}$Ti$_{16.67}$, and amorphous
Cu$_{50}$Zr$_{50}$, and show that even surfaces of homogeneous materials
develop a self-affine structure. By characterizing subsurface deformation, we
connect the self-affinity of the surface to the spatial correlation of
deformation events occurring within the bulk and present scaling relations for
the evolution of roughness with strain. | cond-mat_mtrl-sci |
Strain and order-parameter coupling in Ni-Mn-Ga Heusler alloys from
resonant ultrasound spectroscopy: Resonant ultrasound spectroscopy and magnetic susceptibility experiments have
been used to characterize strain coupling phenomena associated with structural
and magnetic properties of the shape-memory Heusler alloy series
Ni$_{50+x}$Mn$_{25-x}$Ga$_{25}$ ($x=0$, 2.5, 5.0, and 7.5). All samples exhibit
a martensitic transformation at temperature $T_M$ and ferromagnetic ordering at
temperature $T_C$, while the pure end member ($x=0$) also has a premartensitic
transition at $T_{PM}$, giving four different scenarios: $T_C>T_{PM}>T_M$,
$T_C>T_M$ without premartensitic transition, $T_C\approx T_M$, and $T_C<T_M$.
Fundamental differences in elastic properties i.e., stiffening versus
softening, are explained in terms of coupling of shear strains with three
discrete order parameters relating to magnetic ordering, a soft mode and the
electronic instability responsible for the large strains typical of martensitic
transitions. Linear-quadratic or biquadratic coupling between these order
parameters, either directly or indirectly via the common strains, is then used
to explain the stabilities of the different structures. Acoustic losses are
attributed to critical slowing down at the premartensite transition, to the
mobility of interphases between coexisting phases at the martensitic transition
and to mobility of some aspect of the twin walls under applied stress down to
the lowest temperatures at which measurements were made. | cond-mat_mtrl-sci |
Status and direction of atom probe analysis of frozen liquids: Imaging of liquids and cryogenic biological materials by electron microscopy
has been recently enabled by innovative approaches for specimen preparation and
the fast development of optimised instruments for cryo-enabled electron
microscopy (cryo-EM). Yet, Cryo-EM typically lacks advanced analytical
capabilities, in particular for light elements. With the development of
protocols for frozen wet specimen preparation, atom probe tomography (APT)
could advantageously complement insights gained by cryo-EM. Here, we report on
different approaches that have been recently proposed to enable the analysis of
relatively large volumes of frozen liquids from either a flat substrate or the
fractured surface of a wire. Both allowed for analysing water ice layers which
are several microns thick consisting of pure water, pure heavy-water and
aqueous solutions. We discuss the merits of both approaches, and prospects for
further developments in this area. Preliminary results raise numerous
questions, in part concerning the physics underpinning field evaporation. We
discuss these aspects and lay out some of the challenges regarding the APT
analysis of frozen liquids. | cond-mat_mtrl-sci |
$\textit{Ab Initio}$ Theory of the Impact from Grain Boundaries and
Substitutional Defects on Superconducting Nb$_3$Sn: Grain boundaries play a critical role in applications of superconducting
Nb$_3$Sn: in dc applications, grain boundaries preserve the material's
inherently high critical current density by pinning flux, while in ac
applications grain boundaries can provide weak points for flux entry and lead
to significant dissipation. We present the first $\textit{ab initio}$ study to
investigate the physics of different boundary types in Nb$_3$Sn using density
functional theory. We identify an energetically favorable selection of tilt and
twist grain boundaries of distinct orientations. We find that clean grain
boundaries free of point defects reduce the Fermi-level density of states by a
factor of two, an effect that decays back to the bulk electronic structure
$\sim1-1.5$ nm from the boundary. We further calculate the binding
free-energies of tin substitutional defects to multiple boundaries, finding a
strong electronic interaction that extends to a distance comparable to that of
the reduction of density of states. Associated with this interaction, we
discover a universal trend in defect electronic entropies near a boundary. We
probe the effects of defect segregation on grain boundary electronic structure
and calculate the impact of substitutional impurities on the Fermi-level
density of states in the vicinity of a grain boundary, finding that titanium
and tantalum have little impact regardless of placement, whereas tin, copper,
and niobium defects each have a significant impact but only on sites away from
the boundary core. Finally, we consider how all of these effects impact the
local superconducting transition temperature $T_\textrm{c}$ as a function of
distance from the boundary plane. | cond-mat_mtrl-sci |
Simulations of Magnetization Reversal in FM/AFM Bilayers With THz
Frequency Pulses: It is widely known that antiferromagnets (AFMs) display a high frequency
response in the terahertz (THz) range, which opens up the possibility for
ultrafast control of their magnetization for next generation data storage and
processing applications. However, because the magnetization of the different
sublattices cancel, their state is notoriously difficult to read. One way to
overcome this is to couple AFMs to ferromagnets - whose state is trivially read
via magneto-resistance sensors. Here we present conditions, using theoretical
modelling, that it is possible to switch the magnetization of an AFM/FM bilayer
using THz frequency pulses with moderate field amplitude and short durations,
achievable in experiments. Consistent switching is observed in the phase
diagrams for an order of magnitude increase in the interface coupling and a
tripling in the thickness of the FM layer. We demonstrate a range of reversal
paths that arise due to the combination of precession in the materials and the
THz-induced fields. Our analysis demonstrates that the AFM drives the switching
and results in a much higher frequency dynamics in the FM due to the exchange
coupling at the interface. The switching is shown to be robust over a broad
range of temperatures relevant for device applications. | cond-mat_mtrl-sci |
Mott- versus Slater-type Insulating Nature of Two-Dimensional Sn Atom
Lattice on SiC(0001): Semiconductor surfaces with narrow surface bands provide unique playgrounds
to search for Mott-insulating state. Recently, a combined experimental and
theoretical study [Phys. Rev. Lett. 114, 247602 (2015)] of the two-dimensional
(2D) Sn atom lattice on a wide-gap SiC(0001) substrate proposed a Mott-type
insulator driven by strong on-site Coulomb repulsion U. Our systematic
density-functional theory (DFT) study with local, semilocal, and hybrid
exchange-correlation functionals shows that the Sn dangling-bond state largely
hybridizes with the substrate Si 3p and C 2p states to split into three surface
bands due to the crystal field. Such a hybridization gives rise to the
stabilization of the antiferromagnetic order via superexchange interactions.
The band gap and the density of states predicted by the hybrid DFT calculation
agree well with photoemission data. Our findings not only suggest that the
Sn/SiC(0001) system can be represented as a Slater-type insulator driven by
long-range magnetism, but also have an implication that taking into account
long-range interactions beyond the on-site interaction would be of importance
for properly describing the insulating nature of Sn/SiC(0001). | cond-mat_mtrl-sci |
Atmospheric doping effects in epitaxial graphene: correlation of local
and global electrical measurements: We directly correlate the local (20-nm scale) and global electronic
properties of a device containing mono-, bi- and tri-layer epitaxial graphene
(EG) domains on 6H-SiC(0001) by simultaneously performing local surface
potential measurements using Kelvin probe force microscopy and global transport
measurements. Using well-controlled environmental conditions, where the
starting state of the surface can be reproducibly defined, we investigate the
doping effects of N2, O2, water vapour and NO2 at concentrations representative
of the ambient air. We show that presence of O2, water vapour and NO2 leads to
p-doping of all EG domains. However, the thicker layers of EG are significantly
less affected by the atmospheric dopants. Furthermore, we demonstrate that the
general consensus of O2 and water vapour present in ambient air providing
majority of the p-doping to graphene is a common misconception. We
experimentally show that even the combined effect of O2, water vapour, and NO2
at concentrations higher than typically present in the atmosphere does not
fully replicate the state of the EG surface in ambient air. All doping effects
can be reproducibly reversed by vacuum annealing. Thus, for EG gas sensors it
is essential to consider naturally occurring environmental effects and properly
separate them from those coming from targeted species. | cond-mat_mtrl-sci |
Polarization-driven band topology evolution in twisted MoTe$_2$ and
WSe$_2$: Motivated by recent experimental observations of opposite Chern numbers in
$R$-type twisted MoTe$_2$ and WSe$_2$ homobilayers, we perform large-scale
density-functional-theory (DFT) calculations with machine learning force fields
to investigate moir\'e band topology from large to small twist angels in both
materials. We find that the Chern numbers of the moir\'e frontier bands change
sign as a function of twist angle, and this change is driven by the competition
between the in-plane piezoelectricity and the out-of-plane ferroelectricity.
Our large-scale calculations, enabled by machine learning methods, reveal
crucial insights into interactions across different scales in twisted bilayer
systems. The interplay between atomic-level relaxation effects and
moir\'e-scale electrostatic potential variation opens new avenues for the
design of intertwined topological and correlated states. | cond-mat_mtrl-sci |
Understanding and Optimizing the Sensitization of Anatase Titanium
Dioxide Surface with Hematite Clusters: The presence of small hematite (Fe2O3) clusters at low coverage on titanium
dioxide (TiO2) surface has been observed to enhance photocatalytic activity,
while excess loading of hematite is detrimental. We conduct a comprehensive
density functional theory study of Fe2O3 clusters adsorbed on the anatase TiO2
(101) surface to investigate the effect of Fe2O3 on TiO2. Our study shows that
TiO2 exhibits improved photocatalytic properties with hematite clusters at low
coverage, as evidenced by a systematic study conducted by increasing the number
of cluster adsorbates. The adsorption of the clusters generates impurity states
in the band gap improving light absorption and consequently affecting the
charge transfer dynamics. Furthermore, the presence of hematite clusters
enhances the activity of TiO2 in the hydrogen evolution reaction. The Fe
valence mixing present in some clusters leads to a significant increase in H2
evolution rate compared with the fixed +3 valence of Fe in hematite. We also
investigate the effect of oxygen defects and find extensive modifications in
the electronic properties and local magnetism of the TiO2 - Fe2O3 system,
demonstrating the wide-ranging effect of oxygen defects in the combined system. | cond-mat_mtrl-sci |
Selective observation of surface and bulk bands in polar WTe2 by
laser-based spin- and angle-resolved photoemission spectroscopy: The electronic state of WTe2, a candidate of type-II Weyl semimetal, is
investigated by using laser-based spin- and angle-resolved photoemission
spectroscopy (SARPES). We prepare the pair of WTe2 samples, one with (001)
surface and the other with (00-1) surface, by "sandwich method", and measure
the band structures of each surface separately. The Fermi arcs are observed on
both surfaces. We identify that the Fermi arcs on the two surfaces are both
originating from surface states. We further find a surface resonance band,
which connects with the Fermi-arc band, forming a Dirac-cone-like band
dispersion. Our results indicate that the bulk electron and hole bands are much
closer in momentum space than band calculations. | cond-mat_mtrl-sci |
Observation of Fermi arc spin texture in TaAs: We have investigated the spin texture of surface Fermi arcs in the recently
discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission
spectroscopy. The experimental results demonstrate that the Fermi arcs are
spin-polarized. The measured spin texture fulfills the requirement of mirror
and time reversal symmetries and is well reproduced by our first-principles
calculations, which gives strong evidence for the topologically nontrivial Weyl
semimetal state in TaAs. The consistency between the experimental and
calculated results further confirms the distribution of chirality of the Weyl
nodes determined by first-principles calculations. | cond-mat_mtrl-sci |
Towards printed magnetic sensors based on organic diodes: We report the study of magnetotransport properties of regio-regular poly
(3-hexyl thiophene) based organic diodes. The devices were fabricated using two
different techniques of spin coating and inkjet printing. Positive
magnetoresistance (MR) effect was observed at room temperature in all the
devices. The highest MR magnitude reached up to 16% for some spin-coated
devices and up to 10% in inkjet printed devices. The MR magnitude and line
shapes were found to depend strongly on the measuring current. We observed
deviation from the theoretically predicted Lorentzian or non-Lorentzian line
shape of the MR traces, which is discussed in detail in the article. Although,
the printed devices exhibit MR response as high as for the spin coated ones,
they still need to be optimized in terms of performance and yield for large
scale applications as magnetic sensors. | cond-mat_mtrl-sci |
Synthesis and structure of tetragonal Bi12.5Nd1.5ReO24.5: The Bi12.5Nd1.5ReO24.5 tetragonal phase has been synthesized and lattice cell
parameters have been determined. According to X-ray data the phase has I4/m
sym-metry with lattice parameters a = 0.86742 (12) nm, c =1.7408 (3) nm. | cond-mat_mtrl-sci |
Absence of long-range order in an $XY$ pyrochlore antiferromagnet
Er$_2$AlSbO$_7$: Rare-earth pyrochlores are known to exhibit exotic magnetic phenomena. We
report a study of crystal growth and characterizations of a new rare-earth
compound Er$_2$AlSbO$_7$, in which Al$^{3+}$ and Sb$^{5+}$ ions share the same
positions with a random distribution. The magnetism are studied by magnetic
susceptibility, specific heat and thermal conductivity measurements at low
temperatures down to several tens of milli-kelvin. Different from the other
reported Er-based pyrochlores exhibiting distinct magnetically ordered states,
a spin-freezing transition is detected in Er$_2$AlSbO$_7$ below 0.37 K, which
is primarily ascribed to the inherent structural disorder. A cluster spin-glass
state is proposed in view of the frequency dependence of the peak position in
the ac susceptibility. In addition, the temperature and field dependence of
thermal conductivity indicates rather strong spin fluctuations which is
probably due to the phase competition. | cond-mat_mtrl-sci |
Mechanisms of magnetoelectricity in manganese doped incipient
ferroelectrics: We report magnetization measurements and magnetic resonance data for SrTiO3
doped by manganese. We show that the recently reported coexistent spin and
dipole glass (multiglass) behaviours are strongly affected by the distribution
of Mn ions between the Sr and Ti sites. Motivated by this finding we calculate
the magnetic interactions between Mn impurities of different kinds. Both LSDA+U
and many-body perturbation theory evidence that magnetic and magnetoelectric
interactions are mediated by Mn$_B^{4+}$ ions substituting for Ti. We propose
two microscopic magnetoelectric coupling mechanisms, which can be involved in
all magnetoelectric systems based on incipient ferroelectrics. In the first
one, the electric field modifies the spin susceptibility via spin-strain
coupling of Mn$_{B}^{4+}$. The second mechanism concerns Mn pairs coupled by
the position-dependent exchange interaction. | cond-mat_mtrl-sci |
Emergent impervious band crossing in the bulk in topological nodal line
semimetal ZrAs$_2$: Topological nodal line semimetals (TSMs) represent a unique class of
materials with intriguing electronic structures and rich of symmetries, hosting
electronic states with non-trivial topological properties. Among these,
ZrAs$_2$ stands out, characterized by its nodal lines forming continuous loops
in momentum space, governed by non-symmorphic symmetries. This study integrates
angle-resolved photoemission spectroscopy (ARPES) with density functional
theory (DFT) calculations to explore the electronic states of ZrAs$_2$. In
ARPES scans, we observed a distinctive nodal loop structure observed at lower
excitation energies of 30 and 50 eV. Our results, supported by calculations
based on DFT, unveil symmetry-enforced Dirac-like band crossings anchored at
specific points in the Brillouin zone, with particular emphasis on the S point.
Surface bands and bulk states near the crossing are elucidated through slab
calculations, corroborating experimental findings. DFT calculations also show
the existence of several spin-orbit coupling (SOC) resilient semi-Dirac
crossings pinned at Z point. This comprehensive investigation sheds light on
the intricate electronic behaviors of ZrAs$_2$ with the involved symmetries,
important for fundamental understanding of topological nodal line semimetals. | cond-mat_mtrl-sci |
Sliding on a Nanotube: Interplay of Friction, Deformations and Structure: The frictional properties of individual carbon nanotubes (CNTs) are studied
by sliding an atomic force microscopy tip across and along its principle axis.
This direction-dependent frictional behavior is found to correlate strongly
with the presence of structural defects, surface chemistry, and CNT chirality.
This study shows that it is experimentally possible to tune the
frictional/adhesion properties of a CNT by controlling the CNT structure and
surface chemistry, as well as use friction force to predict its structural and
chemical properties. | cond-mat_mtrl-sci |
Effect of Sm-, Gd- codoping on structural modifications in
aluminoborosilicate glasses under beta-irradiation: Two series of Sm-, Gd-codoped aluminoborosilicate glasses with different
total rare earth content have been studied in order to examine the codoping
effect on the structural modifications of beta-irradiated glasses. The data
obtained by Electron Paramagnetic Resonance spectroscopy indicated that
relative amount of Gd3+ ions located in network former position reveals
non-linear dependence on Sm/Gd ratio. Besides, codoping leads to the evolution
of the EPR signal attributed to defects created by irradiation: superhyperfine
structure of boron oxygen hole centres EPR line becomes less noticeable and
resolved with increase of Gd amount. This fact manifests that Gd3+ ions are
mainly diluted in vicinity of the boron network. By Raman spectroscopy, we
showed that the structural changes induced by the irradiation also reveal
non-linear behaviour with Sm/Gd ratio. In fact, the shift of the Si-O-Si
bending vibration modes has a clear minimum for the samples containing equal
amount of Sm and Gd (50:50) in both series of the investigated glasses. In
contrast, for single doped glass there is no influence of dopant's content on
Si-O-Si shift (in case of Gd) or its diminution (in case of Sm) occurs which is
explained by the reduction process influence. At the same time, no noticeable
effect of codoping on Sm3+ intensity as well as on Sm2+ emission or on Sm
reduction process was observed. | cond-mat_mtrl-sci |
Interface characterization of Co2MnGe/Rh2CuSn Heusler multilayers: All-Heusler multilayer structures have been investigated by means of high
kinetic x-ray photoelectron spectroscopy and x-ray magnetic circular dichroism,
aiming to address the amount of disorder and interface diffusion induced by
annealing of the multilayer structure. The studied multilayers consist of
ferromagnetic Co$_2$MnGe and non-magnetic Rh$_2$CuSn layers with varying
thicknesses. We find that diffusion begins already at comparably low
temperatures between 200 $^{\circ}$C and 250 $^{\circ}$C, where Mn appears to
be most prone to diffusion. We also find evidence for a 4 {\AA} thick
magnetically dead layer that, together with the identified interlayer
diffusion, are likely reasons for the small magnetoresistance found for
current-perpendicular-to-plane giant magneto-resistance devices based on this
all-Heusler system. | cond-mat_mtrl-sci |
Fast Lithium Ion Diffusion in Brownmillerite
$\mathrm{Li}_{x}\mathrm{{Sr}_{2}{Co}_{2}{O}_{5}}$: Ionic conductors have great potential for interesting tunable physical
properties via ionic liquid gating and novel energy storage applications such
as all-solid-state lithium batteries. In particular, low migration barriers and
high hopping attempt frequency are the keys to achieve fast ion diffusion in
solids. Taking advantage of the oxygen-vacancy channel in
$\mathrm{Li}_{x}\mathrm{{Sr}_{2}{Co}_{2}{O}_{5}}$, we show that migration
barriers of lithium ion are as small as 0.28~0.17eV depending on the lithium
concentration rates. Our first-principles calculation also investigated hopping
attempt frequency and concluded the room temperature ionic diffusivity and ion
conductivity is high as ${10}^{-7}\sim{10}^{-6}~\mathrm{{cm}^{2}~s^{-1}}$ and
${10}^{-3}\sim{10}^{-2}~\mathrm{S\cdot{cm}^{-1}}$ respectively, which
outperform most of perovskite-type, garnet-type and sulfide Li-ion solid-state
electrolytes. This work proves
$\mathrm{Li}_{x}\mathrm{{Sr}_{2}{Co}_{2}{O}_{5}}$ as a promising solid-state
electrolyte. | cond-mat_mtrl-sci |
Confinement-induced metal-to-insulator transition in strained
LaNiO$_3$/LaAlO$_3$ superlattices: Using density functional theory calculations including a Hubbard $U$ term we
explore the effect of strain and confinement on the electronic ground state of
superlattices containing the band insulator LaAlO$_3$ and the correlated metal
LaNiO$_3$. Besides a suppression of holes at the apical oxygen, a central
feature is the asymmetric response to strain in single unit cell superlattices:
For tensile strain a band gap opens due to charge disproportionation at the Ni
sites with two distinct magnetic moments of 1.45$\mu_{\rm B}$ and 0.71$\mu_{\rm
B}$. Under compressive stain, charge disproportionation is nearly quenched and
the band gap collapses due to overlap of $d_{3z^2-r^2}$ bands through a
semimetallic state. This asymmetry in the electronic behavior is associated
with the difference in octahedral distortions and rotations under tensile and
compressive strain. The ligand hole density and the metallic state are quickly
restored with increasing thickness of the (LaAlO$_3$)$_n$/(LaNiO$_3$)$_n$
superlattice from $n=1$ to $n=3$. | cond-mat_mtrl-sci |
Exact value of the correlation factor for the divacancy mechanism in FCC
crystals: the end of a long quest: The correlation factor for diffusion by the divacancy mechanism in an FCC
lattice has been the target of a long quest. Amounting to 0.54 at the
beginning, its value was progressively decreased down to 0.4582 with a
confidence interval of 0.0005. The limitations of previous studies came from
the matrix method at work which confined the defect trajectories to a volume of
finite extension around the tracer atom, or, more basically, from the
availability of computing time for the numerical simulations. The present study
determines the exact value thanks to a double Laplace-Fourier transform of the
transport equation, which takes into account all the divacancy trajectories
without any limitation of their lengths. The exact value is lower than all the
preceding evaluations, but falls within the confidence interval which was
estimated from the best evaluations based on simulations. One interesting
byproduct of our analytical approach is the proof of the equality t11=t22. | cond-mat_mtrl-sci |
Anomalous Dirac Plasmons in 1D Topological Electrides: Plasmon opens up the possibility to efficiently couple light and matter at
sub-wavelength scales. In general, the plasmon frequency is dependent of
carrier density. This dependency, however, renders fundamentally a weak plasmon
intensity at low frequency, especially for Dirac plasmon (DP) widely studied in
graphene. Here we demonstrate a new type of DP, excited by a Dirac
nodal-surface state, which exhibits an anomalously density-independent
frequency. Remarkably, we predict realization of anomalous DP (ADP) in 1D
topological electrides, such as Ba3CrN3 and Sr3CrN3, by first-principles
calculations. The ADPs in both systems have a density-independent frequency and
high intensity, and their frequency can be tuned from terahertz to mid-infrared
by changing the excitation direction. Furthermore, the intrinsic weak
electron-phonon coupling of anionic electrons in electrides affords an added
advantage of ultra-low phonon-assisted damping and hence a long lifetime of the
ADPs. Our work paves the way to developing novel plasmonic and optoelectronic
devices by combining topological physics with electride materials. | cond-mat_mtrl-sci |
Analysis of Lithiation and Delithiation Kinetics in Silicon: Analysis of lithiation and delithiation kinetics in pulse-laser-deposited
crystalline thin-film silicon (Si) electrodes is presented. Data from
open-circuit relaxation experiments are used in conjunction with a model based
on Tafel kinetics and double-layer capacitance to estimate the apparent
transfer coefficients ({\alpha}a, {\alpha}c), and exchange current density to
capacitance ratio (i0/Cdl) for lithiation and delithiation reactions in a
lithiated silicon (LixSi) system. Parameters estimated from data sets obtained
during first-cycle amorphization of crystalline Si, as well as from cycled
crystalline Si and amorphous Si thin-film electrodes do not show much
variation, indicating that they are intrinsic to lithiation/delithiation in Si.
A methodology to estimate the side-reaction rate and its role in the evolution
of the open-circuit potential of the LixSi system are discussed. We conclude
that the large potential offset between lithiation and delithiation reactions
at any given state of charge is partially caused by a large kinetic resistance
(i.e., small i0). Using the estimated parameters, the model is shown to predict
successfully the behavior of the system under galvanostatic lithiation and
delithiation. | cond-mat_mtrl-sci |
Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N2
Plasma Exposure: Controllable doping of two-dimensional materials is highly desired for ideal
device performance in both hetero- and p-n homo-junctions. Herein, we propose
an effective strategy for doping of MoS2 with nitrogen through a remote N2
plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2
plasma exposure using in-situ X-ray photoelectron spectroscopy, we identified
the presence of covalently bonded nitrogen in MoS2, where substitution of the
chalcogen sulfur by nitrogen is determined as the doping mechanism.
Furthermore, the electrical characterization demonstrates that p-type doping of
MoS2 is achieved by nitrogen doping, in agreement with theoretical predictions.
Notably, we found that the presence of nitrogen can induce compressive strain
in the MoS2 structure, which represents the first evidence of strain induced by
substitutional doping in a transition metal dichalcogenide material. Finally,
our first principle calculations support the experimental demonstration of such
strain, and a correlation between nitrogen doping concentration and compressive
strain in MoS2 is elucidated. | cond-mat_mtrl-sci |
Direct observation of ordered configurations of hydrogen adatoms on
graphene: Ordered configurations of hydrogen adatoms on graphene have long been
proposed, calculated and searched for. Here we report direct observation of
several ordered configurations of H adatoms on graphene by scanning tunneling
microscopy. On the top side of the graphene plane, H atoms in the
configurations appear to stick to carbon atoms in the same sublattice. A gap
larger than 0.6 eV in the local density of states of the configurations was
revealed by scanning tunneling spectroscopy measurements. These findings can be
well explained by density functional theory calculations based on double sided
H configurations. In addition, factors that may influence H ordering are
discussed. | cond-mat_mtrl-sci |
Spatially dispersive circular photogalvanic effect in a Weyl semimetal: Weyl semimetals are gapless topological states of matter with broken
inversion and/or time reversal symmetry, which can support unconventional
responses to externally applied electrical, optical and magnetic fields. Here
we report a new photogalvanic effect in type-II WSMs, MoTe2 and Mo0.9W0.1Te2,
which are observed to support a circulating photocurrent when illuminated by
circularly polarized light at normal incidence. This effect occurs exclusively
in the inversion broken phase, where crucially we find that it is associated
with a spatially varying beam profile via a new dispersive contribution to the
circular photogalvanic effect (s-CPGE). The response functions derived for
s-CPGE reveal the microscopic mechanism of this photocurrent, which are
controlled by terms that are allowed in the absence of inversion symmetry,
along with asymmetric carrier excitation and relaxation. By evaluating this
response for a minimal model of a Weyl semimetal, we obtain the frequency
dependent scaling behavior of this form of photocurrent. These results
demonstrate opportunities for controlling photoresponse by patterning optical
fields to store, manipulate and transmit information over a wide spectral
range. | cond-mat_mtrl-sci |
Infrared dielectric function, phonon modes and free-charge carrier
properties of high-Al-content Al$_x$Ga$_{1-x}$N alloys determined by
mid-infrared spectroscopic ellipsometry and optical Hall effect: The phonon mode parameters and anisotropic mid-infrared dielectric function
tensor components of high- Al-content Al$_x$Ga$_{1-x}$N alloys in dependence of
the Al content $x$ are precisely determined from mid-infrared spectroscopic
ellipsometry measurements for a set of high-quality Si-doped Al$_x$Ga$_{1-x}$N
epitaxial layers on 4H-SiC substrates. Two-mode behavior of the $E_1$(TO) modes
and one-mode behavior of the $A_1$(LO) mode are found in agreement with
previous Raman scattering spectroscopy reports. The composition dependencies of
the IR active phonon frequency parameters are established and a discussion on
the silent $B_1$ mode that may be disorder activated is provided. The static
dielectric constants in dependence of $x$ are determined by using the
best-match model derived phonon mode frequency and high-frequency dielectric
constant parameters and applying the Lydanne-Sachs-Teller relation. The
effective mass parameter in high-Al-content Al$_x$Ga$_{1-x}$N alloys and its
composition dependence are determined from mid-infrared optical Hall effect
measurements. Furtheremore, the free electron concentration $N$ and mobility
parameters $\mu$ of Al$_x$Ga$_{1-x}$N films with similar Si doping levels are
investigated as function of the Al content, $x$ and discussed. | cond-mat_mtrl-sci |
Strain localization and shear banding in ductile materials: A model of a shear band as a zero-thickness nonlinear interface is proposed
and tested using finite element simulations. An imperfection approach is used
in this model where a shear band, that is assumed to lie in a ductile matrix
material (obeying von Mises plasticity with linear hardening), is present from
the beginning of loading and is considered to be a zone in which yielding
occurs before the rest of the matrix. This approach is contrasted with a
perturbative approach, developed for a J$_2$-deformation theory material, in
which the shear band is modelled to emerge at a certain stage of a uniform
deformation. Both approaches concur in showing that the shear bands
(differently from cracks) propagate rectilinearly under shear loading and that
a strong stress concentration should be expected to be present at the tip of
the shear band, two key features in the understanding of failure mechanisms of
ductile materials. | cond-mat_mtrl-sci |
Ferromagnetic frozen structures from the dipolar hard spheres fluid at
moderate and small volume fractions: We study the magnetic phase diagram of an ensemble of dipolar hard spheres
(DHS) with or without uniaxial anisotropy and frozen in position on a
disordered structure by tempered Monte Carlo simulations. The crucial point is
to consider an anisotropic structure, obtained from the liquid state of the
dipolar hard spheres fluid, frozen in its polarized state at low temperature.
The freezing inverse temperature $\beta_f$ determines the degree of anisotropy
of the structure which is quantified through a structural nematic order
parameter, $\lambda_s$. The case of the non zero uniaxial anisotropy is
considered only in its infinitely strong strength limit where the system
transforms in a dipolar Ising model (DIM). The important finding of this work
is that both the DHS and the DIM with a frozen structure build in this way
present a ferromagnetic phase at volume fractions below the threshold value
where the corresponding isotropic DHS systems exhibit a spin glass phase at low
temperature. | cond-mat_mtrl-sci |
Perfect separation of intraband and interband excitations in PdCoO$_2$: The temperature dependence of the optical properties of the delafossite
PdCoO$_2$ has been measured in the a-b planes over a wide frequency range. The
optical conductivity due to the free-carrier (intraband) response falls well
below the interband transitions, allowing the plasma frequency to be determined
from the $f$-sum rule. Drude-Lorentz fits to the complex optical conductivity
yield estimates for the free-carrier plasma frequency and scattering rate. The
in-plane plasma frequency has also been calculated using density functional
theory. The experimentally-determined and calculated values for the plasma
frequencies are all in good agreement; however, at low temperature the
optically-determined scattering rate is much larger than the estimate for the
transport scattering rate, indicating a strong frequency-dependent
renormalization of the optical scattering rate. In addition to the expected
in-plane infrared-active modes, two very strong features are observed that are
attributed to the coupling of the in-plane carriers to the out-of-plane
longitudinal optic modes. | cond-mat_mtrl-sci |
Polaronic hole-trapping in doped $\rm BaBiO_3$: The present {\em ab initio} study shows that in BaBiO$_3$, Bi$^{3+}$ sites
can trap two holes from the valence band to form Bi$^{5+}$ cations. The
trapping is accompanied by large local lattice distortions, therefore the
composite particle consisting of the electronic-hole and the local lattice
phonon field forms a polaron. Our study clearly shows that even $sp$ elements
can trap carriers at lattice sites, if local lattice relaxations are
sufficiently large to screen the localised hole. The derived model describes
all relevant experimental results, and settles the issue of why hole doped
BaBiO$_3$ remains semiconducting upon moderate hole doping. | cond-mat_mtrl-sci |
In-plane orientation effects on the electronic structure, stability and
Raman scattering of monolayer graphene on Ir(111): We employ angle-resolved photoemission spectroscopy (ARPES) to investigate
the electronic structures of two rotational variants of epitaxial, single-layer
graphene on Ir(111). As grown, the more-abundant R0 variant is nearly
charge-neutral, with strong hybridization between graphene and Ir bands near
the Fermi level. The graphene Fermi surface and its replicas exactly coincide
with Van Hove singularities in the Ir Fermi surface. Sublattice symmetry
breaking introduces a small gap-inducing potential at the Dirac crossing, which
is revealed by n-doping the graphene using K atoms. The energy gaps between
main and replica bands (originating from the moir\'e interference pattern
between graphene and Ir lattices) is shown to be non-uniform along the mini-
zone boundary due to hybridization with Ir bands. An electronically mediated
interaction is proposed to account for the stability of the R0 variant. The
variant rotated 30{\deg} in-plane, R30, is p-doped as grown and K doping
reveals no band gap at the Dirac crossing. No replica bands are found in ARPES
measurements. Raman spectra from the R30 variant exhibit the characteristic
phonon modes of graphene, while R0 spectra are featureless. These results show
that the film/substrate interaction changes from chemisorption (R0) to
physisorption (R30) with in-plane orientation. Finally, graphene-covered Ir has
a work function lower than the clean substrate but higher than graphite. | cond-mat_mtrl-sci |
Cooperative response of Pb(ZrTi)O$_3$ nanoparticles to curled electric
fields: Using first-principles based effective Hamiltonian and finite temperature
Monte Carlo simulations we investigate cooperative responses, as well as
microscopic mechanism for vortex switching, in zero-dimensional
Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ nanoparticles under curled electric fields. We
find that the generally accepted domain coexistence mechanism is not valid for
toroid switching. Instead dipoles are shown to display unusual collective
behaviors by forming a new vortex with perpendicular (but not opposite) toroid
moment. The strong correlation between the new and original vortices is
revealed to be critical for reversing toroid moment. Microscopic insight for
the puzzling collective response is discussed. Based on our finding, we further
describe a technological approach that is able to drastically reduce the
magnitude of the curled electric field needed for vortex switching. | cond-mat_mtrl-sci |
Andreev reflection in ferrimagnetic CoFe2O4/SrRuO3 spin filters: We have performed point contact spectroscopy measurements on a sample
constituted by a metallic ferromagnetic oxide (SrRuO_3) bottom electrode and a
tunnel ferrimagnetic (CoFe_2O_4) barrier. Andreev reflection is observed across
the tunnel barrier. From the comparison of Andreev reflection in SrRuO3 and
across the CoFe_2O_4 barrier we infer that the ferrimagnetic barrier has a spin
filter efficiency not larger than +13%. The observation of a moderate and
positive spin filtering is discussed in the context of the microstructure of
the barriers and symmetry-related spin filtering effects. | cond-mat_mtrl-sci |
Disassembling one-dimensional chains in molybdenum oxides: The dimensionality of quantum materials strongly affects their physical
properties. Although many emergent phenomena, such as charge-density wave and
Luttinger liquid behavior, are well understood in one-dimensional (1D) systems,
the generalization to explore them in higher dimensional systems is still a
challenging task. In this study, we aim to bridge this gap by systematically
investigating the crystal and electronic structures of molybdenum-oxide family
compounds, where the contexture of 1D chains facilitates rich emergent
properties. While the quasi-1D chains in these materials share general
similarities, such as the motifs made up of MoO6 octahedrons, they exhibit vast
complexity and remarkable tunability. We disassemble the 1D chains in
molybdenum oxides with different dimensions and construct effective models to
excellently fit their low-energy electronic structures obtained by ab initio
calculations. Furthermore, we discuss the implications of such chains on other
physical properties of the materials and the practical significance of the
effective models. Our work establishes the molybdenum oxides as simple and
tunable model systems for studying and manipulating the dimensionality in
quantum systems. | cond-mat_mtrl-sci |
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