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Pentacene islands grown on ultra-thin SiO2: Ultra-thin oxide (UTO) films were grown on Si(111) in ultrahigh vacuum at
room temperature and characterized by scanning tunneling microscopy. The
ultra-thin oxide films were then used as substrates for room temperature growth
of pentacene. The apparent height of the first layer is 1.57 +/- 0.05 nm,
indicating standing up pentacene grains in the thin-film phase were formed.
Pentacene is molecularly resolved in the second and subsequent molecular
layers. The measured in-plane unit cell for the pentacene (001) plane (ab
plane) is a=0.76+/-0.01 nm, b=0.59+/-0.01 nm, and gamma=87.5+/-0.4 degrees. The
films are unperturbed by the UTO's short-range spatial variation in tunneling
probability, and reduce its corresponding effective roughness and correlation
exponent with increasing thickness. The pentacene surface morphology follows
that of the UTO substrate, preserving step structure, the long range surface
rms roughness of ~0.1 nm, and the structural correlation exponent of ~1. | cond-mat_mtrl-sci |
The Structure of Eu-III: Previous x-ray diffraction studies have reported Eu to transform from the hcp
structure to a new phase, Eu-III, at 18 GPa. Using x-ray powder diffraction we
have determined that Eu remains hcp up to 33 GPa, and that the extra peaks that
appear at 18 GPa are from an impurity phase with space group R-3c . Above 33
GPa the diffraction pattern becomes very much more complex, signalling a
transition to a phase with a distorted hcp structure. | cond-mat_mtrl-sci |
Nitrogen-vacancy centers created by N$^+$ ion implantation through
screening SiO$_2$ layers on diamond: We report on an ion implantation technique utilizing a screening mask made of
SiO$_2$ to control both the depth profile and the dose. By appropriately
selecting the thickness of the screening layer, this method fully suppresses
the ion channeling, brings the location of the highest NV density to the
surface, and effectively reduces the dose by more than three orders of
magnitude. With a standard ion implantation system operating at the energy of
10 keV and the dose of 10$^{11}$ cm$^2$ and without an additional etching
process, we create single NV centers close to the surface with coherence times
of a few tens of $\mu$s. | cond-mat_mtrl-sci |
Tunable magnon-magnon coupling in synthetic antiferromagnets: In this work, we study magnon-magnon coupling in synthetic antiferromagnets
(SyAFs) using microwave spectroscopy at room temperature. Two distinct
spin-wave modes are clearly observed and are hybridised at degeneracy points.
We provide a phenomenological model that captures the coupling phenomena and
experimentally demonstrate that the coupling strength is controlled by the
out-of-plane tilt angle as well as the interlayer exchange field. We
numerically show that a spin-current mediated damping in SyAFs plays a role in
influencing the coupling strength. | cond-mat_mtrl-sci |
Transverse circular photogalvanic effect associated with
Lorentz-violating Weyl fermions: Nonlinear optical responses of quantum materials have recently undergone
dramatic developments to unveil nontrivial geometry and topology. A remarkable
example is the quantized longitudinal circular photogalvanic effect (CPGE)
associated with the Chern number of Weyl fermions, while the physics of
transverse CPGE in Weyl semimetals remains exclusive. Here, we show that the
transverse CPGE of Lorentz invariant Weyl fermions is forced to be zero. We
find that the transverse photocurrents of Weyl fermions are associated not only
with the Chern numbers but also with the degree of Lorentz-symmetry breaking in
condensed matter systems. Based on the generic two-band model analysis, we
provide a new powerful equation to calculate the transverse CPGE based on the
tilting and warping terms of Weyl fermions. Our results are more capable in
designing large transverse CPGE of Weyl semimetals in experiments and are
applied to more than tens of Weyl materials to estimate their photocurrents.
Our method paves the way to study the CPGE of massless or massive
quasiparticles to design next-generation quantum optoelectronics. | cond-mat_mtrl-sci |
Directional carrier transport in micrometer-thick gallium oxide films
for high-performance deep-ultraviolet photodetection: Incorporating emerging ultrawide bandgap semiconductors with a
metal-semiconductor-metal (MSM) architecture is highly desired for
deep-ultraviolet (DUV) photodetection. However, synthesis-induced defects in
semiconductors complicate the rational design of MSM DUV photodetectors due to
their dual role as carrier donors and trap centers, leading to a commonly
observed trade-off between responsivity and response time. Here, we demonstrate
a simultaneous improvement of these two parameters in {\epsilon}-Ga2O3 MSM
photodetectors by establishing a low-defect diffusion barrier for directional
carrier transport. Specifically, using a micrometer thickness far exceeding its
effective light absorption depth, the {\epsilon}-Ga2O3 MSM photodetector
achieves over 18-fold enhancement of responsivity and simultaneous reduction of
the response time, which exhibits a state-of-the-art photo-to-dark current
ratio near 10^8, a superior responsivity of >1300 A/W, an ultrahigh detectivity
of >10^16 Jones and a decay time of 123 ms. Combined depth-profile
spectroscopic and microscopic analysis reveals the existence of a broad
defective region near the lattice-mismatched interface followed by a more
defect-free dark region, while the latter one serves as a diffusion barrier to
assist frontward carrier transport for substantially enhancing the
photodetector performance. This work reveals the critical role of the
semiconductor defect profile in tuning carrier transport for fabricating
high-performance MSM DUV photodetectors. | cond-mat_mtrl-sci |
Tunable polarization components and electric field induced
crystallization in polyvinylidenefluoride (PVDF); a piezo polymer: Polyvinylidenefluoride (PVDF) a semicrystalline pieozoelectric polymer was
synthesized with varying process conditions and its ferroelectric domain
orientations were studied using piezoresponse force microscope (PFM). PVDF thin
films fabricated using tape casting technique with precursor solutions of
varying viscosities reveal that the polarization components transform from a
dominant planar component to an out-of-plane polarization components with
increase in viscosity. Interestingly the planar components possessed a head to
head or tail to tail kind of paired domains separated by a distance of ~
380-400nm. The electrostatic energies computed by numerically solving the
electrostatic equilibrium equation for the electrically inhomogeneous system
are in good correlation with the experiments. On increment of electric field,
the domains were observed to grow in size and shape which indicates amorphous
to crystalline transformation in the case of PVDF. Such transformation was
evident from x-ray diffraction studies performed in-situ in the presence of an
applied electric field. | cond-mat_mtrl-sci |
Understanding and tuning magnetism in layered Ising-type antiferromagnet
FePSe3 for potential 2D magnet: Recent development in two-dimensional (2D) magnetic materials have motivated
the search for new van der Waals magnetic materials, especially Ising-type
magnets with strong magnetic anisotropy. Fe-based MPX3 (M = transition metal, X
= chalcogen) compounds such as FePS3 and FePSe3 both exhibit an Ising-type
magnetic order, but FePSe3 receives much less attention compared to FePS3. This
work focuses on establishing the strategy to engineer magnetic anisotropy and
exchange interactions in this less-explored compound. Through chalcogen and
metal substitutions, the magnetic anisotropy is found to be immune against S
substitution for Se whereas tunable only with heavy Mn substitution for Fe. In
particular, Mn substitution leads to a continuous rotation of magnetic moments
from the out-of-plane direction towards in-plane. Furthermore, the magnetic
ordering temperature displays non-monotonic doping dependence for both
chalcogen and metal substitutions but due to different mechanisms. These
findings provide deeper insight into the Ising-type magnetism in this important
van der Waals material, shedding light on the study of other Ising-type
magnetic systems as well as discovering novel 2D magnets for potential
applications in spintronics. | cond-mat_mtrl-sci |
Diffraction at GaAs/Fe$_{3}$Si core/shell nanowires: the formation of
nanofacets: GaAs/Fe$_{3}$Si core/shell nanowire structures were fabricated by
molecular-beam epitaxy on oxidized Si(111) substrates and investigated by
synchrotron x-ray diffraction. The surfaces of the Fe$_3$Si shells exhibit
nanofacets. These facets consist of well pronounced Fe$_3$Si{111} planes.
Density functional theory reveals that the Si-terminated Fe$_3$Si{111} surface
has the lowest energy in agreement with the experimental findings. We can
analyze the x-ray diffuse scattering and diffraction of the ensemble of
nanowires avoiding the signal of the substrate and poly-crystalline films
located between the wires. Fe$_3$Si nanofacets cause streaks in the x-ray
reciprocal space map rotated by an azimuthal angle of 30{\deg} compared with
those of bare GaAs nanowires. In the corresponding TEM micrograph the facets
are revealed only if the incident electron beam is oriented along
[1$\overline{1}$0] in accordance with the x-ray results. Additional maxima in
the x-ray scans indicate the onset of chemical reactions between Fe$_{3}$Si
shells and GaAs cores occurring at increased growth temperatures. | cond-mat_mtrl-sci |
Plasmonic electromagnetically-induced transparency in symmetric
structures: A broken symmetry is generally believed to be a prerequisite of plasmonic
electromagnetically-induced transparency (EIT), since the asymmetry renders the
excitation of the otherwise forbidden dark mode possible. Nevertheless,
according to the picture of magnetic-plasmon resonance (MPR) mediated plasmonic
EIT, we show that the plasmonic EIT can be achieved even in the symmetric
structures based on the second-order MPR. This sharpens our understanding of
the existing concept, but also a profound insight into the plasmonic coherent
interference in the near-field zone. | cond-mat_mtrl-sci |
Reversible phase transformation and doubly-charged anions at the surface
of simple cubic RbC60: The simple cubic phase of a RbC60 thin film has been studied using
photoelectron spectroscopy. The simple cubic-to-dimer transition is found to be
reversible at the film surface. A sharp Fermi edge is observed and a lower
limit of 0.5 eV is found for the surface Hubbard U, pointing to a
strongly-correlated metallic character of thin-film simple cubic RbC60. A
molecular charge state is identified in the valence band and core level
photoemission spectra which arises from C602- anions and contributes to the
spectral intensity at the Fermi level. | cond-mat_mtrl-sci |
Quantum anomalous Hall effect in two-dimensional magnetic insulator
heterojunctions: Recent years have witnessed tremendous success in the discovery of
topological states of matter. Particularly, sophisticated theoretical methods
in time-reversal-invariant topological phases have been developed, leading to
the comprehensive search of crystal database and the prediction of thousands of
new topological materials. In contrast, the discovery of magnetic topological
phases that break time reversal is still limited to several exemplary materials
because the coexistence of magnetism and topological electronic band structure
is rare in a single compound. To overcome this challenge, we propose an
alternative approach to realize the quantum anomalous Hall (QAH) effect, a
typical example of magnetic topological phase, via engineering two-dimensional
(2D) magnetic van der Waals heterojunctions. Instead of a single magnetic
topological material, we search for the combinations of two 2D (typically
trivial) magnetic insulator compounds with specific band alignment so that they
can together form a type-III heterojunction with topologically non-trivial band
structure. By combining the data-driven materials search, first principles
calculations, and the symmetry-based analytical models, we identify 8 type-III
heterojunctions consisting of 2D ferromagnetic insulator materials from a
family of 2D monolayer MXY compounds (M = metal atoms, X = S, Se, Te, Y = F,
Cl, Br, I) as a set of candidates for the QAH effect. In particular, we
directly calculate the topological invariant (Chern number) and chiral edge
states in the MnNF/MnNCl heterojunction with ferromagnetic stacking. This work
illustrates how data-driven material science can be combined with
symmetry-based physical principles to guide the search for novel
heterojunction-based quantum materials hosting the QAH effect and other exotic
quantum states in general. | cond-mat_mtrl-sci |
Dual-band metacomposites containing hybrid Fe and Co-based ferromagnetic
microwires: We investigated the microwave properties of polymer based metacomposites
containing hybridized parallel Fe- and Co-based microwire arrays. A dual-band
left-handed feature was observed in the frequency bands of 1.5 to 5.5 GHz and 9
to 17 GHz, indicated by two transmission windows associated with ferromagnetic
resonance of Fe-based microwires and long range dipolar resonance between the
wire arrays. The plasma frequency after hybridization is significantly
increased due to the enhanced effective diameter through the wire-wire
interactions between the Fe- and Co- microwire couples. These results offer
essential perspectives in designing the multi-band metamaterial for microwave
applications such as sensors and cloaking devices. | cond-mat_mtrl-sci |
Molecular Tuning of the Magnetic Response in Organic Semiconductors: The tunability of high-mobility organic semi-conductors (OSCs) holds great
promise for molecular spintronics. In this study, we show this extreme
variability - and therefore potential tunability - of the molecular
gyromagnetic coupling ("g-") tensor with respect to the geometric and
electronic structure in a much studied class of OSCs. Composed of a structural
theme of phenyl- and chalcogenophene (group XVI element containing,
five-membered) rings and alkyl functional groups, this class forms the basis of
several intensely studied high-mobility polymers and molecular OSCs. We show
how in this class the g-tensor shifts, $\Delta g$, are determined by the
effective molecular spin-orbit coupling (SOC), defined by the overlap of the
atomic spin-density and the heavy atoms in the polymers. We explain the
dramatic variations in SOC with molecular geometry, chemical composition,
functionalization, and charge life-time using a first-principles theoretical
model based on atomic spin populations. Our approach gives a guide to tuning
the magnetic response of these OSCs by chemical synthesis. | cond-mat_mtrl-sci |
Intrinsic anomalous Hall effect in Ni-substituted magnetic Weyl
semimetal Co3Sn2S2: Topological materials have recently attracted considerable attention among
materials scientists as their properties are predicted to be protected against
perturbations such as lattice distortion and chemical substitution. However,
any experimental proof of such robustness is still lacking. In this study, we
experimentally demonstrate that the topological properties of the ferromagnetic
kagome compound Co3Sn2S2 are preserved upon Ni substitution. We systematically
vary the Ni content in Co3Sn2S2 single crystals and study their magnetic and
anomalous transport properties. For the intermediate Ni substitution, we
observe a remarkable increase in the coercive field while still maintaining
significant anomalous Hall conductivity. The large anomalous Hall conductivity
of these compounds is intrinsic, consistent with first-principle calculations,
which proves its topological origin. Our results can guide further studies on
the chemical tuning of topological materials for better understanding. | cond-mat_mtrl-sci |
On the impact of capillarity for strength at the nanoscale: The interior of nanoscale crystals experiences stress that compensates the
capillary forces and that can be large, in the order of 1 GPa. Various studies
have speculated on whether and how this surface-induced stress affects the
stability and plasticity of small crystals. Yet, experiments have so far failed
to discriminate between the surface contribution and other, bulk-related size
effects. In order to clarify the issue, we study the variation of the flow
stress of a nanomaterial while distinctly different variations of the two
capillary parameters surface tension and surface stress are imposed under
control of an applied electric potential. Our theory qualifies the suggested
impact of $\textit{surface stress}$ as not forceful and instead predicts a
significant contribution of the surface energy, as measured by the
$\textit{surface tension}$. The predictions for the combined potential- and
size dependence of the flow stress are quantitatively supported by the
experiment. Previous suggestions, favoring the surface stress as the relevant
capillary parameter, are not consistent with the experiment. | cond-mat_mtrl-sci |
Spin-phonon coupling in BaFe12O19 M-type hexaferrite: The spin-phonon coupling in magnetic materials is due to the modulation of
the exchange integral by lattice vibrations. BaFe12O19 M-type hexaferrite,
which is the most used magnetic material as permanent magnet, transforms into
ferromagnet at high temperatures, but no spin-phonon coupling was previously
observed at this transition. In this letter, we investigated the
temperature-dependent Raman spectra of polycrystalline BaFe12O19 M-type
hexaferrite from room temperature up to 780 K to probe spin-phonon coupling at
the ferrimagnetic transition. An anomaly was observed in the position of the
phonon attributed to the Fe(4)O6 octahedra, evidencing the presence of a
spin-phonon coupling in BaM in the ferrimagnetic transition at 720 K. The
results also confirmed the spin-phonon coupling is different for each phonon
even when they couple with the same spin configuration. | cond-mat_mtrl-sci |
Multiferroic RMnO3 thin films: Multiferroic materials have received an astonishing attention in the last
decades due to expectations that potential coupling between distinct ferroic
orders could inspire new applications and new device concepts. As a result, a
new knowledge on coupling mechanisms and materials science has dramatically
emerged. Multiferroic RMnO3 perovskites are central to this progress providing
a suitable platform to tailor spin-spin and spin-lattice interactions. With
views towards applications, development of thin films of multiferroic materials
have also progressed enormously and nowadays thin film manganites are available
with properties mimicking those of bulk compounds. Here we review achievements
on the growth and characterization of magnetic and ferroelectric properties of
hexagonal and orthorhombic RMnO3 epitaxial thin films, discuss some challenging
issues and we suggest some guidelines for future research and developments. | cond-mat_mtrl-sci |
Cooling rate dependence of the antiferromagnetic domain structure of a
single crystalline charge ordered manganite: The low temperature phase of single crystals of Nd$_{0.5}$Ca$_{0.5}$MnO$_3$
and Gd$_{0.5}$Ca$_{0.5}$MnO$_3$ manganites is investigated by squid
magnetometry. Nd$_{0.5}$Ca$_{0.5}$MnO$_3$ undergoes a charge-ordering
transition at $T_{CO}$=245K, and a long range CE-type antiferromagnetic state
is established at $T_N$=145K. The dc-magnetization shows a cooling rate
dependence below $T_N$, associated with a weak spontaneous moment. The
associated excess magnetization is related to uncompensated spins in the
CE-type antiferromagnetic structure, and to the presence in this state of
fully orbital ordered regions separated by orbital domain walls. The observed
cooling rate dependence is interpreted to be a consequence of the rearrangement
of the orbital domain state induced by the large structural changes occurring
upon cooling. | cond-mat_mtrl-sci |
Electrodynamics of magnetoelectric media and magnetoelectric fields: The relationship between magnetoelectricity and electromagnetism is a subject
of a strong interest and numerous discussions in microwave and optical wave
physics and material sciences. The definition of the energy and momentum of the
electromagnetic (EM) field in a magnetoelectric (ME) medium is not a trivial
problem. The question of whether electromagnetism and magnetoelectricity can
coexist without an extension of Maxwell theory arises when we study the effects
of EM energy propagation and consider group velocity of the waves in a ME
medium. The energy balance equation reveals unusual topological structure of
fields in ME materials. Together with certain constraints on the constitutive
parameters of a medium, definite constraints on the local field structure
should be imposed. Analyzing the EM phenomena inside a ME material, we should
answer the question: what kind of the near fields arising from a sample of such
a material can we measure? Visualization of the ME states requires an
experimental technique that is based on an effective coupling to the violation
of spatial as well as temporal inversion symmetry. To observe the ME energy in
a subwavelength region, it is necessary to assume the existence of first
principle near fields, the ME fields. These are non Maxwellian near fields with
specific properties of violation of spatial and temporal inversion symmetry. A
particular interest to the ME fields arises in studies of metamaterials with
artificial atoms ME elements. | cond-mat_mtrl-sci |
Strain-induced stabilization of Al functionalization in graphene oxide
nanosheet for enhanced NH3 storage: Strain effects on the stabilization of Al ad-atom on graphene
oxide(GO)nanosheet as well as its implications for NH3 storage have been
investigated using first-principles calculations.The binding energy of Al
ad-atom on GO is found to be a false indicator of its stability.Tensile strain
is found to be very effective in stabilizing the Al ad-atom on GO.It
strengthens the C-O bonds through an enhanced charge transfer from C to O
atoms. Interestingly,C-O bond strength is found to be the correct index for
Al's stability.Optimally strained Al-functionalized GO binds up to 6 NH3
molecules,while it binds no NH3 molecule in unstrained condition. | cond-mat_mtrl-sci |
A novel coupled RPL/OSL system to understand the dynamics of the
metastable states: Metastable states form by charge (electron and hole) capture in defects in a
solid. They play an important role in dosimetry, information storage, and many
medical and industrial applications of photonics. Despite many decades of
research, the exact mechanisms resulting in luminescence signals such as
optically/thermally stimulated luminescence (OSL or TL) or long persistent
luminescence through charge transfer across the metastable states remain poorly
understood. Our lack of understanding owes to the fact that such luminescence
signals arise from a convolution of several steps such as charge (de)trapping,
transport and recombination, which are not possible to track individually. Here
we present a novel coupled RPL(radio-photoluminescence)/OSL system based on an
electron trap in a ubiquitous, natural, geophotonic mineral called feldspar
(aluminosilicate). RPL/OSL allows understanding the dynamics of the trapped
electrons and trapped holes individually. We elucidate for the first time trap
distribution, thermal eviction, and radiation-induced growth of trapped
electron and holes. The new methods and insights provided here are crucial for
next generation model-based applications of luminescence dating in Earth and
environmental sciences, e.g. thermochronometry and photochronometry. | cond-mat_mtrl-sci |
The nano-structural inhomogeneity of dynamic hydrogen bond network of
water: In the present study, water is considered as a dynamic network between
molecules at distances not exceeding 3.2 angstroms. The instantaneous
configurations obtained by using the molecular dynamics method have been
sequentially analyzed, the mutual orientation of each molecule with its
neighboring molecules has been studied and the interaction energy of each pair
of neighbor molecules has been calculated. The majority of mutual orientation
angles between molecules lie in the interval [10, 30] degrees. It has been
shown that more than 85% of the molecular pairs in each instantaneous
configuration form H-bonds and the H-bond network includes all water molecules
in the temperature range 233-293 K. The number of H-bonds fluctuates near the
mean value and increases with decreasing temperature, and the energy of the
vast majority of such bonds is much higher than the thermal energy. The
interaction energy of 80% of the H-bonding molecular pairs lies in the interval
[-7; -4] kcal/mol. The interaction energy of pairs that do not satisfy the
H-bond angle criterion lies in the interval [-5; 4] kcal/mol, and the number of
such bonds does not exceed 15% and decreases with decreasing temperature. For
the first time it was found that in each instantaneous configuration the H-bond
network contains built-in nanometric structural heterogeneities formed by
shorter H-bonds. The fraction of molecules involved in the structural
heterogeneities increases from 40% to 60% with a temperature decrease from 293
K to 233 K. These heterogeneities have a finite lifetime, but are constantly
present in the water. The number of large heterogeneities (containing more than
20 molecules) increases with decreasing temperature, and the number of small
structural heterogeneities (less than 20) decreases. | cond-mat_mtrl-sci |
Digital Twins solve the mystery of Raman spectra of parental and reduced
graphene oxides: A still amazing identity of the D-G doublet Raman spectra of parental and
reduced graphene oxides is considered from the digital twins viewpoint. About
thirty DTs, presenting different aspects of the GO structure and properties,
were virtually synthesized using atomic spin-density algorithm, which allowed
reliably displaying reasons for this extraordinary spectral feature. In both
cases, it was established that the D-G doublets owe their origin to the sp3-sp2
C-C stretchings, respectively. This outwardly similar community of the doublets
origin of GO and rGO is thoroughly analyzed to reveal different grounds of the
feature in the two cases. Multilayer packing of individual rGO molecules in
stacks, in the first case, and spin-influenced prohibition of the 100%
oxidative reaction, the termination of which is accompanied with a particular
set of highly ordered by length sp3- and sp2 C-C bonds, protecting the carbon
carcass from destruction caused by the stress induced sp2-to-sp3
transformation, in the second, are the main reasons. The DT concept has been
realized on the basis of virtual vibrational spectrometer HF Spectrodyn. | cond-mat_mtrl-sci |
Efficient prediction of grain boundary energies from atomistic
simulations via sequential design: Data based materials science is the new promise to accelerate materials
design. Especially in computational materials science, data generation can
easily be automatized. Usually, the focus is on processing and evaluating the
data to derive rules or to discover new materials, while less attention is
being paid on the strategy to generate the data. In this work, we show that by
a sequential design of experiment scheme, the process of generating and
learning from the data can be combined to discover the relevant sections of the
parameter space. Our example is the energy of grain boundaries as a function of
their geometric degrees of freedom, calculated via atomistic simulations. The
sampling of this grain boundary energy space, or even subspaces of it,
represents a challenge due to the presence of deep cusps of the energy, which
are located at irregular intervals of the geometric parameters. Existing
approaches to sample grain boundary energy subspaces therefore either need a
huge amount of datapoints or a~priori knowledge of the positions of these
cusps. We combine statistical methods with atomistic simulations and a
sequential sampling technique and compare this strategy to a regular sampling
technique. We thereby demonstrate that this sequential design is able to sample
a subspace with a minimal amount of points while finding unknown cusps
automatically. | cond-mat_mtrl-sci |
First-principles Engineering of Charged Defects for Two-dimensional
Quantum Technologies: Charged defects in 2D materials have emerging applications in quantum
technologies such as quantum emitters and quantum computation. Advancement of
these technologies requires rational design of ideal defect centers, demanding
reliable computation methods for quantitatively accurate prediction of defect
properties. We present an accurate, parameter-free and efficient procedure to
evaluate quasiparticle defect states and thermodynamic charge transition levels
of defects in 2D materials. Importantly, we solve critical issues that stem
from the strongly anisotropic screening in 2D materials, that have so far
precluded accurate prediction of charge transition levels in these materials.
Using this procedure, we investigate various defects in monolayer hexagonal
boron nitride (h-BN) for their charge transition levels, stable spin states and
optical excitations. We identify $C_BN_V$ (nitrogen vacancy adjacent to carbon
substitution of boron) to be the most promising defect candidate for scalable
quantum bit and emitter applications. | cond-mat_mtrl-sci |
Accurate polarization within a unified Wannier function formalism: We present an alternative formalism for calculating the maximally localized
Wannier functions in crystalline solids, obtaining an expression which is
extremely simple and general. In particular, our scheme is exactly invariant
under Brillouin zone folding, and therefore it extends trivially to the
Gamma-point case. We study the convergence properties of the Wannier functions,
their quadratic spread and centers as obtained by our simplified technique. We
show how this convergence can be drastically improved by a simple and
inexpensive ``refinement'' step, which allows for very efficient and accurate
calculations of the polarization in zero external field. | cond-mat_mtrl-sci |
Self-Organized Graphene/Graphite Structures Obtained Directly on Paper: We experimentally investigated the properties of graphite layers produced by
an easy and non-conventional method of repeatedly rubbing conventional random
stacked graphite bulk against insulating and semiconductor substrates. The
patterned structure composed of rubbed-off and transferred layers exhibits
properties of a solid-state material with through-thickness anisotropy of
carrier mobility reaching ~10^3 cm^2/V*sec at the surface. The surface of the
structure demonstrates quality of more ordered and optically oriented mono or
few layer graphene shaped by self-organization process due to friction.
Enhanced photoconductivity originating from modification of continuous and
linear valence and conduction bands caused by interaction between 4 graphene
layers made possible obtaining Raman spectra at near infrared excitation
wavelength of 976 nm. | cond-mat_mtrl-sci |
Very low bias stress in n-type organic single crystal transistors: Bias stress effects in n-channel organic field-effect transistors (OFETs) are
investigated using PDIF-CN2 single-crystal devices with Cytop gate dielectric,
both under vacuum and in ambient. We find that the amount of bias stress is
very small as compared to all (p-channel) OFETs reported in the literature.
Stressing the PDIF-CN2 devices by applying 80 V to the gate for up to a week
results in a decrease of the source drain current of only ~1% under vacuum and
~10% in air. This remarkable stability of the devices leads to characteristic
time constants, extracted by fitting the data with a stretched exponential -
that are \tau ~ 2\cdot10^9 s in air and \tau ~ 5\cdot10^9 s in vacuum -
approximately two orders of magnitude larger than the best values reported
previously for p-channel OFETs. | cond-mat_mtrl-sci |
Microstructural Effects of Chemical Island Templating in Patterned
Matrix-Pillar Oxide Nanocomposites: The ability to pattern the location of pillars in epitaxial matrix-pillar
nanocomposites is a key challenge to develop future technologies using these
intriguing materials. One such model system employs a ferrimagnetic
CoFe$_{2}$O$_{4}$ (CFO) pillar embedded in a ferroelectric BiFeO$_{3}$ (BFO)
matrix, which has been proposed as a possible memory or logic system. These
composites self-assemble spontaneously with pillars forming through nucleation
at a random location when grown via physical vapor deposition. Recent results
have shown that if an island of the pillar material is pre-patterned on the
substrate, it is possible to control the nucleation process and determine the
locations where pillars form. In this work, we employ electron microscopy and
x-ray diffraction to examine the chemical composition and microstructure of
patterned CFO-BFO nanocomposites. Cross-sectional transmission electron
microscopy is used to examine the nucleation effects at the interface between
the template island and resulting pillar.Evidence of grain boundaries and
lattice tilting in the templated pillars is also presented and attributed to
the microstructure of the seed island. | cond-mat_mtrl-sci |
Submolecular resolution by variation of IETS amplitude and its relation
to AFM/STM signal: Here we show scanning tunnelling microscopy (STM), non-contact atomic force
microscopy (AFM) and inelastic electron tunnelling spectroscopy (IETS)
measurements on organic molecule with a CO- terminated tip at 5K. The
high-resolution contrast observed simultaneously in all channels unam-
biguously demonstrates the common imaging mechanism in STM/AFM/IETS, related to
the lateral bending of the CO-functionalized tip. The IETS spectroscopy reveals
that the submolecular con- trast at 5K consists of both renormalization of
vibrational frequency and variation of the amplitude of IETS signal. This
finding is also corroborated by first principles simulations. We extend accord-
ingly the probe-particle AFM/STM/IETS model to include these two main
ingredients necessary to reproduce the high-resolution IETS contrast. We also
employ the first principles simulations to get more insight into different
response of frustrated translation and rotational modes of CO-tip during
imaging. | cond-mat_mtrl-sci |
Dielectric response of BaTiO3 electronic states under AC fields via
microsecond time-resolved X-ray absorption spectroscopy: For the first time, the dielectric response of a BaTiO3 thin film under an AC
electric field is investigated using time-resolved X-ray absorption
spectroscopy at the Ti K-edge to clarify correlated contributions of each
constituent atom on the electronic states. Intensities of the pre-edge eg peak
and shoulder structure just below the main edge increase with an increase in
the amplitude of the applied electric field, whereas that of the main peak
decreases in an opposite manner. Based on the multiple scattering theory, the
increase and decrease of the eg and main peaks are simulated for different Ti
off-center displacements. Our results indicate that these spectral features
reflect the inter- and intra-atomic hybridization of Ti 3d with O 2p and Ti 4p,
respectively. In contrast, the shoulder structure is not affected by changes in
the Ti off-center displacement but is susceptible to the effect of the corner
site Ba ions. This is the first experimental verification of the dynamic
electronic contribution of Ba to polarization reversal. | cond-mat_mtrl-sci |
Sliding induced multiple polarization states in two-dimensional
ferroelectrics: When the atomic layers in a non-centrosymmetric van der Waals structure slide
against each other, the interfacial charge transfer results in a reversal of
the structures spontaneous polarization. This phenomenon is known as sliding
ferroelectricity and it is markedly different from conventional ferroelectric
switching mechanisms relying on ion displacement. Here, we present layer
dependence as a new dimension to control sliding ferroelectricity. By
fabricating 3R MoS2 of various thicknesses into dual-gate field-effect
transistors, we obtain anomalous intermediate polarization states in multilayer
3R MoS2. Using results from ab initio density functional theory calculations,
we propose a generalized model to describe the ferroelectric switching process
in multilayer 3R MoS2 and to explain the formation of these intermediate
polarization states. This work reveals the critical roles that layer number and
interlayer dipole coupling play in sliding ferroelectricity and presents a new
strategy for the design of novel sliding ferroelectric devices. | cond-mat_mtrl-sci |
Breakdown-limited endurance in HZO FeFETs: mechanism and improvement
under bipolar stress: Breakdown is one of main failure mechanisms that limit write endurance of
ferroelectric devices using hafnium oxide-based ferroelectric materials. In
this study, we investigate the gate current and breakdown characteristics of
Hf0.5Zr0.5O2/Si ferroelectric field-effect transistors (FeFETs) by using
carrier separation measurements to analyze electron and hole leakage currents
during time-dependent dielectric breakdown (TDDB) tests. Rapidly increasing
substrate hole currents and stress-induced leakage current (SILC)-like electron
currents can be observed before the breakdown of the ferroelectric gate
insulator of FeFETs. This apparent degradation under voltage stress is
recovered and the time-to-breakdown is significantly improved by interrupting
the TDDB test with gate voltage pulses with the opposite polarity, suggesting
that defect redistribution, rather than defect generation, is responsible for
the trigger of hard breakdown. | cond-mat_mtrl-sci |
Anomalous electronic conductance in quasicrystals: Generic quantum interference effects occuring in 1D-quasicrystals are
reviewed with emphasis on the joint effect of phason disorder on electronic
localization and propagation modes. In close conjunction with properties of
real materials, the contributions of quantum interferences in several regimes
close to the metal insulator transition are outlined. | cond-mat_mtrl-sci |
Van der Waals density functionals applied to solids: The van der Waals density functional (vdW-DF) of Dion et al. [Phys. Rev.
Lett. 92, 246401 (2004)] is a promising approach for including dispersion in
approximate density functional theory exchange-correlation functionals. Indeed,
an improved description of systems held by dispersion forces has been
demonstrated in the literature. However, despite many applications, standard
general tests on a broad range of materials are lacking. Here we calculate the
lattice constants, bulk moduli, and atomization energies for a range of solids
using the original vdW-DF and several of its offspring. We find that the
original vdW-DF overestimates lattice constants in a similar manner to how it
overestimates binding distances for gas phase dimers. However, some of the
modified vdW functionals lead to average errors which are similar to those of
PBE or better. Likewise, atomization energies that are slightly better than
from PBE are obtained from the modified vdW-DFs. Although the tests reported
here are for "hard" solids, not normally materials for which dispersion forces
are thought to be important, we find a systematic improvement in cohesive
properties for the alkali metals and alkali halides when non-local correlations
are accounted for. | cond-mat_mtrl-sci |
A New Kind of Atlas of Zeolite Building Blocks: We have analysed structural motifs in the Deem database of hypothetical
zeolites, to investigate whether the structural diversity found in this
database can be well-represented by classical descriptors such as distances,
angles, and ring sizes, or whether a more general representation of atomic
structure, furnished by the smooth overlap of atomic positions (SOAP) method,
is required to capture accurately structure-property relations. We assessed the
quality of each descriptor by machine-learning the molar energy and volume for
each hypothetical framework in the dataset. We have found that SOAP with a
cutoff-length of 6 \AA, which goes beyond near-neighbor tetrahedra, best
describes the structural diversity in the Deem database by capturing relevant
inter-atomic correlations. Kernel principal component analysis shows that SOAP
maintains its superior performance even when reducing its dimensionality to
those of the classical descriptors, and that the first three kernel principal
components capture the main variability in the data set, allowing a 3D point
cloud visualization of local environments in the Deem database. This ``cloud
atlas" of local environments was found to show good correlations with the
contribution of a given motif to the density and stability of its parent
framework. Local volume and energy maps constructed from the
SOAP/machine-learning analyses provide new images of zeolites that reveal
smooth variations of local volumes and energies across a given framework, and
correlations between local volume and energy in a given framework. | cond-mat_mtrl-sci |
Comments on All-electron self-consistent GW in the Matsubara-time
domain: Implementation and benchmarks of semiconductors and insulators: Chu et al. recently reported extensive results of local density approximation
(LDA) and of four (4) different Green function and dressed Coulomb (GW)
approximation calculations of electronic properties of several semiconductors
and insulators [Phys. Rev. B 93, 125210 (2016)]. | cond-mat_mtrl-sci |
Two types of magnetic bubbles in MnNiGa observed via Lorentz microscopy: Magnetic bubbles are remarkable spin structures that developed in uniaxial
magnets with strong magnetocrystalline anisotropy. Several contradictory
reports have been published concerning the magnetic bubble structure in a
metallic magnet MnNiGa: Biskyrmions or type-II bubbles. Lorentz microscopy in
polycrystalline MnNiGa was used to explain the magnetic bubble structure.
Depending on the connection between the magnetic easy axis and the observation
plane, two types of magnetic bubbles were formed. Magnetic bubbles with
180{\deg} domains were formed if the easy axis was away from the direction
perpendicular to the observation plane. The contrast of biskyrmion is
reproduced by this form of a magnetic bubble. When the easy axis was
approximately perpendicular to the observing plane, type-II bubbles were
observed in the same specimen. The findings will fill a knowledge gap between
prior reports on magnetic bubbles in MnNiGa. | cond-mat_mtrl-sci |
Ultralow Work Function of the Electride Sr$_3$CrN$_3$: Electrides have valence electrons that occupy free space in the crystal
structure, making them easier to extract. This feature can be used in catalysis
for important reactions that usually requires a high-temperature and
high-pressure environments, such as ammonia synthesis. In this paper, we use
density functional theory to investigate the behaviour of interstitial
electrons of the 1-dimensional electride Sr$_3$CrN$_3$. We find that the bulk
excess electron density persists on introduction of surface terminations, that
the crystal termination perpendicular to the 1D free-electron channel is highly
stable and we confirm an extremely low work function with hybrid functional
methods. Our results indicate that Sr$_3$CrN$_3$ is a potentially important
novel catalyst, with accessible, directional and extractable free electron
density. | cond-mat_mtrl-sci |
Can single crystal X-ray diffraction determine a structure uniquely?: The diffraction technique is widely used in the determination of crystal
structures and is one of the bases for the modern science and technology. All
related structure determination methods are based on the assumption that
perfect single crystal X-ray diffraction (SXRD) can determine a structure
uniquely. But as the structure factor phases are lost in SXRD and even more
information is lost in powder X-ray diffraction (PXRD), this assumption is
still questionable. In this work, we found that structures with certain
characteristic can have its sister structure with exactly the same PXRD or even
SXRD pattern. A computer program is developed to search the ICSD database, and
about 1000 structures were identified to have this characteristic. The original
structure and its sister structures can have different space groups,
topologies, crystal systems etc. and some may even have multiple sisters. Our
studies indicate that special caution is needed since a structure with
reasonable atomic positions and perfect match of experimental diffraction
intensities could still be wrong. | cond-mat_mtrl-sci |
Next-generation non-local van der Waals density functional: The fundamental ideas for a non-local density functional theory -- capable of
reliably capturing van der Waals interaction -- were already conceived in the
1990's. In 2004, a seminal paper introduced the first practical non-local
exchange-correlation functional called vdW-DF, which has become widely
successful and laid the foundation for much further research. However, since
then, the functional form of vdW-DF has remained unchanged. Several successful
modifications paired the original functional with different (local) exchange
functionals to improve performance and the successor vdW-DF2 also updated one
internal parameter. Bringing together different insights from almost two
decades of development and testing, we present the next-generation non-local
correlation functional called vdW-DF3, in which we change the functional form
while staying true to the original design philosophy. Although many popular
functionals show good performance around the binding separation of van der
Waals complexes, they often result in significant errors at larger separations.
With vdW-DF3, we address this problem by taking advantage of a recently
uncovered and largely unconstrained degree of freedom within the vdW-DF
framework that can be constrained through empirical input, making our
functional semi-empirical. For two different parameterizations, we benchmark
vdW-DF3 against a large set of well-studied test cases and compare our results
with the most popular functionals, finding good performance in general for a
wide array of systems and a significant improvement in accuracy at larger
separations. Finally, we discuss the achievable performance within the current
vdW-DF framework, the flexibility in functional design offered by vdW-DF3, as
well as possible future directions for non-local van der Waals density
functional theory. | cond-mat_mtrl-sci |
Effect of acetylene links on electronic and optical properties of
semiconducting graphynes: The family of graphynes, novel two-dimensional semiconductors with various
and fascinating chemical and physical properties, has attracted great interest
from both science and industry. Currently, the focus of graphynes is on
graphdiyne, or graphyne-2. In this work, we systematically study the effect of
acetylene, i.e., carbon-carbon triple bond, links on the electronic and optical
properties of a series of graphynes (graphyne-n, where n = 1-5, the number of
acetylene bonds) using the ab initio calculations. We find an even-odd pattern,
i.e., n = 1, 3, 5 and n = 2, 4 having different features, which has not be
discovered in studying graphyne or graphdyine only. It is found that as the
number of acetylene bonds increases, the electron effective mass increases
continuously in the low energy range because of the flatter conduction band
induced by the longer acetylene links. Meanwhile, longer acetylene links result
in larger redshift of the imaginary part of the dielectric function, loss
function, and extinction coefficient. In this work, we propose an effective
method to tune and manipulate both the electronic and optical properties of
graphynes for the applications in optoelectronic devices and photo-chemical
catalysis. | cond-mat_mtrl-sci |
Quasi-Two-Dimensional Extraordinary Hall Effect: Quasi-two-dimensional transport is investigated in a system consisting of one
ferromagnetic layer placed between two insulating layers. Using the mechanism
of skew-scattering to describe the Extraordinary Hall Effect (EHE) and
calculating the conductivity tensor, we compare the quasi- two-dimensional Hall
resistance with the resistance of a massive sample. In this study a new
mechanism of EHE (geometric mechanism of EHE) due to non-ideal interfaces and
volume defects is also proposed. | cond-mat_mtrl-sci |
Thermal Conductivity and Mechanical Properties of Nitrogenated Holey
Graphene: Nitrogenated holey graphene (NHG), a two-dimensional graphene-derived
material with a C2N stoichiometry and evenly distributed holes and nitrogen
atoms in its basal plane, has recently been synthesized. We performed first
principles calculations and molecular dynamics simulations to investigate
mechanical and heat transport properties of this novel two-dimensional material
at various temperatures. First principles calculations based on density
functional theory yield an elastic modulus of 400 +/- 5 GPa at 0 K, 10% larger
than predicted by molecular dynamics simulations at low temperatures. We
observed an overall decreasing trend in elastic modulus and tensile strength as
temperature increases. At room temperature, we found that NHG can present a
remarkable elastic modulus of 335 +/- 5 GPa and tensile strength of 60 GPa. We
also investigated the thermal conductivity of NHG via non-equilibrium molecular
dynamics simulations. At 300 K an intrinsic thermal conductivity of 64.8 W/m-K
was found, with an effective phonon mean free path of 34.0 nm, both of which
are smaller than respective values for graphene, and decrease with temperature.
Our modeling-based predictions should serve as guide to experiments concerning
physical properties of this novel material. | cond-mat_mtrl-sci |
Complete characterization of the macroscopic deformations of periodic
unimode metamaterials of rigid bars and pivots: A complete characterization is given of the possible macroscopic deformations
of periodic nonlinear affine unimode metamaterials constructed from rigid bars
and pivots. The materials are affine in the sense that their macroscopic
deformations can only be affine deformations: on a local level the deformation
may vary from cell to cell. Unimode means that macroscopically the material can
only deform along a one dimensional trajectory in the six dimensional space of
invariants describing the deformation (excluding translations and rotations).
We show by explicit construction that any continuous trajectory is realizable
to an arbitrarily high degree of approximation provided at all points along the
trajectory the geometry does not collapse to a lower dimensional one. In
particular, we present two and three dimensional dilational materials having an
arbitrarily large flexibility window. These are perfect auxetic materials for
which a dilation is the only easy mode of deformation. They are free to dilate
to arbitrarily large strain with zero bulk modulus. | cond-mat_mtrl-sci |
The rise of Single-Atom Catalysts: In recent years, single-atom catalysts attracted lots of attention because of
their high catalytic activity, selectivity, stability, maximum atom
utilization, exceptional performance, and low cost. Single-atom catalyst
contains isolated individual atom which are coordinated with the surface atoms
of support such as a metal oxide or 2d - materials. In this review article, we
present the advancement in single-atom catalysis in recent years with a focus
on the various synthesis methods and their application in catalytic reactions.
We also demonstrate the reaction mechanism of a single-atom catalyst for
different catalytic reactions from theoretical aspects using density functional
theory. | cond-mat_mtrl-sci |
GPU Acceleration of Large-Scale Full-Frequency GW Calculations: Many-body perturbation theory is a powerful method to simulate electronic
excitations in molecules and materials starting from the output of density
functional theory calculations. By implementing the theory efficiently so as to
run at scale on the latest leadership high-performance computing systems it is
possible to extend the scope of GW calculations. We present a GPU acceleration
study of the full-frequency GW method as implemented in the WEST code.
Excellent performance is achieved through the use of (i) optimized GPU
libraries, e.g., cuFFT and cuBLAS, (ii) a hierarchical parallelization strategy
that minimizes CPU-CPU, CPU-GPU, and GPU-GPU data transfer operations, (iii)
nonblocking MPI communications that overlap with GPU computations, and (iv)
mixed-precision in selected portions of the code. A series of performance
benchmarks have been carried out on leadership high-performance computing
systems, showing a substantial speedup of the GPU-accelerated version of WEST
with respect to its CPU version. Good strong and weak scaling is demonstrated
using up to 25920 GPUs. Finally, we showcase the capability of the GPU version
of WEST for large-scale, full-frequency GW calculations of realistic systems,
e.g., a nanostructure, an interface, and a defect, comprising up to 10368
valence electrons. | cond-mat_mtrl-sci |
Hydrogen trapping and embrittlement in high-strength Al-alloys: Ever more stringent regulations on greenhouse gas emissions from
transportation motivate efforts to revisit materials used for vehicles.
High-strength Al-alloys often used in aircrafts could help reduce the weight of
automobiles, but are susceptible to environmental degradation. Hydrogen (H)
"embrittlement" is often pointed as the main culprit, however, the mechanisms
underpinning failure are elusive: atomic-scale analysis of H inside an alloy
remains a challenge, and this prevents deploying alloy design strategies to
enhance the materials' durability. Here we successfully performed near-atomic
scale analysis of H trapped in second-phase particles and at grain boundaries
in a high-strength 7xxx Al-alloy. We used these observations to guide atomistic
ab-initio calculations which show that the co-segregation of alloying elements
and H favours grain boundary decohesion, while the strong partitioning of H
into the second-phases removes solute H from the matrix, hence preventing
H-embrittlement. Our insights further advance the mechanistic understanding of
H-assisted embrittlement in Al-alloys, emphasizing the role of H-traps in
retarding cracking and guiding new alloy design. | cond-mat_mtrl-sci |
Magnetic behavior of a spin-1 dimer: model system for homodinuclear
nickel (II) complexes: Magnetic behavior of a spin-1 Heisenberg dimer is analysed in dependence on
both uniaxial single-ion anisotropy and XXZ exchange anisotropy in a zero- as
well as non-zero longitudinal magnetic field. A complete set of eigenfunctions
and eigenvalues of the total Hamiltonian is presented together with an exact
analytical expression for the Gibbs free energy, longitudinal magnetization,
longitudinal and transverse susceptibility. The obtained theoretical results
are compared with the relevant experimental data of
[Ni2(Medpt)2(ox)(H2O)2](ClO4)2.2H2O (Medpt = methyl-bis(3-aminopropyl)amine). | cond-mat_mtrl-sci |
Electronic and energetic properties of Ge(110) pentagons: The electronic and energetic properties of the elementary building block,
i.e. a five-membered atom ring (pentagon), of the Ge(110) surface was studied
by scanning tunneling microscopy and spectroscopy at room temperature. The
Ge(110) surface is composed of three types of domains: two ordered domains
((16x2) and c(8x10)) and a disordered domain. The elementary building block of
all three domains is a pentagon. Scanning tunneling spectra recorded on the
(16x2), c(8x10) and disordered domains are very similar and reveal three
well-defined electronic states. Two electronic states are located 1.1 eV and
0.3 eV below the Fermi level respectively, whereas the third electronic state
is located 0.4 eV above the Fermi level. The electronic states at -0.3 eV and
0.4 eV can be ascribed to the pentagons, whilst we tentatively assigned the
electronic state at -1.1 eV to a Ge-Ge back bond or trough state. In addition,
we have analyzed the straight [1-12] oriented step edges. From the kink density
and kink-kink distance distributions we extracted the nearest neighbor
interaction energy between the pentagons, which exhibit a strong preference to
occur in twins, as well as the strain relaxation energy along the pentagon-twin
chains. | cond-mat_mtrl-sci |
Traversal of pulses through negative ($\varepsilon$, $μ$) materials: We study the traversal times of electromagnetic pulses across dispersive
media with negative dielectric permittivity ($\varepsilon$) and magnetic
permeability ($\mu$) parameters. First we investigate the transport of optical
pulses through an electrical plasma and a negative refractive index medium
(NRM) of infinite and semi-infinite extents where no resonant effects come into
play. The total delay time of the pulse constitutes of the group delay time and
the reshaping delay time as analyzed by Peatross et al \cite{peatross}. For
evanescent waves, even with broadband width, the total delay time is negative
for an infinite medium whereas it is positive for the semi-infinite case.
Evidence of the Hartman effect is seen for small propagation distance compared
to the free space pulse length. The reshaping delay mostly dominates the total
delay time in NRM whereas it vanishes when $\varepsilon(\omega)=\mu(\omega)$.
Next we present results on the propagation times through a dispersive slab.
While both large bandwidth and large dissipation have similar effects in
smoothening out the resonant features that appear due to Fabry-P\'{e}rot
resonances, large dissipation can result in very small or even negative
traversal times near the resonant frequencies. We investigate the traversal and
the Wigner delay times for obliquely incident pulses. The coupling of
evanescent waves to slab plasmon polariton modes results in large traversal
times at the resonant conditions. We also find that the group velocity mainly
contributes to the delay time for pulse propagating across a slab with
refractive index (n) = -1. The traversal times are positive and subluminal for
pulses with sufficiently large bandwidths. | cond-mat_mtrl-sci |
Elastic properties and mechanical stability of bilayer graphene:
Molecular dynamics simulations: Graphene has become in last decades a paradigmatic example of two-dimensional
and so-called van-der-Waals layered materials, showing large anisotropy in
their physical properties. Here we study the elastic properties and mechanical
stability of graphene bilayers in a wide temperature range by molecular
dynamics simulations. We concentrate on in-plane elastic constants and
compression modulus, as well as on the atomic motion in the out-of-plane
direction. Special emphasis is placed upon the influence of anharmonicity of
the vibrational modes on the physical properties of bilayer graphene. We
consider the excess area appearing in the presence of ripples in graphene
sheets at finite temperatures. The in-plane compression modulus of bilayer
graphene is found to decrease for rising temperature, and results to be higher
than for monolayer graphene. We analyze the mechanical instability of the
bilayer caused by an in-plane compressive stress. This defines a spinodal
pressure for the metastability limit of the material, which depends on the
system size. Finite-size effects are described by power laws for the
out-of-plane mean-square fluctuation, compression modulus, and spinodal
pressure. Further insight into the significance of our results for bilayer
graphene is gained from a comparison with data for monolayer graphene and
graphite. | cond-mat_mtrl-sci |
New ultrahigh pressure phases of H2O ice predicted using an adaptive
genetic algorithm: We propose three new phases of H2O under ultrahigh pressure. Our structural
search was performed using an adaptive genetic algorithm which allows an
extensive exploration of crystal structure. The new sequence of
pressure-induced transitions beyond ice X at 0 K should be ice X - Pbcm - Pbca
- Pmc21 - P21 - P21/c phases. Across the Pmc21 - P21 transition, the
coordination number of oxygen increases from 4 to 5 with a significant increase
of density. All stable crystalline phases have nonmetallic band structures up
to 7 TPa. | cond-mat_mtrl-sci |
Low temperature investigations and surface treatments of colloidal
narrowband fluorescent nanodiamonds: We report fluorescence investigations and Raman spectroscopy on colloidal
nanodiamonds (NDs) obtained via bead assisted sonic disintegration (BASD) of a
polycrystalline chemical vapor deposition film. The BASD NDs contain in situ
created silicon vacancy (SiV) centers. Whereas many NDs exhibit emission from
SiV ensembles, we also identify NDs featuring predominant emission from a
single bright SiV center. We demonstrate oxidation of the NDs in air as a tool
to optimize the crystalline quality of the NDs via removing damaged regions
resulting in a reduced ensemble linewidth as well as single photon emission
with increased purity. We furthermore investigate the temperature dependent
zero-phonon-line fine-structure of a bright single SiV center as well as the
polarization properties of its emission and absorption. | cond-mat_mtrl-sci |
Detecting Chiral Orbital Angular Momentum by Circular Dichroism ARPES: We show, by way of tight-binding and first-principles calculations, that a
one-to-one correspondence between electron's crystal momentum k and non-zero
orbital angular momentum (OAM) is a generic feature of surface bands. The OAM
forms a chiral structure in momentum space much as its spin counterpart in
Rashba model does, as a consequence of the inherent inversion symmetry breaking
at the surface but not of spin-orbit interaction. Circular dichroism (CD)
angle-resolved photoemission (ARPES) experiment is an efficient way to detect
this new order, and we derive formulas explicitly relating the CD-ARPES signal
to the existence of OAM in the band structure. The cases of degenerate p- and
d-orbital bands are considered. | cond-mat_mtrl-sci |
Nano-thermodynamics of chemically induced graphene-diamond
transformation: Nearly two-dimensional diamond, or diamane, is coveted as ultrathin
$sp^3$-carbon film with unique mechanics and electro-optics. The very thinness
($~h$) makes it possible for the surface chemistry, e.g. adsorbed atoms, to
shift the bulk phase thermodynamics in favor of diamond, from multilayer
graphene. Thermodynamic theory coupled with atomistic first principles
computations predicts not only the reduction of required pressure
($p/p_{\infty}>1-h_0/h$), but also the nucleation barriers, definitive for the
kinetic feasibility of diamane formation. Moreover, the optimal adsorbent
chair-pattern on a bilayer graphene results in a cubic diamond lattice, while
for thicker precursors the adsorbent boat-structure tends to produce hexagonal
diamond (lonsdaleite), if graphene was in AA` stacking to start with. As
adsorbents, H and F are conducive to diamond formation, while Cl appears
sterically hindered. | cond-mat_mtrl-sci |
Microscopic theory of ionic motion in solid electrolytes: We propose a microscopic, first-principles description of the ionic
conduction in crystals. This formalism allows us to gain new insights into the
ideal characteristics of general ionic conducting materials and, in particular,
solid electrolytes. Using \textit{ab initio} calculations, we show that our
formalism results in ionic mobilities consistent with experiments for several
materials. Our work opens the possibility of developing solid electrolytes
based on fundamental physical principles rather than empirical descriptions of
the underlying processes. | cond-mat_mtrl-sci |
Limitations of ab initio methods to predict the electronic-transport
properties of two-dimensional materials: The computational example of
2H-phase transition metal dichalcogenides: Over the last few years, $ab~initio$ methods have become an increasingly
popular tool to evaluate intrinsic carrier transport properties in 2D
materials. The lack of experimental information, and the progress made in the
development of DFT tools to evaluate electronic band structures, phonon
dispersions, and electron-phonon scattering matrix-elements, have made them a
favored choice. However, a large discrepancy is observed in the literature
among the $ab~initio$ calculated carrier mobility in 2D materials. Some of the
discrepancies are a result of the physical approximations made in calculating
the electron-phonon coupling constants and the carrier mobility. These
approximations can be avoided by using a sophisticated transport model.
However, despite using appropriate transport models, the uncertainty in the
reported carrier mobility is still quite large in some materials. The major
differences observed between these refined model calculations are the `flavors'
of DFT (exchange-correlation functional, pseudopotential, and the effect of
spin-orbit coupling) used. Here, considering several monolayer 2H-TMDs as
examples, we calculate the low- and high-field transport properties using
different `flavors' of DFT, and calculate a range for the electron mobility
values. We observe that in some materials the values differ by orders of
magnitude (For example, in monolayer WS$_{2}$ the electron low-field mobility
varies between 37 cm$^{2}$/(V$\cdot$s) and 767 cm$^{2}$/(V$\cdot$s)). We
analyze critically these discrepancies, and try to understand the limitations
of the current $ab~initio$ methods in calculating carrier transport properties. | cond-mat_mtrl-sci |
Magnetic kagome materials RETi3Bi4 family with weak interlayer
interactions: Kagome materials have attracted a surge of research interest recently,
especially for the ones combining with magnetism, and the ones with weak
interlayer interactions which can fabricate thin devices. However, kagome
materials combining both characters of magnetism and weak interlayer
interactions are rare. Here we investigate a new family of titanium based
kagome materials RETi3Bi4 (RE = Eu, Gd and Sm). The flakes of nanometer
thickness of RETi3Bi4 can be obtained by exfoliation due to the weak interlayer
interactions. According to magnetic measurements, out-of-plane ferromagnetism,
out-of-plane anti-ferromagnetism, and in-plane ferromagnetism are formed for RE
= Eu, Gd, and Sm respectively. The magnetic orders are simple and the
saturation magnetizations can be relatively large since the rare earth elements
solely provide the magnetic moments. Further by angle-resolved photoemission
spectroscopy (ARPES) and first-principles calculations, the electronic
structures of RETi3Bi4 are investigated. The ARPES results are consistent with
the calculations, indicating the bands characteristic with kagome sublattice in
RETi3Bi4. We expect these materials to be promising candidates for observation
of the exotic magnetic topological phases and the related topological quantum
transport studies. | cond-mat_mtrl-sci |
Electronic Janus lattice and kagome-like bands in coloring-triangular
MoTe2 monolayers: Polymorphic structures of transition-metal dichalcogenides (TMDs) host exotic
electronic states, like charge density wave and superconductivity. However, the
number of these structures is limited by crystal symmetries, which poses a
challenge to achieve tailored lattices and properties both theoretically and
experimentally. Here, we report a coloring-triangle (CT) latticed MoTe2
monolayer, termed CT-MoTe2, constructed by controllably introducing uniform and
ordered mirror-twin-boundaries into a pristine monolayer in molecular beam
epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy
(STM/STS) together with theoretical calculations reveal that the monolayer has
an electronic Janus lattice, i.e., an energy-dependent atomic-lattice and a
pseudo-Te sublattice, and shares the identical geometry with the Mo5Te8 layer.
Dirac-like and flat electronic bands inherently existing in the CT lattice are
identified by two broad and two prominent peaks in STS spectra, respectively,
and verified with density-functional-theory calculations. Two types of
intrinsic domain boundaries were observed, in one of which the
electronic-Janus-lattice feature maintains, implying potential applications as
an energy-tunable electron-tunneling barrier in future functional devices. | cond-mat_mtrl-sci |
Pressure tuning the Fermi-surface topology of the Weyl semimetal NbP: We report on the pressure evolution of the Fermi surface topology of the Weyl
semimetal NbP, probed by Shubnikov-de Haas oscillations in the
magnetoresistance combined with ab-initio calculations of the band-structure.
Although we observe a drastic effect on the amplitudes of the quantum
oscillations, the frequencies only exhibit a weak pressure dependence up to 2.8
GPa. The pressure-induce variations in the oscillation frequencies are
consistent with our band-structure calculations. Furthermore, we can relate the
changes in the amplitudes to small modifications in the shape of the Fermi
surface. Our findings evidenced the stability of the electronic band structure
of NbP and demonstrate the power of combining quantum-oscillation studies and
band-structure calculations to investigate pressure effects on the
Fermi-surface topology in Weyl semimetals. | cond-mat_mtrl-sci |
Recent advances in the internal functionalization of carbon nanotubes:
synthesis, optical, and magnetic resonance studies: The hollow inside of single-wall carbon nanotubes (SWCNT) provides a unique
degree of freedom to investigate chemical reactions inside this confined
environment and to study the tube properties. It is reviewed herein, how
encapsulating fullerenes, magnetic fullerenes, $^{13}$C isotope enriched
fullerenes and organic solvents inside SWCNTs enables to yield unprecedented
insight into their electronic, optical, and interfacial properties and to study
their growth. Encapsulated C$_{60}$ fullerenes are transformed to inner tubes
by a high temperature annealing. The unique, low defect concentration of inner
tubes makes them ideal to study the effect of diameter dependent treatments
such as opening and closing of the tubes. The growth of inner tubes is achieved
from $^{13}$C enriched encapsulated organic solvents, which shows that
fullerenes do not have a distinguished role and it opens new perspectives to
explore the in-the-tube chemistry. Encapsulation of magnetic fullerenes, such
as N@C$_{60}$ and C$_{59}$N is demonstrated using ESR. Growth of inner tubes
from $^{13}$C enriched fullerenes provides a unique isotope engineered
heteronuclear system, where the outer tubes contain natural carbon and the
inner walls are controllably $^{13}$C isotope enriched. The material enables to
identify the vibrational modes of inner tubes which otherwise strongly overlap
with the outer tube modes. The $^{13}$C NMR signal of the material is specific
for the small diameter SWCNTs. Temperature and field dependent $^{13}$C $T_1$
studies show a uniform metallic-like electronic state for all inner tubes and a
low energy, ~3 meV gap is observed that is assigned to a long sought Peierls
transition. | cond-mat_mtrl-sci |
Native point defects in few-layer phosphorene: Using hybrid density functional theory combined with a semiempirical van der
Waals dispersion correction, we have investigated the structural and electronic
properties of vacancies and self-interstitials in defective few-layer
phosphorene. We find that both a vacancy and a self-interstitial defect are
more stable in the outer layer than in the inner layer. The formation energy
and transition energy of both a vacancy and a self-interstitial P defect
decrease with increasing film thickness, mainly due to the upward shift of the
host valence band maximum in reference to the vacuum level. Consequently, both
vacancies and self-interstitials could act as shallow acceptors, and this well
explains the experimentally observed p-type conductivity in few-layer
phosphorene. On the other hand, since these native point defects have moderate
formation energies and are stable in negatively charged states, they could also
serve as electron compensating centers in n-type few-layer phosphorene. | cond-mat_mtrl-sci |
${\it Ab\ initio}$ thermodynamic properties of certain compounds in
Nd-Fe-B system: In this work, we report the results of \emph{ab initio} calculations of
thermochemical properties of several compounds in the Fe-Nd, B-Nd and B-Fe-Nd
systems. We have performed DFT+U calculations to compute the enthalpy of
formation of the compounds NdB$_6$, NdB$_4$, Nd$_2$B$_5$, Nd$_2$Fe$_{17}$ and
Nd$_5$Fe$_2$B$_6$. It was found that the values obtained with an effective
Hubbard $U$ correction have better agreement with the experimental data. We
have also computed the vibrational contribution to the heat capacity ($C_p$) of
the compounds as a function of temperature was computed using the quasharmonic
approximation. For most of the compounds these properties have not been
experimentally determined until now. Hence, the computed \emph{ab initio}
thermodynamic properties will serve as useful input for the Gibbs energy model
parameter assessment using the CALPHAD method. | cond-mat_mtrl-sci |
Jacutingaite-family: a class of topological materials: Jacutingate, a recently discovered Brazilian naturally occurring mineral, has
shown to be the first experimental realization of the Kane-Mele topological
model. In this letter we have unveiled a class of materials $M_2NX_3$ ($M$=Ni,
Pt, Pd; $N$=Zn, Cd, Hg; and $X$=S, Se, Te), sharing jacutingaite's key
features, i.e., high stability, and topological phase. By employing
first-principles calculations we extensively characterize the energetic
stability of this class while showing a common occurrence of the Kane-Mele
topological phase. Here we found Pt-based materials surpassing jacutingaite's
impressive topological gap and lower exfoliation barrier while retaining its
stability. | cond-mat_mtrl-sci |
Challenges for density functional theory in simulating metal-metal
singlet bonding: a case study of dimerized VO2: VO2 is renowned for its electric transition from an insulating monoclinic
(M1) phase characterized by V-V dimerized structures, to a metallic rutile (R)
phase above 340 Kelvin. This transition is accompanied by a magnetic change:
the M1 phase exhibits a non-magnetic spin-singlet state, while the R phase
exhibits a state with local magnetic moments. Simultaneous simulation of the
structural, electric, and magnetic properties of this compound is of
fundamental importance, but the M1 phase alone has posed a significant
challenge to density functional theory (DFT). In this study, we show none of
the commonly used DFT functionals, including those combined with on-site
Hubbard U to better treat 3d electrons, can accurately predict the V-V dimer
length. The spin-restricted method tends to overestimate the strength of the
V-V bonds, resulting in a small V-V bond length. Conversely, the
spin-symmetry-breaking method exhibits the opposite trends. Each
bond-calculation method underscores one of the two contentious mechanisms,
i.e., Peierls or Mott, involved in the metal-insulator transition in VO2. To
elucidate the challenges encountered in DFT, we also employ an effective
Hamiltonian that integrates one-dimensional magnetic sites, thereby revealing
the inherent difficulties linked with the DFT computations. | cond-mat_mtrl-sci |
An ab-initio study on physical properties of Pd2+ incorporated double
perovskites CaPd3B4O12 (B = Ti, V): Numerous physical properties of CaPd3Ti4O12 (CPTO) and CaPd3V4O12 (CPVO)
double perovskites have been explored based on density functional theory (DFT).
The calculated structural parameters fairly agree with the experimental data to
confirm their stability. The mechanical stability of these two compounds was
clearly observed by the Born stability criteria. To rationalize the mechanical
behavior, we investigate elastic constants, bulk, shear and Young's modulus,
Pugh's ratio, Poisson's ratio and elastic anisotropy index. The ductility index
confirms that both materials are ductile in nature. The electronic band
structure of CPTO and CPVO reveals the direct band gap semiconducting in nature
and metallic characteristics, respectively. The calculated partial density of
states indicates the strong hybridization between Pd 4d and O 2p orbital
electrons for CPTO and Pd 4d and V 3d O 2p for CPVO. The study of electronic
charge density map confirms the coexistence of covalent, ionic and metallic
bonding for both compounds. Fermi surface calculation of CPVO ensures both
electron and hole like surfaces indicating the multiple band nature. In the
midst of optical properties, photoconductivity and absorption coefficient of
both compounds reveal well qualitative compliance with consequences of band
structure computations. Among the thermodynamic properties, the Debye
temperature has been calculated to correlate its topical features including
thermoelectric behavior. The studied thermoelectric transport properties of
CPTO yielded the Seebeck coefficient (186 microVK-1), power factor (11.9
microWcm-1K-2) and figure of merit (ZT) value of about 0.8 at 800 K indicate
that this material could be a promising candidate for thermoelectric device
application. | cond-mat_mtrl-sci |
Study of anharmonicity in Zirconium Hydrides using inelastic neutron
scattering and ab-initio computer modeling: The anharmonic phenomena in Zirconium Hydrides and Deuterides, including
{\epsilon}-ZrH2, {\gamma}-ZrH, and {\gamma}-ZrD, have been investigated from
aspects of inelastic neutron scattering (INS) and lattice dynamics calculations
within the framework of density functional theory (DFT). The observed multiple
sharp peaks below harmonic multi-phonon bands in the experimental spectra of
all three materials did not show up in the simulated INS spectra based on the
harmonic approximation, indicating the existence of strong anharmonicity in
those materials and the necessity of further explanations. We present a
detailed study on the anharmonicity of zirconium hydrides/deuterides by
exploring the 2D potential energy surface of hydrogen/deuterium atoms, and
solving the corresponding 2D single-particle Schrodinger equation to get the
eigenfrequencies. The obtained results well describe the experimental INS
spectra and show harmonic behavior in the fundamental modes and strong
anharmonicity at higher energies. | cond-mat_mtrl-sci |
Spin wavepackets in the Kagome ferromagnet Fe$_3$Sn$_2$: propagation and
precursors: The propagation of spin waves in magnetically ordered systems has emerged as
a potential means to shuttle quantum information over large distances.
Conventionally, the arrival time of a spin wavepacket at a distance, $d$, is
assumed to be determined by its group velocity, $v_g$. He we report
time-resolved optical measurements of wavepacket propagation in the Kagome
ferromagnet Fe$_3$Sn$_2$ that demonstrate the arrival of spin information at
times significantly less than $d/v_g$. We show that this spin wave "precursor"
originates from the interaction of light with the unusual spectrum of
magnetostatic modes in Fe$_3$Sn$_2$. Related effects may have far-reaching
consequences toward realizing long-range, ultrafast spin wave transport in both
ferromagnetic and antiferromagnetic systems. | cond-mat_mtrl-sci |
Controllable thickness inhomogeneity and Berry-curvature-engineering of
anomalous Hall effect in SrRuO3 ultrathin films: In quantum matters hosting electron-electron correlation and spin-orbit
coupling, spatial inhomogeneities, arising from competing ground states, can be
essential for determining and understanding topological properties. A prominent
example is Hall anomalies observed in SrRuO3 films, which were interpreted in
terms of either magnetic skyrmion-induced topological Hall effect (THE) or
inhomogeneous anomalous Hall effect (AHE). To clarify this ambiguity, we
systematically investigated the AHE of SrRuO3 ultrathin films with controllable
inhomogeneities in film thickness (tSRO). By harnessing the step-flow growth of
SrRuO3 films, we induced microscopically-ordered stripes with one-unit-cell
differences in tSRO. The resultant spatial distribution of momentum-space Berry
curvatures enables a two-channel AHE, which shows hump-like anomalies similar
to the THE and can be continuously engineered via sub-unit-cell control of
tSRO. In these inhomogeneous SRO films, we microscopically identified a
two-step magnetic switching and stripe-like ferromagnetic domains. These
features are fingerprints for distinguishing the two-channel AHE from the
skyrmion-induced THE. | cond-mat_mtrl-sci |
Simple prediction of immiscible metal alloying based on metastability
analysis: It has been known that even though two elemental metals, $X$ and $Y$, are
immiscible, they can form alloys on surfaces of other metal $Z$. In order to
understand such surface alloying of immiscible metals, we study the energetic
stability of binary alloys, $XZ$ and $YZ$, in several structures with various
coordination numbers (CNs). By analyzing the formation energy modified to
enhance the subtle energy difference between metastable structures, we find
that $XZ$ and $YZ$ with B2-type structure (CN$=$8) become energetically stable
when the $X$ and $Y$ metals form an alloy on the $Z$ metal surface. This is
consistent with the experimental results for Pb-Sn alloys on metal surfaces
such as Rh(111) and Ru(0001). Some suitable metal substrates are also predicted
to form Pb-Sn alloys. | cond-mat_mtrl-sci |
Melting of graphene: from two to one dimension: The high temperature behaviour of graphene is studied by atomistic
simulations based on an accurate interatomic potential for carbon. We find that
clustering of Stone-Wales defects and formation of octagons are the first steps
in the process of melting which proceeds via the formation of carbon chains.
The molten state forms a three-dimensional network of entangled chains rather
than a simple liquid. The melting temperature estimated from the
two-dimensional Lindemann criterion and from extrapolation of our simulation
for different heating rates is about 4900 K. | cond-mat_mtrl-sci |
Exploring conformational energy landscape of glassy disaccharides by
CPMAS 13C NMR and DFT/GIAO simulations. II. Enhanced molecular flexibility in
amorphous trehalose: This paper deals with the comparative use of the chemical shift surfaces to
simulate experimental 13C CPMAS data on amorphous solid state disaccharides,
paying particular attention to -1-1 linkage of trehalose, to
-1,4 linkage between pyranose rings (lactose) and to linkage implying a
furanose ring (sucrose). The combination of molecular mechanics with DFT/GIAO
ab-initio methods provides reliable structural information on the
conformational distribution in the glass. The results are interpreted in terms
of an enhanced flexibility that trehalose experiences in amorphous solid state
compared to the other sugars. An attempt to relate this property to the balance
between intra- and inter-molecular hydrogen bonding network in the glass is
presented. | cond-mat_mtrl-sci |
Topological Crystalline Insulator Nanostructures: Topological crystalline insulators are topological insulators whose surface
states are protected by the crystalline symmetry, instead of the time reversal
symmetry. Similar to the first generation of three-dimensional topological
insulators such as Bi2Se3 and Bi2Te3, topological crystalline insulators also
possess surface states with exotic electronic properties such as spin-momentum
locking and Dirac dispersion. Experimentally verified topological crystalline
insulators to date are SnTe, Pb1-xSnxSe, and Pb1-xSnxTe. Because topological
protection comes from the crystal symmetry, magnetic impurities or in-plane
magnetic fields are not expected to open a gap in the surface states in
topological crystalline insulators. Additionally, because they are cubic
structure instead of layered structure, branched structures or strong coupling
with other materials for large proximity effects are possible, which are
difficult with layered Bi2Se3 and Bi2Te3. Thus, additional fundamental
phenomena inaccessible in three-dimensional topological insulators can be
pursued. In this review, topological crystalline insulator SnTe nanostructures
will be discussed. For comparison, experimental results based on SnTe thin
films will be covered. Surface state properties of topological crystalline
insulators will be discussed briefly. | cond-mat_mtrl-sci |
Phase locking of vortex based spin transfer oscillators to a microwave
current: Phase locking experiments on vortex based spin transfer oscillators with an
external microwave current are performed. We present clear evidence of phase
locking, frequency pulling, as well as fractional synchronization in this
system, with a minimum peak linewidth of only 3 kHz in the locked state. We
find that locking ranges of the order of 1/3 of the oscillator frequency are
easily achievable because of the large tunability $\partial f/\partial I_{dc}$
observed in our vortex based systems. Such large locking ranges allow us to
demonstrate the simultaneous phase locking of two independent oscillators
connected in series with the external source. | cond-mat_mtrl-sci |
Quasi-equilibrium optical nonlinearities in spin-polarized GaAs: Semiconductor Bloch equations, which microscopically describe the dynamics of
a Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in
two limits: the coherent and the quasi-equilibrium regime. These equations have
been recently extended to include the spin degree of freedom, and used to
explain spin dynamics in the coherent regime. In the quasi-equilibrium limit,
one solves the Bethe-Salpeter equation in a two-band model to describe how
optical absorption is affected by Coulomb interactions within a
spin-unpolarized plasma of arbitrary density. In this work, we modified the
solution of the Bethe-Salpeter equation to include spin-polarization and light
holes in a three-band model, which allowed us to account for spin-polarized
versions of many-body effects in absorption. The calculated absorption
reproduced the spin-dependent, density-dependent and spectral trends observed
in bulk GaAs at room temperature, in a recent pump-probe experiment with
circularly polarized light. Hence our results may be useful in the microscopic
modelling of density-dependent optical nonlinearities in spin-polarized
semiconductors. | cond-mat_mtrl-sci |
Second-phase nucleation on an edge dislocation: A model for nucleation of second phase at or around dislocation in a
crystalline solid is considered. The model employs the Ginzburg-Landau theory
of phase transition comprising the sextic term in order parameter in the Landau
free energy. The ground state solution of the linearized time-independent
Ginzburg-Landau equation has been derived, through which the spatial variation
of the order parameter has been delineated. Moreover, a generic phase diagram
indicating a tricritical behavior near and away from the dislocation is
depicted. The relation between the classical nucleation theory and the
Ginzburg-Landau approach has been discussed, for which the critical formation
energy of nucleus is related to the maximal of the Landau potential energy. A
numerical example illustrating the application of the model to the case of
nucleation of hydrides in zirconium alloys is provided. | cond-mat_mtrl-sci |
Giant thermoelectric figure of merit in multivalley
high-complexity-factor LaSO: We report a giant thermoelectric figure of merit $ZT$ (up to 6 at 1100 K) in
$n$-doped lanthanum oxysulphate LaSO. Thermoelectric coefficients are computed
from ab initio bands within Bloch-Boltzmann theory in an energy-, chemical
potential- and temperature-dependent relaxation time approximation. The lattice
thermal conductivity is estimated from a model employing the ab initio phonon
and Gr\"uneisen-parameter spectrum. The main source of the large $ZT$ is the
significant power factor which correlates with a large band complexity factor.
We also suggest a possible $n$-type dopant for the material based on ab initio
calculations. | cond-mat_mtrl-sci |
Static Hopf Solitons and Knotted Emergent Fields in Solid-State
Noncentrosymmetric Magnetic Nanostructures: Two-dimensional topological solitons, commonly called Skyrmions, are
extensively studied in solid-state magnetic nanostructures and promise many
spintronics applications. However, three-dimensional topological solitons
dubbed hopfions have not been demonstrated as stable spatially localized
structures in solid-state magnetic materials. Here we model the existence of
such static solitons with different Hopf index values in noncentrosymmetric
solid magnetic nanostructures with a perpendicular interfacial magnetic
anisotropy. We show how this surface anisotropy, along with the
Dzyaloshinskii-Moriya interactions and the geometry of nanostructures,
stabilize hopfions. We demonstrate knots in emergent field lines and computer
simulate Lorentz transmission electron microscopy images of such solitonic
configurations to guide their experimental discovery in magnetic solids. | cond-mat_mtrl-sci |
Creating and modulating electronic states on noble metal surfaces:
ultrathin Ag islands on Si(111)-7$\times$7 as a prototype: Various-thickness Ag islands were prepared on Si(111)-7$\times$7 using the
one-step deposition at a high substrate temperature. An electronic state
centered at -0.40$\sim$-0.15eV versus E$_{Fermi}$, detectable on the surface of
the Ag islands thinner than 9 layers, was created by the electronic
hybridization between Ag and Si at the Ag-Si interface. Scanning tunneling
microscopy/spectroscopy and density functional theory revealed that the
thickness of Ag islands determined the strength of the hybridization, leading
to a modulation to the energy and intensity of the state on the surface. | cond-mat_mtrl-sci |
Ground-state polariton condensation in 2D-GaAs semiconductor
microcavities: We observe ground-state polariton condensation in a two dimensional GaAs/AlAs
semiconductor microcavity under non resonant pulsed optical excitation. We
resolve the formation of a polariton condensate by studying the spatial,
angular, coherence, energy and transient dynamics of polariton
photoluminescence. For high excitation densities we also observe a transition
from the weak- to the strong-coupling regime in the time-domain and resolve the
build-up of a coherent polariton state. | cond-mat_mtrl-sci |
Optical Rotatory Dispersion of $α$-quartz: It is shown that some formulae describing optical rotatory dispersion of
$\alpha$-quartz with the aid of two Drude's terms reduce to the combined
formula containing one Drude's and one Chandrasekhar's term. Comparison of
various formulae describing the experimental data of $\alpha$-quartz leads to
the conclusion that the optical activity of this crystal is due to its crystal
structure only, that means the optical activity is not of molecular origin.
Further the rotatory strengths are discussed with the regard to coupled
oscillator model and to the structure of $\alpha$-quartz. | cond-mat_mtrl-sci |
Theory of volumetric capacitance of an electric double-layer
supercapacitor: Electric double layer supercapacitors are a fast-rising class of high-power
energy storage devices based on porous electrodes immersed in a concentrated
electrolyte or ionic liquid. As of yet there is no microscopic theory to
describe their surprisingly large capacitance per unit volume (volumetric
capacitance) of ~ 100 F/cm^3, nor is there a good understanding of the
fundamental limits on volumetric capacitance. In this paper we present a
non-mean-field theory of the volumetric capacitance of a supercapacitor that
captures the discrete nature of the ions and the exponential screening of their
repulsive interaction by the electrode. We consider analytically and via
Monte-Carlo simulations the case of an electrode made from a good metal and
show that in this case the volumetric capacitance can reach the record values.
We also study how the capacitance is reduced when the electrode is an imperfect
metal characterized by some finite screening radius. Finally, we argue that a
carbon electrode, despite its relatively large linear screening radius, can be
approximated as a perfect metal because of its strong nonlinear screening. In
this way the experimentally-measured capacitance values of ~ 100 F/cm^3 may be
understood. | cond-mat_mtrl-sci |
A comparison of Monte-Carlo simulations using RESTRAX and McSTAS with
experiment on IN14: Monte-Carlo simulations of a focusing supermirror guide after the
monochromator on the IN14 cold neutron three-axis spectrometer, I.L.L. were
carried out using the instrument simulation programs RESTRAX and McSTAS. The
simulations were compared to experiment to check their accuracy. Comparisons of
the flux ratios over both a 100 mm2 and a 1600 mm2 area at the sample position
compare well, and there is very close agreement between simulation and
experiment for the energy spread of the incident beam. | cond-mat_mtrl-sci |
Molecular Beam Epitaxy of a Half-Heusler Topological Superconductor
Candidate YPtBi: The search for topological superconductivity has motivated investigations
into materials that combine topological and superconducting properties. The
half-Heusler compound YPtBi appears to be such a material, however experiments
have thus far been limited to bulk single crystals, drastically limiting the
scope of available experiments. This has made it impossible to investigate the
potential topological nature of the superconductivity in this material.
Experiments to access details about the superconducting state require
sophisticated lithographic structures, typically based on thin films. Here we
report on the establishment of high crystalline quality epitaxial thin films of
YPtBi(111), grown using molecular beam epitaxy on Al2O3(0001) substrates. A
robust superconducting state is observed, with both critical temperature and
critical field consistent with that previously reported for bulk crystals.
Moreover we find that AlOx capping sufficiently protects the sample surface
from degradation to allow for proper lithography. Our results pave a path
towards the development of advanced lithographic structures, that will allow
the exploration of the potentially topological nature of superconductivity in
YPtBi. | cond-mat_mtrl-sci |
Sn delta-doping in GaAs: We have prepared a number of GaAs structures delta-doped by Sn using the
well-known molecular beam epitaxy growth technique. The samples obtained for a
wide range of Sn doping densities were characterised by magnetotransport
experiments at low temperatures and in high magnetic fields up to 38 T.
Hall-effect and Shubnikov-de Haas measurements show that the electron densities
reached are higher than for other delta-dopants, like Si and Be. The maximum
carrier density determined by the Hall effect equals 8.4x10^13 cm^-2. For all
samples several Shubnikov-de Haas frequencies were observed, indicating the
population of multiple subbands. The depopulation fields of the subbands were
determined by measuring the magnetoresistance with the magnetic field in the
plane of the delta-layer. The experimental results are in good agreement with
selfconsistent bandstructure calculations. These calculation shows that in the
sample with the highest electron density also the conduction band at the L
point is populated. | cond-mat_mtrl-sci |
Low-temperature Raman scaterring of PMN-PT close to the morphotropic
phase boundary: This paper has been withdrawn by the author due to a crucial sign error in
equation. | cond-mat_mtrl-sci |
Energy landscape of relaxed amorphous silicon: We analyze the structure of the energy landscape of a well-relaxed 1000-atom
model of amorphous silicon using the activation-relaxation technique (ART
nouveau). Generating more than 40,000 events starting from a single minimum, we
find that activated mechanisms are local in nature, that they are distributed
uniformly throughout the model and that the activation energy is limited by the
cost of breaking one bond, independently of the complexity of the mechanism.
The overall shape of the activation-energy-barrier distribution is also
insensitive to the exact details of the configuration, indicating that
well-relaxed configurations see essentially the same environment. These results
underscore the localized nature of relaxation in this material. | cond-mat_mtrl-sci |
Effects of the Position Reversal of Friction Pairs on the Strength of
Tribocharging and Tribodischarging: The friction-induced charging (i.e., tribocharging) and the following
discharging (referred here as tribodischarging) are always believed to have
negative effects on the daily life and on the industrial production. Thus, how
to inhibit the tribocharging and the tribodischarging has caused wide public
concern. Because the discharge caused by the electrical breakdown of the
ambient gas is generally accompanied with the generation of light, we
investigated here the tribocharging and the tribodischarging by observing the
light emitted during friction. We found that the position reversal of the
friction pair has a dramatic impact on the intensity of the tribo-induced
light. Experimental results show that an intense light is produced when a
stationary Al2O3 disk is sliding on a rotating SiO2 disk, but only a weak light
is observed for the case of a stationary SiO2 disk and a rotating Al2O3 disk.
This means that the process of the tribocharging and the tribodischarging can
be significantly influenced owing to the change in the relative position of the
friction couple. The experimentally measured polarities of the tribo-induced
charge on the friction surfaces further indicated that the strong discharging
occurs when the rotating surface is negatively charged. The reason for the
difference in the intensity of the tribocharging and tribodischarging can be
attributed to the combined effects of the contact potential difference and the
temperature gradient between the contacting surfaces on the charge transfer
when friction. Finally, a simple, low cost, yet effective approach, i.e., just
keep the friction partner whose surface is tribo-induced negatively charged as
the stationary one, can be utilized to suppress the intensity of the
tribocharging and the tribodischarging. This work may provide potential
applications in numerous areas of science and engineering and also in the
everyday life. | cond-mat_mtrl-sci |
Influence of surface anisotropy on the hysteresis of magnetic
nanoparticles: We present the results of Monte Carlo simulations of the magnetic properties
of individual spherical nanoparticles with the aim to explain the role played
by surface anisotropy on their low temperature magnetization processes. Phase
diagrams for the equilibrium configurations have been obtained, showing a
change from quasi-uniform magnetization state to a state with hedgehog-like
structures at the surface as $k_S$ increases. Through the simulated hysteresis
loops and the analysis of spin configurations along them, we have identified a
change in the magnetization reversal mechanism from quasi-uniform rotation at
low $k_S$ values, to a non-uniform switching process at high $k_S$. Results for
the dependence of the coercive field and remanence on $k_S$ and particle size
are also reported. | cond-mat_mtrl-sci |
A One-Dimensional Coordination Polymer, BBDTA-InCl4; Possible
Spin-Peierls Transition with High Critical Temperature of 108 K: We have studied the crystal structure and magnetic properties of the organic
radical cation salt, BBDTA-InCl4. This material formed a one-dimensional
coordination polymer, whose structure was characteristic of inorganic
spin-Peierls materials. Magnetic measurements indicated the spin-Peierls
transition like behavior at 108 K, which was higher than those typically
observed for the other organic spin-Peierls materials. The structural aspects
of the lattice distortion from X-ray diffraction measurements at 50 K have been
discussed. | cond-mat_mtrl-sci |
Twin interaction with $Σ$11 tilt grain boundaries in BCC Fe :
Formation of new grain boundaries: It is well known that the twinning is an important mode of plastic
deformation in nanocrystalline materials. As a result, it is expected that the
twin can interact with different grain boundaries (GBs) during the plastic
deformation. Understanding these twin-GB interactions is crucial for our
understanding of mechanical behavior of materials. In this work, the twin
interaction with different $\Sigma$11 symmetric and asymmetric tilt GBs has
been investigated in BCC Fe using molecular dynamics (MD) simulations. The
results indicate that twin nucleate from the crack or GB and, its interaction
with $\Sigma$11 asymmetric tilt GBs leads to the formation of a new GB. This
new GB consist of $<$100$>$ Cottrell type immobile dislocations. The detailed
atomistic mechanisms responsible for this new GB formation have been revealed
using atomistic simulations. Interestingly, the new GB formation has not been
observed in the case of twin interaction with $\Sigma$11 symmetric tilt GBs. | cond-mat_mtrl-sci |
Simulation of stress-impedance effects in low magnetostrictive films: A theoretical study of stress-impedance effect based on the solution of
Landau-Lifsitz-Gilbert equation has been carried out. The results show that
stress impedance effects depend largely on several extrinsic (external bias
field, external frequency) and intrinsic (orientation and magnitude of uniaxial
anisotropy, damping) parameters. | cond-mat_mtrl-sci |
Patterns and driving forces of dimensionality-dependent charge density
waves in 2H-type transition metal dichalcogenides: Two-dimensional (2D) materials have become a fertile playground for the
exploration and manipulation of novel collective electronic states. Recent
experiments have unveiled a variety of robust 2D orders in highly-crystalline
materials ranging from magnetism to ferroelectricity and from superconductivity
to charge density wave (CDW) instability. The latter, in particular, appears in
diverse patterns even within the same family of materials with isoelectronic
species. Furthermore, how they evolve with dimensionality has so far remained
elusive. Here we propose a general framework that provides a unfied picture of
CDW ordering in the 2H polytype of four isoelectronic transition metal
dichalcogenides 2H-MX$_2$ (M=Nb, Ta and X=S, Se). We first show experimentally
that whilst NbSe$_2$ exhibits a strongly enhanced CDW order in the 2D limit,
the opposite trend exists for TaSe$_2$ and TaS$_2$, with CDW being entirely
absent in NbS$_2$ from its bulk to the monolayer. Such distinct behaviours are
then demonstrated to be the result of a subtle, yet profound, competition
between three factors: ionic charge transfer, electron-phonon coupling, and the
spreading extension of the electronic wave functions. Despite its simplicity,
our approach can, in essence, be applied to other quasi-2D materials to account
for their CDW response at different thicknesses, thereby shedding new light on
this intriguing quantum phenomenon and its underlying mechanisms. | cond-mat_mtrl-sci |
Phonon-mediated sticking of electrons at dielectric surfaces: We study phonon-mediated temporary trapping of an electron in
polarization-induced external surface states (image states) of a dielectric
surface. Our approach is based on a quantum-kinetic equation for the occupancy
of the image states. It allows us to distinguish between prompt and kinetic
sticking. Because the depth of the image potential is much larger than the
Debye energy multi-phonon processes are important. Taking two-phonon processes
into account in cases where one-phonon processes yield a vanishing transition
probability, as it is applicable, for instance, to graphite, we analyze the
adsorption scenario as a function of potential depth and surface temperature
and calculate prompt and kinetic sticking coefficients. We find rather small
sticking coefficients, at most of the order of $10^{-3}$, and a significant
suppression of the kinetic sticking coefficient due to a relaxation bottleneck
inhibiting thermalization of the electron with the surface at short timescales. | cond-mat_mtrl-sci |
The effect of intrinsic point defects on ferroelectric polarization
behavior of SrTiO$_3$: The effect of a variety of intrinsic defects and defect clusters in bulk and
thin films of SrTiO$_3$ on ferroelectric polarization and switching mechanism
is investigated by means of density-functional-theory (DFT) based calculations
and the Berry phase approach. Our results show that both the titanium
Ti$_\mathrm{Sr}^{\bullet \bullet}$ and strontium Sr$_\mathrm{Ti}^{"}$ antisite
defects induce ferroelectric polarization in SrTiO$_3$, with the
Ti$_\mathrm{Sr}^{\bullet \bullet}$ defect causing a more pronounced spontaneous
polarization and higher activation barriers of polarization reversal than
Sr$_\mathrm{Ti}^{"}$. The presence of oxygen vacancies bound to the antisite
defects can either enhance or diminish polarization depending on the
configuration of the defect pair, but it always leads to larger activation
barriers of polarization switching as compared to the antisite defects with no
oxygen vacancies. We also show that the magnitude of spontaneous polarization
in SrTiO$_3$ can be tuned by controlling the degree of Sr/Ti nonstroichiometry.
Other intrinsic point defects such as Frenkel defect pairs and electron small
polarons also contribute to the emergence of ferroelectric polarization in
SrTiO$_{3}$. | cond-mat_mtrl-sci |
Time-, spin-, and angle-resolved photoemission spectroscopy with a 1-MHz
10.7-eV pulse laser: We describe a setup of time-, spin-, and angle-resolved photoemission
spectroscopy (tr-SARPES) employing a 10.7-eV ($\lambda$=115.6 nm) pulse laser
at 1-MHz repetition rate as a probe photon source. This equipment effectively
combines technologies of a high-power Yb:fiber laser, ultraviolet-driven
harmonic generation in Xe gas, and a SARPES apparatus equipped with
very-low-energy-electron-diffraction (VLEED) spin detectors. A high repetition
rate (1 MHz) of the probe laser allows experiments with the photoemission
space-charge effects significantly reduced, despite a high flux of 10$^{13}$
photons/s on the sample. The relatively high photon energy (10.7 eV) also
brings the capability of observing a wide momentum range that covers the entire
Brillouin zone of many materials while ensuring high momentum resolution. The
experimental setup overcomes a low efficiency of spin-resolved measurements,
which gets even more severe for the pump-probed unoccupied states, and affords
for investigating ultrafast electron and spin dynamics of modern quantum
materials with energy and time resolutions of 25 meV and 360 fs, respectively. | cond-mat_mtrl-sci |
Understanding correlations in BaZrO3:Structure and dynamics on the
nano-scale: Barium zirconate BaZrO3 is one of few perovskites that is claimed to retain
an average cubic structure down to 0K at ambient pressure, while being
energetically very close to a tetragonal phase obtained by condensation of a
soft phonon mode at the R-point. Previous studies suggest, however, that the
local structure of BaZrO3 may change at low temperature forming nanodomains or
a glass-like phase. Here, we investigate the global and local structure of
BaZrO3 as a function of temperature and pressure via molecular dynamics
simulations using a machine-learned potential with near density functional
theory (DFT) accuracy. We show that the softening of the octahedral tilt mode
at the R-point gives rise to weak diffuse superlattice reflections at low
temperatures and ambient pressure, which are also observed experimentally.
However, we do not observe any static nanodomains but rather soft dynamic
fluctuations of the ZrO6 octahedra with a correlation length of 2 to 3nm over
time-scales of about 1ps. This soft dynamic behaviour is the precursor of a
phase transition and explains the emergence of weak superlattice peaks in
measurements. On the other hand, when increasing the pressure at 300K we find a
phase transition from the cubic to the tetragonal phase at around 16GPa, also
in agreement with experimental studies. | cond-mat_mtrl-sci |
First principles calculations of steady-state voltage-controlled
magnetism: application to x-ray absorption spectroscopy experiment: Recent x-ray absorption experiments have demonstrated the possibility to
accurately monitor the magnetism of metallic hetero-structures controlled via a
time-independent perturbation caused for example by a static electric field.
Using a first-principles, non-equilibrium Green function scheme, we show how
the measured dichroic signal for the corresponding steady-state situation can
be related to the underlying electronic structure and its response to the
external stimulus. The suggested approach works from the infinitesimal limit of
linear response to the regime of strong electric field effects, which is
realized in present experimental high sensitivity investigations. | cond-mat_mtrl-sci |
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