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Tuning the two-dimensional electron gas at the LaAlO3/SrTiO3(001)
interface by metallic contacts: First principles calculations reveal that adding a metallic overlayer on
LaAlO3/SrTiO3(001) eliminates the electric field within the polar LaAlO3 film
and thus suppresses the thickness-dependent insulator-to-metal transition
observed in uncovered films. Independent of the LaAlO3 thickness both the
surface and the interface are metallic, with an enhanced interface carrier
density relative to LaAlO3/SrTiO3(001) after the metallization transition.
Moreover, a monolayer thick metallic Ti-contact exhibits a finite magnetic
moment and for a thin SrTiO3-substrate induces a spin-polarized 2D electron gas
at the n-type interface due to confinement effects. A diagram of band alignment
in M/LaAlO3/SrTiO3(001) and Schottky barriers for M=Ti, Al, and Pt are
provided. | cond-mat_mtrl-sci |
Early stage formation of graphene on the C-face of 6H-SiC: An investigation of the early stage formation of graphene on the C-face of
6H-SiC is presented. We show that the sublimation of few atomic layers of Si
out of the SiC substrate is not homogeneous. In good agreement with the results
of theoretical calculations it starts from defective sites, mainly dislocations
that define nearly circular flakes, which have a pyramidal, volcano-like, shape
with a center chimney where the original defect was located. At higher
temperatures, complete conversion occurs but, again, it is not homogeneous.
Within the sample surface the intensity of the Raman G and 2D bands, evidences
non-homogeneous thickness. | cond-mat_mtrl-sci |
3-omega method for thermal properties of thin film multilayers: Short review on the different models for the electro-thermal 3-omega method.
We present the deduction of the fundamental relation between the 3-omega
voltage with the temperature rise to determine the thermal conductivity. The
usage of the anisotropy of the films allows a smooth transition between 1D and
2D models. A comparison between the multilayer methods and analytical solutions
are presented. | cond-mat_mtrl-sci |
Efficient Photon Upconverters with Ionic Liquids: This paper presents the development and characterization of photon
upconverters fabricated with ionic liquids (ILs), which are novel fluids that
are recently drawing attention due to their unique properties, such as
negligible vapor pressures and high thermal stabilities. The upconverters in
this study are based on triplet-triplet annihilation (TTA) between excited
polycyclic aromatic molecules, and TTA requires a fluidic media to allow for
the molecules to collide with each other for energy transfer. This process,
TTA-based photon upconversion (TTA-UC), was therefore mainly accomplished with
organic solvents previously. It is found that the molecules used for TTA-UC,
which are non-polar or weakly polar, are stably solvated in a certain class of
ILs. The mechanism of the observed solvation is proposed and discussed. The
upconversion quantum yields (UC-QYs) measured by continuous wave (CW) light
excitation reach as high as 10 % with moderate excitation intensity (~ 6
W/cm2), which is considerably higher than those in previous TTA-UC studies
performed with organic solvents (up to ~ 4 % with CW light excitations). It is
found that the value of UC-QY starts to saturate as the excitation power
increases for all the cases, even within the moderate CW excitation power range
in this study. An analytical model that describes the UC-QY vs. excitation
power relationship is derived and compared with the experimental results. The
agreement between them suggests that the donor-acceptor energy transfer in this
system is highly efficient. Based on these experimental and analytical
findings, it is found that efficient energy transfer between the molecules is
possible in ILs and therefore ILs are not actually viscous media for the
purpose of TTA-UC. | cond-mat_mtrl-sci |
Simulation of XANES spectroscopy and the calculation of total energies
for N-heterocyclic carbenes on Au(111): It has recently been demonstrated that N-heterocyclic carbenes (NHCs) form
self-assembled monolayers (SAMs) on metal surfaces. Consequently, it is
important to both characterize and understand their binding modes to fully
exploit NHCs in functional surface systems. To assist with this effort, we have
performed {\it first-principles} total energy calculations for NHCs on Au(111)
and simulations of X-ray absorption near edge structure (XANES). The NHCs we
have considered are N,N-dimethyl-, N,N-diethyl-,
N,N-diisopropylbenzimidazolylidene ($^B$NHC$^X$, with X=Me, Et, and iPr,
respectively) and the bis-$^B$NHC$^X$ complexes with Au derived from these
molecules. We present a comprehensive analysis of the energetic stability of
both the $^B$NHC$^X$ and the complexes on Au(111) and, for the former, examine
the role of the wing group in determining the attachment geometry. Further
structural characterization is performed by calculating the nitrogen K-edge
X-ray absorption spectra. Our simulated XANES results give insight into (i) the
relationship between the $^B$NHC$^X$/Au geometry and the N($1s$) $\rightarrow$
$\pi^\ast/\sigma^\ast$, pre-edge/near-edge, absorption intensities, and (ii)
the contributions of the molecular deformation and molecule-surface electronic
interaction to the XANES spectrum. Our simulations are compared with recent
experimental results. | cond-mat_mtrl-sci |
Numerical Study of Crystal Size Distribution in Polynuclear Growth: The crystal size distribution in polynuclear growth is numerically studied
using a coupled map lattice model. The width of the size distribution depends
on c/D, where c is the growth rate at interface sites and $D$ is the diffusion
constant. When c/D is sufficiently small, the width W increases linearly with
c/D and saturates at large c/D. Monodisperse square and cubic crystals are
obtained respectively on square and cubic lattices when c/D is sufficiently
small for a small kinetic parameter b. The linear dependence of W on c/D in a
parameter range of small c/D is explained by the eigenfunction for the first
eigenvalue in a two-dimensional model and a mean-field model. For the
mean-field model, the slope of the linear dependence is evaluated
theoretically. | cond-mat_mtrl-sci |
A survey of energies from pure metals to multi-principal element alloys: In materials science, a wide range of properties of materials are governed by
various types of energies, including thermal, physicochemical, structural, and
mechanical energies. In 2005, Dr. Frans Spaepen used crystalline
face-centered-cubic (fcc) copper as an example to discuss a variety of
phenomena that are associated with energies. Inspired by his pioneering work,
we broaden our analysis to include a selection of representative pure metals
with fcc, hexagonal close-packed (hcp), and body-centered cubic (bcc)
structures. Additionally, we extend our comparison to energies between pure
metals and equiatomic binary, ternary, and multi-principal element alloys
(sometimes also known as high-entropy alloys). Through an extensive collection
of data and calculations, we compile energy tables that provide a comprehensive
view of how structure and alloying influence the energy profiles of these
metals and alloys. We highlight the significant impact of constituent elements
on the energies of alloys compared to pure metals and reveal a notable
disparity in mechanical energies among materials in fcc-, hcp- and
bcc-structured metals and alloys. Furthermore, we discuss the underlying
mechanisms behind these patterns and discuss the implications for structural
transformations, providing insights into the broader context of these energy
variations. | cond-mat_mtrl-sci |
Possible high-temperature superconductors predicted from electronic
structure and data-filtering algorithms: We report here the completion of the electronic structure of the majority of
the known stoichiometric inorganic compounds, as listed in the International
Crystal Structure Data-base (ICSD). We make a detailed comparison of the
electronic structure, crystal geometry and chemical bonding of cuprate high
temperature superconductors, with the calculated over sixty thousand electronic
structures. Based on compelling similarities of the electronic structures in
the normal state and a data-filtering technique, we propose that high
temperature superconductivity is possible for electron- or hole-doping in a
much larger group of materials than previously considered. The indentified
materials are composed of over one hundred layered compounds, most which
hitherto are untested with respect to their super conducting properties. Of
particular interest are the following materials; Ca$_2$(CuBr$_2$O$_2$),
K$_2$CoF$_4$, Sr$_2$(MoO$_4$) and Sr$_4$V$_3$O$_{10}$, which are discussed in
detail. | cond-mat_mtrl-sci |
Modeling Heterogeneous Materials via Two-Point Correlation Functions:
II. Algorithmic Details and Applications: In the first part of this series of two papers, we proposed a theoretical
formalism that enables one to model and categorize heterogeneous materials
(media) via two-point correlation functions S2 and introduced an efficient
heterogeneous-medium (re)construction algorithm called the "lattice-point"
algorithm. Here we discuss the algorithmic details of the lattice-point
procedure and an algorithm modification using surface optimization to further
speed up the (re)construction process. The importance of the error tolerance,
which indicates to what accuracy the media are (re)constructed, is also
emphasized and discussed. We apply the algorithm to generate three-dimensional
digitized realizations of a Fontainebleau sandstone and a boron
carbide/aluminum composite from the two- dimensional tomographic images of
their slices through the materials. To ascertain whether the information
contained in S2 is sufficient to capture the salient structural features, we
compute the two-point cluster functions of the media, which are superior
signatures of the micro-structure because they incorporate the connectedness
information. We also study the reconstruction of a binary laser-speckle pattern
in two dimensions, in which the algorithm fails to reproduce the pattern
accurately. We conclude that in general reconstructions using S2 only work well
for heterogeneous materials with single-scale structures. However, two-point
information via S2 is not sufficient to accurately model multi-scale media.
Moreover, we construct realizations of hypothetical materials with desired
structural characteristics obtained by manipulating their two-point correlation
functions. | cond-mat_mtrl-sci |
Thermodynamic properties of a tetramer
ferro-ferro-antiferro-antiferromagnetic Ising-Heisenberg bond alternating
chain as a model system for Cu(3-Clpy)$_2$(N$_3$)$_2$: Thermodynamic properties of a tetramer
ferro-ferro-antiferro-antiferromagnetic Ising-Heisenberg bond alternating chain
are investigated by the use of an exact mapping transformation technique. Exact
results for the magnetization, susceptibility and specific heat in the zero as
well as nonzero magnetic field are presented and discussed in detail. The
results obtained from the mapping are compared with the relevant experimental
data of Cu(3-Clpy)$_2$(N$_3$)$_2$ (3-Clpy=3-Chloropyridine). | cond-mat_mtrl-sci |
Laplacian-level density functionals for the kinetic energy density and
exchange-correlation energy: We construct a Laplacian-level meta-generalized gradient approximation
(meta-GGA) for the non-interacting (Kohn-Sham orbital) positive kinetic energy
density $\tau$ of an electronic ground state of density $n$. This meta-GGA is
designed to recover the fourth-order gradient expansion $\tau^{GE4}$ in the
appropiate slowly-varying limit and the von Weizs\"{a}cker expression
$\tau^{W}=|\nabla n|^2/(8n)$ in the rapidly-varying limit. It is constrained to
satisfy the rigorous lower bound $\tau^{W}(\mathbf{r})\leq\tau(\mathbf{r})$.
Our meta-GGA is typically a strong improvement over the gradient expansion of
$\tau$ for atoms, spherical jellium clusters, jellium surfaces, the Airy gas,
Hooke's atom, one-electron Gaussian density, quasi-two dimensional electron
gas, and nonuniformly-scaled hydrogen atom. We also construct a Laplacian-level
meta-GGA for exchange and correlation by employing our approximate $\tau$ in
the Tao, Perdew, Staroverov and Scuseria (TPSS) meta-GGA density functional.
The Laplacian-level TPSS gives almost the same exchange-correlation enhancement
factors and energies as the full TPSS, suggesting that $\tau$ and $\nabla^2 n$
carry about the same information beyond that carried by $n$ and $\nabla n$. Our
kinetic energy density integrates to an orbital-free kinetic energy functional
that is about as accurate as the fourth-order gradient expansion for many real
densities (with noticeable improvement in molecular atomization energies), but
considerably more accurate for rapidly-varying ones. | cond-mat_mtrl-sci |
Quantitative comparison of the magnetic proximity effect in Pt detected
by XRMR and XMCD: X-ray resonant magnetic reflectivity (XRMR) allows for the simultaneous
measurement of structural, optical and magnetooptic properties and depth
profiles of a variety of thin film samples. However, a same-beamtime
same-sample systematic quantitative comparison of the magnetic properties
observed with XRMR and x-ray magnetic circular dichroism (XMCD) is still
pending. Here, the XRMR results (Pt L$_{3}$ absorption edge) for the magnetic
proximity effect in Pt deposited on the two different ferromagnetic materials
Fe and Co$_{33}$Fe$_{67}$ are compared with quantitatively analyzed XMCD
results. The obtained results are in very good quantitative agreement between
the absorption-based (XMCD) and reflectivity-based (XRMR) techniques taking
into account an ab initio calculated magnetooptic conversion factor for the
XRMR analysis. Thus, it is shown that XRMR provides quantitative reliable spin
depth profiles important for spintronic and spin caloritronic transport
phenomena at this type of magnetic interfaces. | cond-mat_mtrl-sci |
Site-Selective Oxygen Vacancy Formation Derived from the Characteristic
Crystal Structures of in Sn-Nb complex Oxides: Divalent tin oxides have attracted considerable attention as novel p-type
oxide semiconductors, which are essential for realizing future oxide electronic
devices. Recently, p-type Sn2Nb2O7 and SnNb2O6 were developed; however,
enhanced hole mobility by reducing defect concentrations is required for
practical use. In this work, we investigate the correlation between the
formation of oxygen vacancy which may reduce the hole-generation efficiency and
hole mobility, and the crystal structure in Sn-Nb complex oxides. Extended
X-ray absorption fine structure spectroscopy and Rietveld analysis of x-ray
diffraction revealed the preferential formation of oxygen vacancy at the O site
bonded to the Sn ions in both the tin niobates. Moreover, a large amount of
oxygen vacancy around the Sn ions were found in the p-type Sn2Nb2O7, thereby
indicating the effect of oxygen vacancy to the low hole-generation efficiency.
The dependence of the formation of oxygen vacancy on the crystal structure can
be elucidated from the Sn-O bond strength that is evaluated based on the bond
valence sum and Debye temperature. The differences in the bond strengths of the
two Sn-Nb complex oxides are correlated through the steric hindrance of Sn2+
with asymmetric electron density distribution. This suggests the importance of
the material design with a focus on the local structure around the Sn ions to
prevent the formation of oxygen vacancy in p-type Sn2+ oxides. | cond-mat_mtrl-sci |
First-principles prediction of high-entropy-alloy stability: High entropy alloys (HEAs) are multicomponent compounds whose high
configurational entropy allows them to solidify into a single phase, with a
simple crystal lattice structure. Some HEA's exhibit desirable properties, such
as high specific strength, ductility, and corrosion resistance, while
challenging the scientist to make confident predictions in the face of multiple
competing phases. We demonstrate phase stability in the multicomponent alloy
system of Cr-Mo-Nb-V, for which some of its binary subsystems are subject to
phase separation and complex intermetallic-phase formation. Our
first-principles calculation of free energy predicts that the configurational
entropy stabilizes a single body-centered cubic (BCC) phase from T = 1,700K up
to melting, while precipitation of a complex intermetallic is favored at lower
temperatures. We form the compound experimentally and confirm that it forms as
a single BCC phase from the melt, but that it transforms reversibly at lower
temperatures. | cond-mat_mtrl-sci |
High-temperature cyclic oxidation kinetics and microstructural
transition mechanisms of Ti-6Al-4V composites reinforced with hybrid
(TiC+TiB) networks: The microstructural features and high-temperature oxidation resistance of
hybrid (TiC+TiB) networks reinforced Ti-6Al-4V composites were investigated
after fabricated with reaction hot pressing technique. The inhomogeneous
distribution of hybrid reinforcers resulted in a sort of stress-induced grain
refinement for {\alpha}-Ti matrix phase, which was further facilitated by
heterogeneous nucleation upon additive interfaces. HRTEM analyses revealed the
crystallographic orientation relation between TiB and alpha-Ti phases as
(201)TiB//(-1100)alpha-Ti plus [11-2]//[0001] alpha-Ti, while TiC and
{\alpha}-Ti phases maintained the interrelation of (-200)TiC//(-2110)
{\alpha}-Ti and [001]TiC//[01-10] alpha-Ti. The hybridly reinforced
Ti-6Al-4V/(TiC+TiB) composites displayed superior oxidation resistance to both
the sintered matrix alloy and the two composites reinforced solely with TiC or
TiB addition during the cyclic oxidation at 873, 973 and 1073 K respectively
for 100 h. The hybrid reinforcers volume fraction was a more influential factor
to improve oxidation resistance than the matrix alloy powder size. As
temperature rose from 873 to 1073 K, the oxidation kinetics transferred from
the nearly parabolic type through qusilinear tendency into the finally linear
mode. This corresponded to the morphological transition of oxide scales from a
continuous protective film to a partially damaged layer and ended up with the
complete spallation of alternating alumina and rutile multilayers. A
phenomenological model was proposed to elucidate the growth process of oxides
scales. The release of thermal stress, the suppression of oxygen diffusion and
the fastening of oxide adherence were found as the three major mechanisms to
enhance the oxidation resistance of hybrid reinforced composites. | cond-mat_mtrl-sci |
Piezo films with adjustable anisotropic strain for bending actuators
with tunable bending profiles: We present a method to produce in-plane polarized piezo films with a freely
adjustable ratio of the strains in orthogonal in-plane directions. They can be
used in piezo bending actuators with a tunable curvature profile. The strains
are obtained as mean strains from a periodic polarization pattern produced by a
suitable doubly interdigitated electrode structure. This mechanism is
demonstrated for several examples using PZT sheets. We further discuss how this
tuning and the parameters of the electrode layout affect the overall magnitude
of the displacement. | cond-mat_mtrl-sci |
Correlating atom probe tomography with X-Ray and electron spectroscopies
to understand microstructure-activity relationships in electrocatalysts: The search for a new energy paradigm with net-zero carbon emissions requires
new technologies for energy generation and storage that are at the crossroad
between engineering, chemistry, physics, surface and materials sciences. To
keep pushing the inherent boundaries of device performance and lifetime, we
need to step away from a cook-and-look approach and aim to establish the
scientific ground to guide the design of new materials. This requires strong
efforts in establishing bridges between microscopy and spectroscopy techniques,
across multiple scales. Here, we discuss how the complementarities of X-ray-
and electron-based spectroscopies and atom probe tomography can be exploited in
the study of surfaces and sub-surfaces to understand structure-property
relationships in electrocatalysts. | cond-mat_mtrl-sci |
Peierls Distortion in Two-Dimensional Tight-Binding Model: The Peierls distortions in a two-dimensional electron-lattice system
described by a Su-Schrieffer-Heeger type model extended to two-dimensions are
numerically studied for a square lattice. The electronic band is just
half-filled and the nesting vector is ($\pi/a$, $\pi/a$) with $a$ the lattice
constant. In contrast to the previous understanding on the Peierls transition
in two dimensions, the distortions which are determined so as to minimize the
total energy of the system involve not only the Fourier component with the
nesting wave vector but also many other components with wave vectors parallel
to the nesting vector. It is found that such unusual distortions contribute to
the formation of gap in the electronic energy spectrum by indirectly (in the
sense of second order perturbation) connecting two states having wave vectors
differing by the nesting vector from each other. Analyses for different system
sizes and for different electron-lattice coupling constants indicate that the
existence of such distortions is not a numerical artifact. It is shown that the
gap of the electronic energy spectrum is finite everywhere over the Fermi
surface. | cond-mat_mtrl-sci |
Mirror real Chern insulator in two and three dimensions: A real Chern insulator (RCI) featuring a real Chern number and a second-order
boundary mode appears in a two-dimensional (2D) system with the space-time
inversion symmetry (PT ). Here, we propose a kind of RCI: mirror real Chern
insulator (MRCI) which emerges from the system having additional horizontal
mirror symmetry Mz. The MRCI generally is characterized by two independent real
Chern numbers, respectively defined in the two mirror subsystems of the system.
Hence, the MRCI may host the second-order boundary modes different from the
conventional RCI. We show that for spinless systems, the definition of the MRCI
is straightforward, as PT keeps each mirror subsystem invariant. For the
spinful systems with both PT and Mz, the real Chern number for the total system
remain well defined, as MzPT = C2zT , and (C2zT )2= 1. However, since C2zT
exchanges the two mirror subsystems, the definition of the MRCI in spinful
systems requires the help of projective symmetry algebra. We also discuss the
MRCIs in 3D systems, where the MRCI is defined on certain mirror-invariant 2D
planes. Compared with its 2D counterpart, the 3D MRCI can exhibit more abundant
physics when the systems have additional nonsymmorphic operators. Several
concrete MRCI models including 2D and 3D, spinless and spinful models are
constructed to further demonstrate our ideas. | cond-mat_mtrl-sci |
Simulations on the elastic response of amorphous and nanocomposite
carbon: Theoretical calculations of the elastic response of carbon composites and
amorphous carbon are reported. The studied composites consist of crystalline
nanoinclusions, either spherical diamonds or carbon nanotubes, embedded in
amorphous carbon matrices. The elastic constants of the composites were
calculated and found to systematically increase as the density increases. The
elastic recovery under hydrostatic pressure for all structures was also
investigated and was found to be significantly high for both nanocomposite and
amorphous carbon, but decreases as the material becomes more dilute. | cond-mat_mtrl-sci |
Experimental determination and modelling of volume shrinkage in curing
thermosets: This work deals with the characterisation and modelling of the curing process
and its associated volume changes of an epoxy based thermoset resin.
Measurements from differential scanning calorimetry (DSC) define the progress
of the chemical reaction. The related thermochemical volume changes are
recorded by an especially constructed experimental setup based on Archimedes
principle. Information on measuring procedure and data processing are provided.
This includes investigations on compensation of environmental influences,
long-term stability and resolution. With the aim of simulating the adhesives
curing process, constitutive models representing the reaction kinetics and
thermochemical volume changes are presented and the model parameters are
identified. | cond-mat_mtrl-sci |
New type of incommensurate magnetic ordering in Mn3TeO6: The complex metal oxide Mn3TeO6 exhibits a corundum related structure and has
been prepared both in forms of single crystals by chemical transport reactions
and of polycrystalline powders by a solid state reaction route. The crystal
structure and magnetic properties have been investigated using a combination of
X-ray and neutron powder diffraction, electron microscopy, calorimetric and
magnetic measurements. At room temperature this compound adopts a trigonal
structure, space group R3 with a = 8.8679(1) {\AA}, c = 10.6727(2) {\AA}. A
long-range magnetically ordered state is identified below 23 K. An unexpected
feature of this magnetic structure is several types of Mn-chains. Under the
action of the incommensurate magnetic propagation vector k = [0, 0, 0.4302(1)]
the unique Mn site is split into two magnetically different orbits. One orbit
forms a perfect helix with the spiral axis along the c-axis while the other
orbit has a sine wave character along the c-axis. | cond-mat_mtrl-sci |
Two-dimensional ferroelectrics from high throughput computational
screening: We report a high throughput computational search for two-dimensional
ferroelectric materials. The starting point is 252 pyroelectric materials from
the computational 2D materials database (C2DB) and from these we identify 64
ferroelectric materials by explicitly constructing adiabatic paths connecting
states of reversed polarization. In particular we find 49 materials with
in-plane polarization, 8 materials with out-of-plane polarization and 6
materials with coupled in-plane and out-of-plane polarization. Most of the
known 2D ferroelectrics are recovered by the screening and the far majority of
the new predicted ferroelectrics are known as bulk van der Waals bonded
compounds, which implies that these could be experimentally accessible by
direct exfoliation. For roughly 25{\%} of the materials we find a metastable
state in the non-polar structure, which could have important consequences for
the thermodynamical properties and may imply a first order transition to the
polar phase. Finally, we list the magnetic pyroelectrics extracted from the
C2DB and focus on the case of VAgP$_2$Se$_6$, which exhibits a three-state
switchable polarization vector that is strongly coupled to the magnetic
excitation spectrum. | cond-mat_mtrl-sci |
Acoustic Cyclotron Resonance and Giant High Frequency Magnetoacoustic
Oscillations in Metals with Locally Flattened Fermi Surface: We consider the effect of local flattening on the Fermi surface (FS) of a
metal upon geometric oscillations of the velocity and attenuation of ultrasonic
waves in the neighborhood of the acoustic cyclotron resonance. It is shown that
such peculiarities of the local geometry of the FS can lead to a significant
enhancement of both cyclotron resonance and geometric oscillations.
Characteristic features of the coupling of ultrasound to shortwave cyclotron
waves arising due to the local flattening of the FS are analyzed.
PACS numbers 71.18.+y; 72.15.Gd; 72.15.-v | cond-mat_mtrl-sci |
Spin-wave-induced spin torque in Rashba spin-orbit coupling system: We study the effects of Rashba spin-orbit coupling on the spin torque induced
by spin waves, which are the plane wave dynamics of magnetization. The spin
torque is derived from linear response theory, and we calculate the dynamic
spin torque by considering the impurity-ladder-sum vertex corrections. This
dynamic spin torque is divided into three terms: a damping term, a $distortion$
term, and a correction term for the equation of motion. The $distorting$ torque
describes a phenomenon unique to the Rashba spin-orbit coupling system, where
the distorted motion of magnetization precession is subjected to the
anisotropic force from the Rashba coupling. The oscillation mode of the
precession exhibits an elliptical trajectory, and the ellipticity depends on
the strength of the nesting effects, which could be reduced by decreasing the
electron lifetime. | cond-mat_mtrl-sci |
Overview of phase-field models for fatigue fracture in a unified
framework: In the last ten years, the phase-field method has gained much attention as a
novel method to simulate fracture due to its straightforward way allowing to
cover crack initiation and propagation without additional conditions. More
recently, it has also been applied to fatigue fracture due to cyclic loading.
This publication gives an overview of the main phase-field fatigue models
published to date. We present all models in a unified variational framework for
best comparability. Subsequently, the models are compared regarding their most
important features. It becomes apparent that they can be classified in mainly
two categories according to the way fatigue is implemented in the model - that
is as a gradual degradation of the fracture toughness or with an additional
term in the crack driving force. We aim to provide a helpful guide for choosing
the appropriate model for different applications and for developing existing
models further. | cond-mat_mtrl-sci |
A First Principles Investigation of Native Interstitial Diffusion in
Cr2O3: First principles density functional theory (DFT) investigation of native
interstitials and the associated self-diffusion mechanisms in {\alpha}-Cr2O3
reveals that interstitials are more mobile than vacancies of corresponding
species. Cr interstitials occupy the unoccupied Cr sublattice sites that are
octahedrally coordinated by 6 O atoms, and O interstitials form a dumbbell
configuration orientated along the [221] direction (diagonal) of the corundum
lattice. Calculations predict that neutral O interstitials are predominant in
O-rich conditions and Cr interstitials in +2 and +1 charge states are the
dominant interstitial defects in Cr-rich conditions. Similar to that of the
vacancies, the charge transition levels of both O and Cr interstitials are
located deep within the bandgap. Transport calculations reveal a rich variety
of interstitial diffusion mechanisms that are species, charge, and orientation
dependent. Cr interstitials diffuse preferably along the diagonal of corundum
lattice in a two step process via an intermediate defect complex comprising a
Cr interstitial and an adjacent Cr Frenkel defect in the neighboring Cr
bilayer. This mechanism is similar to that of the vacancy mediated Cr diffusion
along the c-axis with intermediate Cr vacancy and Cr Frenkel defect
combination. In contrast, O interstitials diffuse via bond switching mechanism.
O interstitials in -1 and -2 charge states have very high mobility compared to
neutral O interstitials. | cond-mat_mtrl-sci |
Study of spin-phonon coupling and magnetic field induced spin
reorientation in polycrystalline multiferroic $GdFeO_3$: The present work reports the preparation of polycrystalline multiferroic
$GdFeO_3$ (GdFO) and characterization with x-ray diffraction (XRD),
magnetization, temperature dependent Raman spectroscopy, temperature and
magnetic field dependent $^{57}Fe$ M$\ddot{o}$ssbauer spectroscopy
measurements. The sample is found to be phase pure from Rietveld refinement of
XRD pattern. The M$\ddot{o}$ssbauer spectra measured in the presence of
external magnetic field show the signatures of field induced spin reorientation
transition, which are corroborated by magnetization measurements. From the
temperature dependent variation of internal hyperfine field, N$\grave{e}$el
transition temperature ($T_{N,Fe}$) of 672.5$\pm$0.2 K and critical exponent
($\beta$) of 0.333$\pm$0.003 is obtained. Temperature dependent (300 - 760 K)
Raman spectroscopy measurements show the signatures of spin-phonon coupling and
local structural re-arrangement across $T_{N,Fe}$. | cond-mat_mtrl-sci |
Edge Modes and Asymmetric Wave Transport in Topological Lattices:
Experimental Characterization at Finite Frequencies: Although topological mechanical metamaterials have been extensively studied
from a theoretical perspective, their experimental characterization has been
lagging. To address this shortcoming, we present a systematic laser-assisted
experimental characterization of topological kagome lattices, aimed at
elucidating their in-plane phononic and topological characteristics. We
specifically explore the continuum elasticity limit, which is established when
the ideal hinges that appear in the theoretical models are replaced by
ligaments capable of supporting bending deformation, as observed for instance
in realistic physical lattices. We reveal how the zero-energy floppy edge modes
predicted for ideal configurations morph into finite-frequency phonon modes
that localize at the edges. By probing the lattices with carefully designed
excitation signals, we are able to extract and characterize all the features of
a complex low-frequency acoustic regime in which bulk modes and topological
edge modes overlap and entangle in response. The experiments provide
unequivocal evidence of the existence of strong asymmetric wave transport
regimes at finite frequencies. | cond-mat_mtrl-sci |
Revealing Correlation of Valence State with Nanoporous Structure in
Cobalt Catalyst Nanoparticles by in Situ Environmental TEM: Simultaneously probing the electronic structure and morphology of materials
at the nanometer or atomic scale while a chemical reaction proceeds is
significant for understanding the underlying reaction mechanisms and optimizing
a materials design. This is especially important in the study of nanoparticle
catalysts, yet such experiments have rarely been achieved. Utilizing an
environmental transmission electron microscope (ETEM) equipped with a
differentially pumped gas cell, we are able to conduct nanoscopic imaging and
electron energy loss spectroscopy (EELS) in situ for cobalt catalysts under
reaction conditions. Analysis revealed quantitative correlation of the cobalt
valence states to the particles' nanoporous structures. The in situ experiments
were performed on nanoporous cobalt particles coated with silica while a 15
mTorr hydrogen environment was maintained at various temperatures
(300-600\degreeC). When the nanoporous particles were reduced, the valence
state changed from cobalt oxide to metallic cobalt and concurrent structural
coarsening was observed. In situ mapping of the valence state and the
corresponding nanoporous structures allows quantitatively analysis necessary
for understanding and improving the mass activity and lifetime of cobalt-based
catalysts, i.e., for Fischer-Tropsch synthesis that converts carbon monoxide
and hydrogen into fuels, and uncovering the catalyst optimization mechanisms. | cond-mat_mtrl-sci |
First-principles study of PbTiO$_3$ under uniaxial strains and stresses: The behavior of PbTiO$_3$ under uniaxial strains and stresses is investigated
from first-principles calculations within density functional theory. We show
that irrespectively of the uniaxial mechanical constraint applied, the system
keeps a purely ferroelectric ground-state, with the polarization aligned either
along the constraint direction ($FE_z$ phase) or along one of the pseudo-cubic
axis perpendicular to it ($FE_x$ phase). This contrasts with the cases of
isotropic or biaxial mechanical constraints for which novel phases combining
ferroelectic and antiferrodistortive motions have been previously reported.
Under uniaxial strain, PbTiO$_3$ switched from a $FE_x$ ground state under
compressive strain to $FE_z$ ground-state under tensile strain, beyond a
critical strain $\eta_{zz}^c \approx +1$\%. Under uniaxial stress, PbTiO$_3$
exhibits either a $FE_x$ ground state under compression ($\sigma_{zz} < 0$) or
a $FE_z$ ground state under tension ($\sigma_{zz} > 0$). Here, however, an
abrupt jump of the structural parameters is also predicted under both
compressive and tensile stresses at critical values $\sigma_{zz} \approx$ $+2$
GPa and $- 8$ GPa. This behavior appears similar to that predicted under
negative isotropic pressure and might reveal practically useful to enhance the
piezoelectric response in nanodevices. | cond-mat_mtrl-sci |
Surface-state-dominated transport in crystals of the topological
crystalline insulator In-doped Pb$_{1-x}$Sn$_x$Te: Three-dimensional topological insulators and topological crystalline
insulators represent new quantum states of matter, which are predicted to have
insulating bulk states and spin-momentum-locked gapless surface states.
Experimentally, it has proven difficult to achieve the high bulk resistivity
that would allow surface states to dominate the transport properties over a
substantial temperature range. Here we report a series of indium-doped
Pb$_{1-x}$Sn$_x$Te compounds that manifest huge bulk resistivities together
with strong evidence of topological surface states, based on
thickness-dependent transport studies and magnetoresistance measurements. For
these bulk-insulating materials, the surface states determine the resistivity
for temperatures approaching 30 K. | cond-mat_mtrl-sci |
Hydrogen Compounds of Group-IV Nanosheets: The structural and electronic properties of the hydrides of silicene and
germanene have been studied using ab initio calculations. The trend for the M-H
(M=C, Si, Ge) bond lengths, and corresponding bond energies, is consistent with
the atomic size trend, and comparable to those of MH_4 hydrides. Band
structures were also obtained for the buckled configuration, which is the
stable form for both silicene and germanene. Upon hydrogenation, both silicane
(indirect gap) and germanane (direct gap) are semiconducting. | cond-mat_mtrl-sci |
Spin-dependent Transparency of Ferromagnet/Superconductor Interfaces: Because the physical interpretation of the spin-polarization of a ferromagnet
determined by point-contact Andreev reflection (PCAR) is non-trivial, we have
carried out parameter-free calculations of PCAR spectra based upon a
scattering-theory formulation of Andreev reflection generalized to
spin-polarized systems and a tight-binding linear muffin tin orbital method for
calculating the corresponding scattering matrices. PCAR is found to measure the
spin-dependent interface transparency rather than the bulk polarization of the
ferromagnet which is strongly overestimated by free electron model fitting. | cond-mat_mtrl-sci |
Wave mechanics in media pinned at Bravais lattice points: The propagation of waves through microstructured media with periodically
arranged inclusions has applications in many areas of physics and engineering,
stretching from photonic crystals through to seismic metamaterials. In the
high-frequency regime, modelling such behaviour is complicated by multiple
scattering of the resulting short waves between the inclusions. Our aim is to
develop an asymptotic theory for modelling systems with arbitrarily-shaped
inclusions located on general Bravais lattices. We then consider the limit of
point-like inclusions, the advantage being that exact solutions can be obtained
using Fourier methods, and go on to derive effective medium equations using
asymptotic analysis. This approach allows us to explore the underlying reasons
for dynamic anisotropy, localisation of waves, and other properties typical of
such systems, and in particular their dependence upon geometry. Solutions of
the effective medium equations are compared with the exact solutions, shedding
further light on the underlying physics. We focus on examples that exhibit
dynamic anisotropy as these demonstrate the capability of the asymptotic theory
to pick up detailed qualitative and quantitative features. | cond-mat_mtrl-sci |
Anisotropic magnetoresistance of spin-orbit coupled carriers scattered
from polarized magnetic impurities: Anisotropic magnetoresistance (AMR) is a relativistic magnetotransport
phenomenon arising from combined effects of spin-orbit coupling and broken
symmetry of a ferromagnetically ordered state of the system. In this work we
focus on one realization of the AMR in which spin-orbit coupling enters via
specific spin-textures on the carrier Fermi surfaces and ferromagnetism via
elastic scattering of carriers from polarized magnetic impurities. We report
detailed heuristic examination, using model spin-orbit coupled systems, of the
emergence of positive AMR (maximum resistivity for magnetization along
current), negative AMR (minimum resistivity for magnetization along current),
and of the crystalline AMR (resistivity depends on the absolute orientation of
the magnetization and current vectors with respect to the crystal axes)
components. We emphasize potential qualitative differences between pure
magnetic and combined electro-magnetic impurity potentials, between short-range
and long-range impurities, and between spin-1/2 and higher spin-state carriers.
Conclusions based on our heuristic analysis are supported by exact solutions to
the integral form of the Boltzmann transport equation in archetypical
two-dimensional electron systems with Rashba and Dresselhaus spin-orbit
interactions and in the three-dimensional spherical Kohn-Littinger model. We
include comments on the relation of our microscopic calculations to standard
phenomenology of the full angular dependence of the AMR, and on the relevance
of our study to realistic, two-dimensional conduction-band carrier systems and
to anisotropic transport in the valence band of diluted magnetic
semiconductors. | cond-mat_mtrl-sci |
Coherent control of spontaneous emission of a three-level atom in a
coherent photonic band gap reservoir: By studying the fluorescence and optical properties of a three-level system,
we propose a new point of view on the coherent control of these spectra. With
the definite phase difference between the fields of the air band and dielectric
band in photonic band gap (PBG) reservoirs, the spectra of spontaneous
emission, absorption, and dispersion exhibit the coherent property and quantum
interference effect. This coherent interference depending on the position of
the embedded atom and the width of band gap causes the coupling of the
free-space light and the PBG light to result in blue shift of spectra and the
appearance of dark lines and kinks. By coherently controlling the
position-dependent dispersion, we can tune the frequency of slow light. | cond-mat_mtrl-sci |
Strain tunability of perpendicular magnetic anisotropy in van der Waals
ferromagnets VI3: Layered ferromagnets with high coercivity have special applications in
nanoscale memory elements in electronic circuits, such as data storage.
Therefore, searching for new hard ferromagnets and effectively tuning or
enhancing the coercivity are the hottest topics in layered magnets today. Here,
we report a strain tunability of perpendicular magnetic anisotropy in van der
Waals (vdW) ferromagnets VI3 using magnetic circular dichroism measurements.
For an unstrained flake, the M-H curve shows a rectangular-shaped hysteresis
loop with perpendicular magnetic anisotropy and a large coercivity (up to 1.775
T at 10 K). Furthermore, the coercivity can be enhanced to a maximum of 2.6 T
at 10 K under a 2.9% in-plane tensile strain. Our DFT calculations show that
the magnetic anisotropy energy (MAE) can be dramatically increased after
applying an in-plain tensile strain, which contributes to the enhancement of
coercivity in the VI3 flake. Meanwhile, the strain tunability on the coercivity
of CrI3, with a similar crystal structure, is limited. The main reason is the
strong spin-orbital coupling in V3+ in VI6 octahedra in comparison with that in
Cr3+. The strain tunability of coercivity in VI3 flakes highlights its
potential for integration into vdW heterostructures, paving the way toward
nanoscale spintronic devices and applications in the future. | cond-mat_mtrl-sci |
Mode-Dependent Damping in Metallic Antiferromagnets Due to
Inter-Sublattice Spin Pumping: Damping in magnetization dynamics characterizes the dissipation of magnetic
energy and is essential for improving the performance of spintronics-based
devices. While the damping of ferromagnets has been well studied and can be
artificially controlled in practice, the damping parameters of
antiferromagnetic materials are nevertheless little known for their physical
mechanisms or numerical values. Here we calculate the damping parameters in
antiferromagnetic dynamics using the generalized scattering theory of
magnetization dissipation combined with the first-principles transport
computation. For the PtMn, IrMn, PdMn and FeMn metallic antiferromagnets, the
damping coefficient associated with the motion of magnetization ($\alpha_m$) is
one to three orders of magnitude larger than the other damping coefficient
associated with the variation of the N\'eel order ($\alpha_n$), in sharp
contrast to the assumptions made in the literature. | cond-mat_mtrl-sci |
The mechanical response of a creased sheet: We investigate the mechanics of thin sheets decorated by non-interacting
creases. The system considered here consists in parallel folds connected by
elastic panels. We show that the mechanical response of the creased structure
is twofold, depending both on the bending deformation of the panels and the
hinge-like intrinsic response of the crease. We show that a characteristic
length scale, defined by the ratio of bending to hinge energies, governs
whether the structure's response consists in angle opening or panel bending
when a small load is applied. The existence of this length scale is a building
block for future works on origami mechanics | cond-mat_mtrl-sci |
Multiferroic and magnetoelectric nature of GaFeO3, AlFeO3 and related
oxides: GaFeO3, AlFeO3 and related oxides are ferrimagnetic exhibiting
magnetodielectric effect. There has been no evidence to date for
ferroelectricity and hence multiferroicity in these oxides. We have
investigated these oxides as well as oxides of the composition
Al1-x-yGaxFe1+yO3 (x = 0.2, y = 0.2) for possible ferroelectricity by carrying
out pyroelectric measurements. These measurements establish the occurrence of
ferroelectricity at low temperatures below the N\`eel temperature in these
oxides. They also exhibit significant magnetoelectric effect. We have tried to
understand the origin of ferroelectricity based on non-centrosymmetric magnetic
ordering and disorder by carrying out first-principles calculations. | cond-mat_mtrl-sci |
Complete Strain Mapping of Nanosheets of Tantalum Disulfide: Quasi-two-dimensional (quasi-2D) materials hold promise for future
electronics because of their unique band structures that result in electronic
and mechanical properties sensitive to crystal strains in all three dimensions.
Quantifying crystal strain is a prerequisite to correlating it with the
performance of the device, and calls for high resolution but spatially resolved
rapid characterization methods. Here we show that using fly-scan nano X-ray
diffraction we can accomplish a tensile strain sensitivity below 0.001% with a
spatial resolution of better than 80 nm over a spatial extent of 100 $\mu$m on
quasi 2D flakes of 1T-TaS2. Coherent diffraction patterns were collected from a
$\sim$ 100 nm thick sheet of 1T-TaS2 by scanning 12keV focused X-ray beam
across and rotating the sample. We demonstrate that the strain distribution
around micron and sub-micron sized 'bubbles' that are present in the sample may
be reconstructed from these images. The experiments use state of the art
synchrotron instrumentation, and will allow rapid and non-intrusive strain
mapping of thin film samples and electronic devices based on quasi 2D
materials. | cond-mat_mtrl-sci |
Highly sensitive NO2 sensors by pulsed laser deposition on graphene: Graphene as a single-atomic-layer material is fully exposed to environment
and has therefore a great potential for creating of sensitive gas sensors.
However, in order to realize this potential for different polluting gases,
graphene has to be functionalized - adsorption centers of different type and
with high affinity to target gases have to be created at its surface. In this
present work, modification of graphene by small amounts of laser ablated
materials is introduced for this purpose as a versatile and precise tool. The
approach was demonstrated with two very different materials chosen for pulsed
laser deposition (PLD), a metal (Ag) and a dielectric oxide (ZrO2). It was
shown that the gas response and its recovery rate can be significantly enhanced
by choosing the PLD target material and deposition conditions. The response to
NO2 gas in air was amplified up to 40 times in case of PLD-modified graphene in
comparison with pristine graphene and reached 7-8% at 40 ppb of NO2 and 20-30%
at 1 ppm of N2. These results were obtained after PLD in gas environment (5 x
10-2 mbar oxygen or nitrogen) and atomic areal densities of deposited materials
of were about 10 15 cm-2. The ultimate level of NO2 detection in air, as
extrapolated from the experimental data obtained at room temperature under mild
UV-excitation, was below 1 ppb. | cond-mat_mtrl-sci |
Impurity-induced transition to a Mott insulator in Sr$_3$Ru$_2$O$_7$: The electrical, magnetic, and structural properties of
Sr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$ (0 $\leq x \leq$ 0.2) are investigated. The
parent compound Sr$_3$Ru$_2$O$_7$ is a paramagnetic metal, critically close to
magnetic order. We have found that, with a Ru-site doping by only a few percent
of Mn, the ground state is switched from a paramagnetic metal to an
antiferromagnetic insulator. Optical conductivity measurements show the opening
of a gap as large as 0.1 eV, indicating that the metal-to-insulator transition
is driven by the electron correlation. The complex low-temperature
antiferromagnetic spin arrangement, reminiscent of those observed in some
nickelates and manganites, suggests a long range orbital order. | cond-mat_mtrl-sci |
Directional Anisotropy of Crack Propagation Along $Σ$3 Grain
Boundary in BCC Fe: Crack growth behaviour along the coherent twin boundary (CTB), i.e.,
$\Sigma$3{112} of BCC Fe is investigated using molecular dynamics (MD)
simulations. The growth of an atomistically sharp crack with {112}$<$110$>$
orientation has been examined along the two opposite $<$111$>$ directions of
CTB under mode-I loading at a constant strain rate. Separate MD simulations
were carried out with crack inserted in the left side, right side and middle of
the specimen model system. The results indicate that the crack grows
differently along the two opposite $<$111$>$ directions. In case of a crack
inserted in the left side, the crack grows in ductile manner, while it
propagates in semi-brittle manner in the case of crack inserted in the right
side. The directional dependence of crack growth along the CTB is also
confirmed by the stress-strain behaviour. This anisotropy in crack growth
behaviour has been attributed to the twinning-antitwinning asymmetry of
1/6$<$111$>$ partial dislocations on {112} planes. | cond-mat_mtrl-sci |
Refined Geometry and Frozen Phonons in KNbO3: In order to arrive at ultimately accurate results available with the LMTO
method in the local density approximation, the stability of full-potential LMTO
predictions for off-center displacements in KNbO3, as depending on the choice
of basis and expansion cutoffs, has been thoroughly investigated. With the
calculation setup thus optimized, supercell frozen phonon calculations aimed at
the study of the chain-structure instability over the Brillouin zone have been
done, and the long-wavelength limit of the LO phonon is discussed. | cond-mat_mtrl-sci |
Anomalous resistivity upturn in the van der Waals ferromagnet
Fe$_5$GeTe$_2$: Fe$_5$GeTe$_2$ (n = 3, 4, 5) have recently attracted increasing attention due
to their two-dimensional van der Waals characteristic and high temperature
ferromagnetism, which make promises for spintronic devices. The Fe(1) split
site is one important structural characteristic of Fe$_5$GeTe$_2$ which makes
it very different from other Fe$_5$GeTe$_2$ (n = 3, 4) systems. The local
atomic disorder and short-range order can be induced by the split site. In this
work, the high-quality van der Waals ferromagnet Fe$_5$GeTe$_2$ were grown to
study the low-temperature transport properties. We found a resistivity upturn
below 10 K. The temperature and magnetic field dependence of the resistivity
are in good agreement with a combination of the theory of disorder-enhanced
three-dimensional electron-electron and single-channel Kondo effect. The Kondo
effect exists only at low magnetic field B < 3 T, while electron-electron
dominates the appearance for the low-temperature resistivity upturn. We believe
that the enhanced three-dimensional electron-electron interaction in this
system is induced by the local atomic structural disorder due to the split site
of Fe(1). Our results indicate that the split site of Fe plays an important
role for the exceptional transport properties. | cond-mat_mtrl-sci |
Magnetic force microscopy investigation of arrays of nickel nanowires
and nanotubes: The magnetic properties of arrays of nanowires (NWs) and nanotubes (NTs), 150
nm in diameter, electrodeposited inside nanoporous polycarbonate membranes are
investigated. The comparison of the nanoscopic magnetic force microscopy (MFM)
imaging and the macroscopic behavior as measured by alternating gradient force
magnetometry (AGFM) is made. It is shown that MFM is a complementary technique
that provides an understanding of the magnetization reversal characteristics at
the microscopic scale of individual nanostructures. The local hysteresis loops
have been extracted by MFM measurements. The influence of the shape of such
elongated nanostructures on the dipolar coupling and consequently on the
squareness of the hysteresis curves is demonstrated. It is shown that the
nanowires exhibit stronger magnetic interactions than nanotubes. The
non-uniformity of the magnetization states is also revealed by combining the
MFM and AGFM measurements. | cond-mat_mtrl-sci |
First principles prediction of structural and electronic properties of
TlxIn(1-x)N alloy: Structural and electronic properties of zinc blende TlxIn(1-x)N alloy have
been evaluated from first principles. The band structures have been obtained
within the density functional theory (DFT), the modified Becke-Johnson (MBJLDA)
approach for the exchange-correlation potential, and fully relativistic
pseudopotentials. The calculated band-gap dependence on Tl content in this
hypothetical alloy exhibits a linear behaviour up to the 25 % of thalium
content where its values become close to zero. In turn, the split-off energy at
the Gamma point of the Brillouin zone, related to the spin-orbit coupling, is
predicted to be comparable in value with the band-gap for relatively low
thalium contents of about 5 %. These findings suggest TlxIn(1-x)N alloy as a
promising material for optoelectronic applications. Furthermore, the band
structure of TlN reveals some specific properties exhibited by topological
insulators. | cond-mat_mtrl-sci |
Observation of sharp metamagnetic transition, Griffiths like phase and
glassy nature in double perovskite Eu2CoMnO6: In the present investigation, some novel magnetic behaviors exhibited by
double perovskite (DP) Eu2CoMnO6(ECMO) has been reported. XRD analysis of ECMO
showed that it has a monoclinic crystal structure (space group P 21/n). A
second-order magnetic phase transition as a sudden jump in the magnetization
curve has been observed at 124.5 K. This is related to the paramagnetic to
ferromagnetic/E*-type antiferromagnetic phase transition due to the competing
Co-O-Mn exchange interactions. A clear low-temperature compensation point
followed by negative magnetization is observed in the zero-field-cooled curve
of the sample, suggesting the formation of canted ferromagnetic domains or
antiparallel spins and clusters that are separated by an antiphase boundary.
The large bifurcation between the ZFC and FC curves has been observed,
suggesting strong spin frustration is present in the system. More
interestingly, sharp multiple steps in magnetization are observed in M-H curve
at 2 K and observed only in the forward field-sweep direction which vanishes on
increasing temperature. Moreover, prominent smaller peaks immediately above the
long-range ordering temperature are observed suggesting the presence of
preformed percolating clusters which eventually gives rise to Griffiths like
phase which is seen in DC as well in AC susceptibility. The real part of AC
susceptibility with DC bias shows an unusual sharp peak near TC that broadens
on increasing field strength and splits into two maxima around 750 Oe, which is
attributed to the presence of critical fluctuations associated with a
continuous transition to the FM state and large magnetic anisotropy in the
system. | cond-mat_mtrl-sci |
Uncooled bolometer response of a low noise La2/3Sr1/3MnO3 thin film: We report measurements of the optical responses of a La2/3Sr1/3MnO3 (LSMO)
sample at a wavelength of 533 nm in the 300-400 K range. The 200 nm thick film
was grown by pulsed laser deposition on (100) SrTiO3 substrate and showed
remarkably low noise. At 335 K the temperature coefficient of the resistance of
a 100 micrometers wide 300 micrometers long LSMO line was 0.017 K-1 and the
normalized Hooge parameter was 9 e-30 m3, which is among the lowest reported
values. We then measured an optical sensitivity at I = 5 mA of 10.4 V.W-1 and
corresponding noise equivalent power (NEP) values of 8.1 e-10 W.Hz-1/2 and 3.3
e-10 W. Hz-1/2 at 30 Hz and above 1kHz, respectively. Simple considerations on
bias current conditions and thermal conductance G are finally given for further
sensitivity improvements using LSMO films. The performances were indeed
demonstrated on bulk substrates with G of 10-3 W.K-1. One could expect a NEP
reduction by three orders of magnitude if a membrane-type geometry was used,
which makes this LSMO device competitive against commercially available
uncooled bolometers. | cond-mat_mtrl-sci |
On the mechanism behind the inverse melting in systems with competing
interactions: Here we present a fundamental comprehension of the microscopic mechanisms
leading to the emergence of inverse melting transitions by considering a
thorough mean-field analysis of a variety of minimal models with different
competing interactions. Through analytical and numerical tools we identify the
specific connections between the characteristic energy of the homogeneous and
modulated phases and the observed reentrant behaviors. In particular, we find
that reentrance is appreciable when the characteristic energy cost of the
homogeneous and modulated phases are comparable to each other, and for systems
in which the local order parameter is limited. In the asymptotic limit of high
energy cost of the homogeneous phase we obtain analytically that the degree of
reentrance of the phase diagram decreases exponentially with the ratio of the
characteristic energy cost of homogeneous and modulated phases. We are also
able to establish theoretical (upper and lower) bounds for the degree of the
reentrance, according to the nature of the competing interactions. Finally, we
confront our mean-field results with Langevin simulations of an effective
coarse grained model, confirming the main results regarding the degree of the
reentrance in the phase diagram. These results shed new light on the many
systems undergoing inverse melting transitions, from magnets to colloids and
vortex matter, by qualitatively improving the understanding of the interplay of
entropy and energy around the inverse melting points. | cond-mat_mtrl-sci |
A Predictive Multiphase Model of Silica Aerogels for Building Envelope
Insulations: This work develops a multiphase thermomechanical model of porous silica
aerogel and implements an uncertainty analysis framework consisting of the
Sobol methods for global sensitivity analyses and Bayesian inference using a
set of experimental data of silica aerogel. A notable feature of this work is
implementing a new noise model within the Bayesian inversion to account for
data uncertainty and modeling error. The hyper-parameters in the likelihood
balance data misfit and prior contribution to the parameter posteriors and
prevent their biased estimation. The results indicate that the uncertainty in
solid conductivity and elasticity are the most influential parameters affecting
the model output variance. Also, the Bayesian inference shows that despite the
microstructural randomness in the thermal measurements, the model captures the
data with 2% error. However, the model is inadequate in simulating the
stress-strain measurements resulting in significant uncertainty in the
computational prediction of a building insulation component. | cond-mat_mtrl-sci |
Effect of strain on electronic and thermoelectric properties of few
layers to bulk MoS$_{2}$: The sensitive dependence of electronic and thermoelectric properties of
MoS$_2$ on the applied strain opens up a variety of applications in the
emerging area of straintronics. Using first principles based density functional
theory calculations, we show that the band gap of few layers of MoS$_2$ can be
tuned by applying i) normal compressive (NC), ii) biaxial compressive (BC), and
iii) biaxial tensile (BT) strain. A reversible semiconductor to metal
transition (S-M transition) is observed under all three types of strain. In the
case of NC strain, the threshold strain at which S-M transition occurs
increases with increasing number of layers and becomes maximum for the bulk. On
the other hand, the threshold strain for S-M transition in both BC and BT
strain decreases with the increase in number of layers. The difference in the
mechanisms for the S-M transition is explained for different types of applied
strain. Furthermore, the effect of strain type and number of layers on the
transport properties are also studied using Botzmann transport theory. We
optimize the transport properties as a function of number of layers and applied
strain. 3L- and 2L-MoS$_2$ emerge as the most efficient thermoelectric material
under NC and BT strain, respectively. The calculated thermopower is large and
comparable to some of the best thermoelectric materials. A comparison between
the feasibility of these three types of strain is also discussed. | cond-mat_mtrl-sci |
Plethora of tunable Weyl fermions in kagome magnet Fe3Sn2 thin films: Interplay of magnetism and electronic band topology in unconventional magnets
enables the creation and fine control of novel electronic phenomena. In this
work, we use scanning tunneling microscopy and spectroscopy to study thin films
of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually
large number of densely-spaced spectroscopic features straddling the Fermi
level. These are consistent with signatures of low-energy Weyl fermions and
associated topological Fermi arc surface states predicted by theory. By
measuring their response as a function of magnetic field, we discover a
pronounced evolution in energy tied to the magnetization direction. Electron
scattering and interference imaging further demonstrates the tunable nature of
a subset of related electronic states. Our experiments provide the first
visualization of how in-situ spin reorientation drives changes in the
electronic density of states of the Weyl fermion band structure. Combined with
previous reports of massive Dirac fermions, flat bands and electronic
nematicity, our work establishes Fe3Sn2 as a unique platform that harbors an
extraordinarily wide array of topological and correlated electron phenomena. | cond-mat_mtrl-sci |
Formation of Core-Shell Precipitates in off-stochiometric Ni-Mn-Sn
Heusler alloys probed through the induced Sn-moment: The Shell-ferromagnetic effect originates from the segregation process in
off-stochiometric Ni-Mn-based Heusler. In this work, we investigate the
precipitation process of L2$_1$-ordered Ni$_2$MnSn and L1$_0$-ordered NiMn in
off-stochiometric Ni$_{50}$Mn$_{45}$Sn$_{5}$ during temper annealing, by X-ray
diffraction (XRD) and $^{119}$Sn M\"ossbauer spectroscopy. While XRD probes
long-range ordering of the lattice structure, M\"ossbauer spectroscopy probes
nearest-neighbour interactions, reflected in the induced Sn magnetic moment. As
shown in this work, the induced magnetic Sn moment can be used as a detector
for microscopic structural changes and is, therefore, a powerful tool for
investigating the formation of nano-precipitates. Similar research can be
performed in the future, for example, on different pinning type magnets like
Sm-Co or Nd-Fe-B. | cond-mat_mtrl-sci |
A systematic study of four series of electron-doped rare earth
manganates, LnxCa1-xMnO3 (Ln=La, Nd, Gd and Y) over the x=0.02-0.25
composition range: Electrical and magnetic properties of four series of manganates LnxCa1-xMnO3
(Ln=La, Nd, Gd and Y) have been studied in the electron doped regime
(x=0.02-0.25) in order to investigate the various inter-dependent phenomena
such as ferromagnetism, phase separation and charge ordering. The general
behavior of all the four series of manganates is similar, with some of the
properties showing dependence on the average radius of the A-site cations, <rA>
and cation size disorder. Thus, all the compositions show increase in
magnetization at 100-120 K (TM) for x<xmax, the magnetization increasing with
increasing x. The value of xmax increases with decreasing <rA>, probably due to
the increased phase separation induced by site disorder. This is also reflected
in the larger width of the hysteresis loops at T<TM for small x or <rA>. In
this regime, the electrical resistivity decreases with increasing x, but
remains low and nearly constant T>TM. The percolative nature of the conduction
mechanism at T<TM is substantiated by the fit of the conductivity data to the
scaling law, s \mu |xc-x|p where p is in the 2-4 range. When x>xmax, the
materials become antiferromagnetic and charge-ordered at a temperature TCA,
accompanied by a marked increase in resistivity. The value of TCA increases
with increase in <rA> and x (upto x=0.3). Thus, all the four series of
manganates are characterized by a phase-separated regime between x=0.02 and
0.1-0.15 and an antiferromagnetic charge-ordered regime at x>0.1-0.15. | cond-mat_mtrl-sci |
Electronic stopping for protons and α particles from
first-principles electron dynamics: The case of silicon carbide: We present the first-principles determination of electronic stopping power
for protons and {\alpha} particles in a semiconductor material of great
technological interest: silicon carbide. The calculations are based on
nonequilibrium simulations of the electronic response to swift ions using
real-time, time-dependent density functional theory (RT-TDDFT). We compare the
results from this first-principles approach to those of the widely used linear
response formalism and determine the ion velocity regime within which linear
response treatments are appropriate. We also use the nonequilibrium electron
densities in our simulations to quantitatively address the longstanding
question of the velocity-dependent effective charge state of projectile ions in
a material, due to its importance in linear response theory. We further examine
the validity of the recently proposed centroid path approximation for reducing
the computational cost of acquiring stopping power curves from RT-TDDFT
simulations. | cond-mat_mtrl-sci |
Two and one-dimensional honeycomb structures of silicon and germanium: Based on first-principles calculations of structure optimization, phonon
modes and finite temperature molecular dynamics, we predict that silicon and
germanium have stable, two-dimensional, low-buckled, honeycomb structures.
Similar to graphene, they are ambipolar and their charge carriers can behave
like a massless Dirac fermions due to their pi- and pi*-bands which are crossed
linearly at the Fermi level. In addition to these fundamental properties, bare
and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and
magnetic properties, which are size and orientation dependent. These properties
offer interesting alternatives for the engineering of diverse nanodevices. | cond-mat_mtrl-sci |
Total energy calculation for the metallic hcp phase of Zn in the bulk,
layered, and quantum dot limits: The structural and electronic properties of the metallic hcp phase of Zn in
the bulk, monolayer, bilayer, and quantum dot limits have been studied by using
total energy calculations. From our calculated density of states and electronic
band structure, in agreement with previous work, bulk hybridization of the
Zn--$4s$, $3p$, and $3d$ orbitals is obtained. Furthermore, we found that this
orbital hybridization is also obtained for the monolayer, bilayer, and quantum
dot systems. At the same time, we found that the Zn monolayer and bilayer
systems show electronic properties characteristic of lamellar systems, while
the quantum dot system shows the behavior predicted for a 0D system. | cond-mat_mtrl-sci |
Possible structural and bond reconstruction in 2D ferromagnetic
semiconductor VSe2 under uniaxial stress: 2D semiconducting transition metal dichalcogenides have been used to make
high-performance electronic, spintronic, and optoelectronic devices. Recently,
room-temperature ferromagnetism and semiconducting property were found in 2D
VSe$_2$ nanoflakes (mechanically exfoliated onto silicon substrates capped with
a oxide layer) and are attributed to the stable 2H-phase of VSe$_2$ in the 2D
limit. Here, our first-principles investigation show that a metastable
semiconducting H' phase can be formed from the H VSe2 monolayer and some other
similar when these 2D H-phase materials are under uniaxial stress or uniaxial
strain. For the uniaxial stress (uniaxial strain) scheme, the H' phase will
become lower in total energy than the H phase at the transition point. The
calculated phonon spectra indicate the dynamical stability of the H' structures
of VSe$_2$, VS$_2$, and CrS$_2$, and the path of phase switching between the H
and H' VSe$_2$ phases is calculated. For VSe$_2$, the H' phase has stronger
ferromagnetism and its Currier temperature can be substantially enhanced by
applying uniaxial stress or strain. Spin-resolved electronic structures, energy
band edges, and effective carrier masses for both of the H and H' phases can be
substantially changed by the applied uniaxial stress or strain, leading to huge
effective masses near the band edge of the strained H' phase. Analysis
indicated that the largest bond length difference between the H' and H phases
can reach -19\% for the Se3-Se3' bond, and there is noticeable covalence for
the Se3-Se3' bond, which switches the valence of the nearby V atoms, leading to
the enhanced ferromagnetism. Therefore, structural and bond reconstruction can
be realized by applying uniaxial stress in 2D ferromagnetic H VSe$_2$ and some
other similar. These can be useful to seeking more insights and phenomena in
such 2D materials for potential applications. | cond-mat_mtrl-sci |
Thermoelectric figure of merit of tau-type conductors of several donors: Dimensionless thermoelectric figure of merit $ZT$ is investigated for
two-dimensional organic conductors $\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$,
$\tau$-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and
$\tau$-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ ($y \le 0.875$), respectively. The
$ZT$ values were estimated by measuring electrical resistivity, thermopower and
thermal conductivity simultaneously. The largest $ZT$ is 2.7 $\times$ 10$^{-2}$
at 155 K for $\tau-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, 1.5 $\times$ 10$^{-2}$
at 180 K for $\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and 5.4 $\times$
10$^{-3}$ at 78 K for $\tau-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, respectively.
Substitution of the donor molecules fixing the counter anion revealed
EDT-S,S-DMEDT-TTF is the best of the three donors to obtain larger $ZT$. | cond-mat_mtrl-sci |
Broken translational and rotational symmetries in LiMn1.5Ni0.5O4 spinel: In condensed matter physics broken symmetries and emergence of
quasi-particles are intimately linked to each other. Whenever a symmetry is
broken, it leaves its fingerprints, and that may be observed indirectly via its
influence on the other quasi-particles. Here, we report the strong signature of
broken rotational symmetry induced due to long range-ordering of spins in Mn -
sublattice of LiMn1.5Ni0.5O4 below Tc ~ 113 K reflected with the marked changes
in the lattice vibrations using Raman scattering. In particular, the majority
of the observed first-order phonon modes show a sharp shift in frequency in the
vicinity of long range magnetic-ordering temperature. Phonons exist in a
crystalline system because of broken translational symmetry, therefore any
renormalization in the phonon-spectrum could be a good gauge for broken
translational symmetry. Anomalous evolution of the few modes associated with
stretching of Mn/NiO6 octahedra in the intermediate temperature range (~ 60-260
K) marked the broken translational symmetry attributed to the charge ordering.
Interestingly same modes also show strong coupling with magnetic degrees of
freedom, suggesting that charge-ordering and magnetic transition may be linked
to each other. | cond-mat_mtrl-sci |
Demystifying magnetic resonance measurements of the true diffusion
propagator: In a recent work, a method for the magnetic resonance (MR) measurement of the
true diffusion propagator was introduced, which was subsequently implemented
and validated for free diffusion on a benchtop MR scanner. Here, we provide a
brief theoretical description of the method and discuss various experimental
regimes. | cond-mat_mtrl-sci |
Intraband divergences in third order optical response of 2D systems: The existence of large nonlinear optical coefficients is one of the
preconditions for using nonlinear optical materials in nonlinear optical
devices. For a crystal, such large coefficients can be achieved by matching
photon energies with resonant energies between different bands, and so the
details of the crystal band structure play an important role. Here we
demonstrate that large third-order nonlinearities can also be generally
obtained by a different strategy: As any of the incident frequencies or the sum
of any two or three frequencies approaches zero, the doped or excited
populations of electronic states lead to divergent contributions in the induced
current density. We refer to these as intraband divergences, by analogy with
the behavior of Drude conductivity in linear response. Physically, such
resonant processes can be associated with a combination of inraband and
interband optical transitions. Current-induced second order nonlinearity,
coherent current injection, and jerk currents are all related to such
divergences, and we find similar divergences in degenerate four wave mixing and
cross-phase modulation under certain conditions. These divergences are limited
by intraband relaxation parameters, and lead to a large optical response from a
high quality sample; we find they are very robust with respect to variations in
the details of the band structure. To clearly track all of these effects, we
analyze gapped graphene, describing the electrons as massive Dirac fermions;
under the relaxation time approximation, we derive analytic expressions for the
third order conductivities, and identify the divergences that arise in
describing the associated nonlinear phenomena. | cond-mat_mtrl-sci |
GaAs(111)A and B in hydrazine sulfide solutions : extreme polarity
dependence of surface adsorption processes: Chemical bonds formed by hydrazine-sulfide treatment of GaAs(111) were
studied by synchrotron photoemission spectroscopy. At the B surface, the top
arsenic atoms are replaced by nitrogen atoms, while GaAs(111)A is covered by
sulfur, also bonded to underlying gallium, despite the sulfide molar
concentration being 103 times smaller than that of the hydrazine. This extreme
dependence on surface polarity is explained by competitive adsorption processes
of HS- and OH- anions and of hydrazine molecules, on Ga- adsorption sites,
which have distinct configurations on the A and B surfaces. | cond-mat_mtrl-sci |
Symmetric Versus Nonsymmetric Structure of the Phosphorus Vacancy on
InP(110): The atomic and electronic structure of positively charged P vacancies on
InP(110) surfaces is determined by combining scanning tunneling microscopy,
photoelectron spectroscopy, and density-functional theory calculations. The
vacancy exhibits a nonsymmetric rebonded atomic configuration with a charge
transfer level 0.75+-0.1 eV above the valence band maximum. The scanning
tunneling microscopy (STM) images show only a time average of two degenerate
geometries, due to a thermal flip motion between the mirror configurations.
This leads to an apparently symmetric STM image, although the ground state
atomic structure is nonsymmetric. | cond-mat_mtrl-sci |
Discovery of stable surfaces with extreme work functions by
high-throughput density functional theory and machine learning: The work function is the key surface property that determines how much energy
is required for an electron to escape the surface of a material. This property
is crucial for thermionic energy conversion, band alignment in
heterostructures, and electron emission devices. Here, we present a
high-throughput workflow using density functional theory (DFT) to calculate the
work function and cleavage energy of 33,631 slabs (58,332 work functions) that
we created from 3,716 bulk materials, including up to ternary compounds. The
number of materials for which we calculated surface properties surpasses the
previously largest database, the Materials Project, by a factor of $\sim$27. On
the tail ends of the work function distribution we identify 34 and 56 surfaces
with an ultra-low (<2 eV) and ultra-high (>7 eV) work function, respectively.
Further, we discover that the $(100)$-Ba-O surface of BaMoO$_3$ and the
$(001)$-F surface of Ag$_2$F have record-low (1.25 eV) and record-high (9.06
eV) steady-state work functions without requiring coatings, respectively. Based
on this database we develop a physics-based approach to featurize surfaces and
use supervised machine learning to predict the work function. We find that
physical choice of features improves prediction performance far more than
choice of model. Our random forest model achieves a mean absolute test error of
0.09 eV, which is more than 6 times better than the baseline and comparable to
the accuracy of DFT. This surrogate model enables rapid predictions of the work
function ($\sim 10^5$ faster than DFT) across a vast chemical space and
facilitates the discovery of material surfaces with extreme work functions for
energy conversion, electronic applications, and contacts in 2-dimensional
devices. | cond-mat_mtrl-sci |
PhySRNet: Physics informed super-resolution network for application in
computational solid mechanics: Traditional approaches based on finite element analyses have been
successfully used to predict the macro-scale behavior of heterogeneous
materials (composites, multicomponent alloys, and polycrystals) widely used in
industrial applications. However, this necessitates the mesh size to be smaller
than the characteristic length scale of the microstructural heterogeneities in
the material leading to computationally expensive and time-consuming
calculations. The recent advances in deep learning based image super-resolution
(SR) algorithms open up a promising avenue to tackle this computational
challenge by enabling researchers to enhance the spatio-temporal resolution of
data obtained from coarse mesh simulations. However, technical challenges still
remain in developing a high-fidelity SR model for application to computational
solid mechanics, especially for materials undergoing large deformation. This
work aims at developing a physics-informed deep learning based super-resolution
framework (PhySRNet) which enables reconstruction of high-resolution
deformation fields (displacement and stress) from their low-resolution
counterparts without requiring high-resolution labeled data. We design a
synthetic case study to illustrate the effectiveness of the proposed framework
and demonstrate that the super-resolved fields match the accuracy of an
advanced numerical solver running at 400 times the coarse mesh resolution while
simultaneously satisfying the (highly nonlinear) governing laws. The approach
opens the door to applying machine learning and traditional numerical
approaches in tandem to reduce computational complexity accelerate scientific
discovery and engineering design. | cond-mat_mtrl-sci |
Direct Measurement of the Electronic Structure and band gap nature of
atomic-layer-thick 2H-MoTe2: The millimeter sized monolayer and bilayer 2H-MoTe2 single crystal samples
are prepared by a new mechanical exfoliation method. Based on such high-quality
samples, we report the first direct electronic structure study on them, using
standard high resolution angle-resolved photoemission spectroscopy (ARPES). A
direct band gap of 0.924eV is found at K in the rubidium-doped monolayer MoTe2.
Similar valence band alignment is also observed in bilayer MoTe2,supporting an
assumption of a analogous direct gap semiconductor on it. Our measurements
indicate a rather large band splitting of 212meV at the valence band maximum
(VBM) in monolayer MoTe2, and the splitting is systematically enlarged with
layer stacking, from monolayer to bilayer and to bulk. Meanwhile, our PBE band
calculation on these materials show excellent agreement with ARPES results.
Some fundamental electronic parameters are derived from the experimental and
calculated electronic structures. Our findings lay a foundation for further
application-related study on monolayer and bilayer MoTe2. | cond-mat_mtrl-sci |
Magnetoelectric coupling in polycrystalline FeVO4: We report coupling between magnetic and electric orders for antiferromagnetic
polycrystalline FeVO4 in which magnetism-induced polarization has been recently
found in noncollinear antiferromagnetic state below the second
antiferromagnetic phase transition at TN2=15.7K. In this low symmetry phase
space group P-1, the magnetic field dependence of electric polarization
evidences a clear magnetoelectric coupling in the noncollinear spin-configured
antiferromagnetic phase. The discontinuity of magnetodielectric effect observed
at the vicinity of the polar to nonpolar transition evidences competition
between different magnetodielectric couplings in the two different
antiferromagnetic states. The existence of thermal expansion anomaly near TN2
and magnetostriction effect support magnetoelastically mediated scenario of the
observed magnetoelectric effect. | cond-mat_mtrl-sci |
Magnetism of two-dimensional defects in Pd: stacking faults, twin
boundaries and surfaces: Careful first-principles density functional calculations reveal the
importance of hexagonal versus cubic stacking of closed packed planes of Pd as
far as local magnetic properties are concerned. We find that, contrary to the
stable face centered cubic phase, which is paramagnetic, the hexagonal
close-packed phase of Pd is ferromagnetic with a magnetic moment of 0.35
$\mu_{B}$/atom. Our results show that two-dimensional defects with local hcp
stacking, like twin boundaries and stacking faults, in the otherwise fcc Pd
structure, increase the magnetic susceptibility. The (111) surface also
increases the magnetic susceptibility and it becomes ferromagnetic in
combination with an individual stacking fault or twin boundary close to it. On
the contrary, we find that the (100) surface decreases the tendency to
ferromagnetism. The results are consistent with the magnetic moment recently
observed in small Pd nanoparticles, with a large surface area and a high
concentration of two-dimensional stacking defects. | cond-mat_mtrl-sci |
Detecting and Directing Single Molecule Binding Events on H-Si(100) with
Application to Ultra-dense Data Storage: Many new material systems are being explored to enable smaller, more capable
and energy efficient devices. These bottom up approaches for atomic and
molecular electronics, quantum computation, and data storage all rely on a
well-developed understanding of materials at the atomic scale. Here, we report
a versatile scanning tunneling microscope (STM) charge characterization
technique, which reduces the influence of the typically perturbative STM tip
field, to develop this understanding even further. Using this technique, we can
now observe single molecule binding events to atomically defined reactive sites
(fabricated on a hydrogen-terminated silicon surface) through electronic
detection. We then developed a new error correction tool for automated hydrogen
lithography, directing molecular hydrogen binding events using these sites to
precisely repassivate surface dangling bonds (without the use of a scanned
probe). We additionally incorporated this molecular repassivation technique as
the primary rewriting mechanism in new ultra-dense atomic data storage designs
(0.88 petabits per in$^{2}$). | cond-mat_mtrl-sci |
Theory of magnetic domains in uniaxial thin films: For uniaxial easy axis films, properties of magnetic domains are usually
described within the Kittel model, which assumes that domain walls are much
thinner than the domains. In this work we present a simple model that includes
a proper description of the magnetostatic energy of domains and domain walls
and also takes into account the interaction between both surfaces of the film.
Our model describes the behavior of domain and wall widths as a function of
film thickness, and is especially well suited for the strong stripe phase. We
prove the existence of a critical value of magneto-crystalline anisotropy above
which stripe domains exist for any film thickness and justify our model by
comparison with exact results. The model is in good agreement with experimental
data for hcp cobalt. | cond-mat_mtrl-sci |
Current correlation functions for chemical sensors based on DNA
decorated carbon nanotube: The current characteristics of DNA decorated carbon nanotubes for different
gas odors are studied. A simple model of charge transfer between the
Gas-DNA-base complex and single wall carbon nanotube (SWCN) is proposed to
explain the current response for different odors. The autocorrelation and
two-point correlation functions are calculated for the current sensitivity
curves. These correlation functions together with the current characteristics
form finger-prints for detection of the odor and DNA sequence. | cond-mat_mtrl-sci |
Measuring Dislocation Density in Aluminum with Resonant Ultrasound
Spectroscopy: Dislocations in a material will, when present in enough numbers, change the
speed of propagation of elastic waves. Consequently, two material samples,
differing only in dislocation density, will have different elastic constants, a
quantity that can be measured using Resonant Ultrasound Spectroscopy.
Measurements of this effect on aluminum samples are reported. They compare well
with the predictions of the theory. | cond-mat_mtrl-sci |
Novel Cyano-Bridged 4f-3d Coordination Polymers with a Unique 2D
Topological Architecture and Unusual Magnetic Behavior: Cyano-bridged bimetallic hybrid Prussian Blue one- to three-dimensional
(1D-3D) coordination polymers based on [M(CN)6]3- (M = Fe, Cr, Mn) have
attracted great attention because of their rich and interesting structures and
magnetic behaviors. The previous study implies that increasing dimensionality
may enhance and improve bulk magnetic properties.Our strategy for the rational
synthesis of high-dimensional network is to use a suitable combination of
cyanide groups and other bridging ligands. Here, 2,2'-bipyrimidine (bpym) was
selected, in preference to 4,4'-bipyrazine and pyrazine, because it is more
capable of transmitting magnetic interactions and its bis(chelating)
coordination modes facilitate connection between lanthanide ions. Unexpectedly,
two novel coordination polymers [NdM(bpym)(H2O)4(CN)6]3H2O (M = Fe 1, Co 2;)
were obtained, which have a unique 2D topological architecture, and exhibit
unusual magnetic behavior. | cond-mat_mtrl-sci |
Trends in Ferromagnetism in Mn doped dilute III-V alloys from a density
functional perspective: Mn doping in dilute III-V alloys has been examined as a route to enhance
ferromagnetic stability. Strong valence band bowing is expected at the dilute
limit, implying a strong modification of the ferromagnetic stability upon
alloying, with even an increase in some cases. Using first principle electronic
structure calculations we show that while codoping with a group V anion
enhances the ferromagnetic stability in some cases when the effects of
relaxation of the lattice are not considered, strong impurity scattering in the
relaxed structure result in a reduction of the ferromagnetic stability. | cond-mat_mtrl-sci |
Magnetism, spin-wave relaxation and spiral exchange in a trilayer
magnetic junction: We study the non-collinear exchange coupling across a trilayer magnetic
junction consisting of two ferromagnets separated by a thin dilute magnetic
semiconductor containing itinerant carriers with finite spin relaxation. It is
remarkable that, by increasing the spin relaxation, the critical temperature is
substantially enhanced and the shape of the magnetization curve becomes more
mean-field like. We attribute these interesting changes to the broken
time-reversal symmetry which suppresses the oscillatory
Ruderman-Kittel-Kasuya-Yosida interaction. Our argument is further strengthened
by the emergence of the non-collinear spiral exchange coupling across the
trilayer magnetic junction with finite spin relaxation. | cond-mat_mtrl-sci |
Observation of magnetocapacitance in ferromagnetic nanowires: The authors have investigated magnetic domain wall induced capacitance
variation as a tool for the detection of magnetic reversal in magnetic
nanowires for in-plane (NiFe) and out-of-plane (Co/Pd) magnetization
configurations. The switching fields in the capacitance measurements match with
that of the magnetoresistance measurements in the opposite sense. The origin of
the magnetocapacitance has been attributed to magnetoresistance. This
magnetocapacitance detection technique can be useful for magnetic domain wall
studies. | cond-mat_mtrl-sci |
The First Synchrotron Infrared Beamlines at the ALS: Spectromicroscopy
and Fast Timing: Two recently commissioned infrared beamlines on the 1.4 bending magnet port
at the Advanced Light Source, LBNL, are described. Using a synchrotron as an IR
source provides three primary advantages: increased brightness, very fast light
pulses, and enhanced far-IR flux. The considerable brightness advantage
manifests itself most beneficially when performing spectroscopy on a
microscopic length scale. Beamline (BL) 1.4.3 is a dedicated FTIR
spectromicroscopy beamline, where a diffraction-limited spot size using the
synchrotron source is utilized. BL 1.4.2 consists of a vacuum FTIR bench with a
wide spectral range and step-scan capability. This BL makes use of the pulsed
nature of the synchrotron light as well as the far-IR flux. Fast timing is
demonstrated by observing the pulses from the electron bunch storage pattern at
the ALS. Results from several experiments from both IR beamlines will be
presented as an overview of the IR research currently being done at the ALS. | cond-mat_mtrl-sci |
SLKMC-II study of self-diffusion of small Ni clusters on Ni (111)
surface: We studied self-diffusion of small 2D Ni islands (consisting of up to 10
atoms) on Ni (111) surface using a self-learning kinetic Monte Carlo (SLKMC-II)
method with an improved pattern-recognition scheme that allows inclusion of
both fcc and hcp sites in the simulations. In an SLKMC simulation, a database
holds information about the local neighborhood of an atom and associated
processes that is accumulated on-the-fly as the simulation proceeds. In this
study, these diffusion processes were identified using the drag method, and
their activation barriers calculated using a semi-empirical interaction
potential based on the embedded-atom method. Although a variety of concerted,
multi-atom and single-atom processes were automatically revealed in our
simulations, we found that these small islands diffuse primarily via concerted
diffusion processes. We report diffusion coefficients for each island size at
various tepmratures, the effective energy barrier for islands of each size and
the processes most responsible for diffusion of islands of various sizes,
including concerted and multi-atom processes that are not accessible under
SLKMC-I or in short time-scale MD simulations. | cond-mat_mtrl-sci |
Spin-filtering and Disorder Induced Giant Magnetoresistance in Carbon
Nanotubes: Ab Initio Calculations: Nitrogen-doped carbon nanotubes can provide reactive sites on the
porphyrin-like defects. It's well known that many porphyrins have transition
metal atoms, and we have explored transition metal atoms bonded to those
porphyrin-like defects in N-doped carbon nanotubes. The electronic structure
and transport are analyzed by means of a combination of density functional
theory and recursive Green's functions methods. The results determined the Heme
B-like defect (an iron atom bonded to four nitrogens) as the most stable and
with a higher polarization current for a single defect. With randomly
positioned Heme B-defects in a few hundred nanometers long nanotubes the
polarization reaches near 100% meaning an effective spin filter. A disorder
induced magnetoresistance effect is also observed in those long nanotubes,
values as high as 20000% are calculated with non-magnectic eletrodes. | cond-mat_mtrl-sci |
Ab initio DFT+U study of He atom incorporation into UO2 crystals: We present and discuss results of a density functional theory (DFT) study of
a perfect UO2 crystals and He atoms in octahedral interstitial positions. We
have calculated basic bulk crystal properties and He incorporation energies
into the low temperature anti-ferromagnetic UO2 phase using several
exchange-correlation functionals within the spin-polarized local density (LDA)
and generalized gradient (GGA) approximations. In all these DFT calculations we
included the on-site correlation corrections using the Hubbard model (DFT+U
approach). We analysed a potential crystalline symmetry reduction and confirmed
the presence of the Jahn-Teller effect in a perfect UO2. We discuss also the
problem of a conducting electronic state arising when He is placed into a
tetragonal antiferromagnetic phase of UO2. Consequently, we found a specific
lattice distortion which allows us to restore the semiconducting state and
properly estimate He incorporation energies. Unlike the bulk properties, the He
incorporation energy strongly depends on several factors, including the
supercell size, the use of spin polarization, the exchange-correlation
functionals and on-site correlation corrections. We compare our results for the
He incorporation with the previous shell model and ab initio DFT calculations. | cond-mat_mtrl-sci |
Magnetic nanographite: Hydrogenated nanographite can display spontaneous magnetism. Recently we
proposed that hydrogenation of nanographite is able to induce finite
magnetization. We have performed theoretical investigation of a graphene ribbon
in which each carbon is bonded to two hydrogen atoms at one edge and to a
single hydrogen atom at another edge. Application of the local-spin-density
approximation to the calculation of the electronic band-structure of the ribbon
shows appearance of a spin-polarized flat band at the Fermi energy. Producing
different numbers of mono-hydrogenated carbons and di-hydrogenated carbons can
create magnetic moments in nanographite. | cond-mat_mtrl-sci |
Static corrections versus dynamic correlation effects in the valence
band Compton profile spectra of Ni: We compute the Compton profile of Ni using the Local Density Approximation of
Density Functional Theory supplemented with electronic correlations treated at
different levels. The total/magnetic Compton profiles show not only
quantitative but also qualitative significant differences depending weather
Hubbard corrections are treated at a mean field +U or in a more sophisticated
dynamic way. Our aim is to discuss the range and capability of electronic
correlations to modify the kinetic energy along specific spatial directions.
The second and the fourth order moments of the difference in the Compton
profiles are discussed as a function of the strength of local Coulomb
interaction $U$. | cond-mat_mtrl-sci |
Temperature and bias voltage dependence of Co/Pd multilayer-based
magnetic tunnel junctions with perpendicular magnetic anisotropy: Temperature- and bias voltage-dependent transport measurements of magnetic
tunnel junctions (MTJs) with perpendicularly magnetized Co/Pd electrodes are
presented. Magnetization measurements of the Co/Pd multilayers are performed to
characterize the electrodes. The effects of the Co layer thickness in the Co/Pd
bilayers, the annealing temperature, the Co thickness at the MgO barrier
interface, and the number of bilayers on the tunneling magneto resistance (TMR)
effect are investigated. TMR-ratios of about 11 % at room temperature and 18.5
% at 13 K are measured and two well-defined switching fields are observed. The
results are compared to measurements of MTJs with Co-Fe-B electrodes and
in-plane anisotropy. | cond-mat_mtrl-sci |
Low-Energy Electron Diffraction With Energy Invariant Carrier Wave
Wavenumber Modulated by Exchange-Correlation Interaction: We present low-energy electron diffraction (LEED) as elastic electron-atom
scattering (EEAS) operating in a target crystal waveguide where a Coulombic
carrier wave is wavenumber modulated by exchange-correlation (XC) interaction.
Carrier potential is designed using a KKR (Korringa-Kohn-Rostoker) muffin-tin
model built on overlapping free atoms. XC potential is constructed using
Sernelius's many-particle theory on electron self-energy. EEAS phase shifts are
derived from Dirac's differential equations, and four recent LEED
investigations are recalculated: Cu(111)+$( 3\!\surd3\times\!\surd3 )
\mathrm{R30^\circ}$-TMB, Ag(111)+$(4\!\times \!4 )$-O, Ag(111)+$(
7\!\times\!\surd3 ) \mathrm{rect}$-$\mathrm{SO}_4$, Ru(0001)+$(
\surd3\!\times\!\surd3 ) \mathrm{R30^\circ}$-C. TMB stands for
1,3,5-tris(4-mercaptophenyl)-benzene with chemical formula
C$_{24}$H$_{15}$S$_3$. We are able to report substantially improved reliability
factors. | cond-mat_mtrl-sci |
Tunability of the optical absorption in small silver cluster-polymer
hybrid systems: We have calculated the absorption characteristics of different hybrid systems
consisting of Ag, Ag2 or Ag3 atomic clusters and poly(methacrylic acid) (PMAA)
using the time-dependent density-functional theory. The polymer is found to
have an extensive structural-dependency on the spectral patterns of the hybrid
systems relative to the bare clusters. The absorption spectrum can be `tuned'
to the visible range for hybrid systems with an odd number of electrons per
silver cluster, whereas for hybrid systems comprising an even number of
electrons, the leading absorption edge can be shifted up to about 4.5 eV. The
results give theoretical support to the experimental observations on the
absorption in the visible range in metal cluster-polymer hybrid structures. | cond-mat_mtrl-sci |
Effect of Gd/Nd doping on the magnetic properties of PrMnO3: A study on temperature dependent magnetic properties of single phase
orthorhombic perovskites system associated with space group Pbnm compounds
Pr1-x(Gd/Nd)xMnO3 (x=0.3, 0.5, 0.7) was carried out. A magnetization reversal
is observed below the Neel temperature (TN), in DC magnetization measurements
(at 50 Oe) in the doped compounds. This may be due to the antiparallel coupling
between the two magnetic sublattices (|Pr+Gd/Nd | and Mn). With lowering of
temperature, the |Pr+ Gd/Nd|) ions begin to polarize under the negative
internal field due to canted moment of Mn moments. The hysteresis plot taken at
50K shows a ferrimagnetic characteristic and the presence of spin canting of
ions in the magnetic sublattices. Arrott plot indicates field induced second
order paramagnetic to ferrimagnetic (PM-FiM) phase transition in this system. | cond-mat_mtrl-sci |
Silicon nanowire band gap modification: Band gap modification for small-diameter (1 nm) silicon nanowires resulting
from the use of different species for surface termination is investigated by
density functional theory calculations. Because of quantum confinement,
small-diameter wires exhibit a direct band gap that increases as the wire
diameter narrows, irrespective of surface termination. This effect has been
observed in previous experimental and theoretical studies for hydrogenated
wires. For a fixed cross-section, the functional group used to saturate the
silicon surface significantly modifies the band gap, resulting in relative
energy shifts of up to an electronvolt. The band gap shifts are traced to
details of the hybridization between the silicon valence band and the frontier
orbitals of the terminating group, which is in competition with quantum
confinement. | cond-mat_mtrl-sci |
The quantum spin Hall effect and topological insulators: In topological insulators, spin-orbit coupling and time-reversal symmetry
combine to form a novel state of matter predicted to have exotic physical
properties. | cond-mat_mtrl-sci |
Ten-million-atom electronic structure calculations on the K computer
with a massively parallel order-N theory: A massively parallel order-N electronic structure theory was constructed by
an interdisciplinary research between physics, applied mathematics and computer
science. (1) A high parallel efficiency with ten-million-atom nanomaterials was
realized on the K computer with upto 98,304 processor cores. The mathematical
foundation is a novel linear algebraic algorithm for the generalized shifted
linear equation. The calculation was carried out by our code ' ELSES '
(www.elses.jp) with modelled (tight-binding-form) systems based on ab initio
calculations. (2) A post-calculation analysis method, called pi-orbital
crystalline orbital Hamiltonian population (pi-COHP) method, is presented,
since the method is ideal for huge electronic structure data distributed among
massive nodes. The analysis method is demonstrated in an sp2-sp3 nano-composite
carbon solid, with an original visualization software 'VisBAR'. The present
research indicates general aspects of computational physics with current or
next-generation supercomputers. | cond-mat_mtrl-sci |
Charged surfaces and slabs in periodic boundary conditions: Plane wave density functional theory codes generally assume periodicity in
all three dimensions. This causes difficulties when studying charged systems,
for instance energies per unit cell become infinite, and, even after being
renormalised by the introduction of a uniform neutralising background, are very
slow to converge with cell size. The periodicity introduces spurious electric
fields which decay slowly with cell size and which also slow the convergence of
other properties relating to the ground state charge density. This paper
presents a simple self-consistent technique for producing rapid convergence of
both energies and charge distribution in the particular geometry of 2D
periodicity, as used for studying surfaces. | cond-mat_mtrl-sci |
Dislocations and cracks in generalized continua: Dislocations play a key role in the understanding of many phenomena in solid
state physics, materials science, crystallography and engineering. Dislocations
are line defects producing distortions and self-stresses in an otherwise
perfect crystal lattice. In particular, dislocations are the primary carrier of
crystal plasticity and in dislocation based fracture mechanics. | cond-mat_mtrl-sci |
Is there something of the MCT in orientationally disordered crystals ?: Molecular Dynamics simulations have been performed on the orientationally
disordered crystal chloroadamantane: a model system where dynamics are almost
completely controlled by rotations. A critical temperature T_c = 225 K as
predicted by the Mode Coupling Theory can be clearly determined both in the
alpha and beta dynamical regimes. This investigation also shows the existence
of a second remarkable dynamical crossover at the temperature T_x > T_c
consistent with a previous NMR and MD study [1]. This allows us to confirm
clearly the existence of a 'landscape-influenced' regime occurring in the
temperature range [T_c-T_x] as recently proposed [2,3]. | cond-mat_mtrl-sci |
Dependence of Magnetic Anisotropy and Magnetoresistance of
Ni81Fe19-Films on Annealing: Permalloy (Py:Ni81Fe19) exhibits an anisotropic magnetoresistance (AMR) which
is very often used to read magnetic signals from storage devices. Py-films of
thickness 20nm were prepared by dc-magnetron sputtering in a magnetic field
onto thermally oxidized Si-wafers and annealed ex situ at temperatures up to
1000K in order to investigate the dependence of the magnetic anisotropy and the
AMR on heat treatments. The films exhibit an uniaxial anisotropy after
preparation which changes during annealing above 520K. The AMR along the former
magnetically easy axis as well as the corresponding field sensitivity are
increased by a heat treatment around 700K reaching maxima of about 8% and a
maximum sensitivity of 1.5%/Oe, respectively. We discuss possible sources for
the change in anisotropy, i.e. strain effects, inhomogeneities, and changes of
the local atomic order. | cond-mat_mtrl-sci |
Spin Dependent Lifetimes and Spin-orbit Hybridization Points in Heusler
Compounds: We present an ab initio calculation of the k and spin-resolved electronic
lifetimes in the half-metallic Heusler compounds Co(2)MnSi and Co(2)FeSi. We
determine the spin-flip and spin-conserving contributions to the lifetimes and
study in detail the behavior of the lifetimes around states that are strongly
spin-mixed by spin-orbit coupling. We find that, for non-degenerate bands, the
spin mixing alone does not determine the energy dependence of the (spin-flip)
lifetimes. Qualitatively, the lifetimes reflect the lineup of electron and hole
bands. We predict that different excitation conditions lead to drastically
different spin-flip dynamics of excited electrons and may even give rise to an
enhancement of the non-equilibrium spin polarization. | cond-mat_mtrl-sci |
Spontaneous anomalous Hall effect arising from an unconventional
compensated magnetic phase in a semiconductor: The anomalous Hall effect, commonly observed in metallic magnets, has been
established to originate from the time-reversal symmetry breaking by an
internal macroscopic magnetization in ferromagnets or by a non-collinear
magnetic order. Here we observe a spontaneous anomalous Hall signal in the
absence of an external magnetic field in an epitaxial film of MnTe, which is a
semiconductor with a collinear antiparallel magnetic ordering of Mn moments and
a vanishing net magnetization. The anomalous Hall effect arises from an
unconventional phase with strong time-reversal symmetry breaking and
alternating spin polarization in real-space crystal structure and
momentum-space electronic structure. The anisotropic crystal environment of
magnetic Mn atoms due to the non-magnetic Te atoms is essential for
establishing the unconventional phase and generating the anomalous Hall effect. | cond-mat_mtrl-sci |
Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping
layer structures: Magnetic tunnel junction (MTJ) based on CoFeB/MgO/CoFeB structures is of
great interest due to its application in the spin-transfer-torque magnetic
random access memory (STT-MRAM). Large interfacial perpendicular magnetic
anisotropy (PMA) is required to achieve high thermal stability. Here we use
first-principles calculations to investigate the magnetic anisotropy energy
(MAE) of MgO/CoFe/capping layer structures, where the capping materials include
5d metals Hf, Ta, Re, Os, Ir, Pt, Au and 6p metals Tl, Pb, Bi. We demonstrate
that it is feasible to enhance PMA by using proper capping materials.
Relatively large PMA is found in the structures with capping materials of Hf,
Ta, Os, Ir and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to
giant PMA (6.09 mJ/m2), which is about three times larger than that of the
MgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the
contributions to MAE from each atomic layer and orbital. These findings provide
a comprehensive understanding of the PMA and point towards the possibility to
achieve advanced-node STT-MRAM with high thermal stability. | cond-mat_mtrl-sci |
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