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Theorems on ground-state phase transitions in Kohn-Sham models given by
the Coulomb density functional: Some theorems on derivatives of the Coulomb density functional with respect
to the coupling constant $\lambda$ are given. Consider an electron density
$n_{GS}({\bf r})$ given by a ground state. A model Fermion system with the
reduced coupling constant, $\lambda<1$, is defined to reproduce $n_{GS}({\bf
r})$ and the ground state energy. Fixing the charge density, possible phase
transitions as level crossings detected in a value of the reduced density
functional happen only at discrete points along the $\lambda$ axis. If the
density is $v$-representable also for $\lambda<1$, accumulation of phase
transition points is forbidden when $\lambda\rightarrow 1$. Relevance of the
theorems for the multi-reference density functional theory is discussed. | cond-mat |
The stochastic Gross-Pitaevskii equation: We show how to adapt the ideas of local energy and momentum conservation in
order to derive modifications to the Gross-Pitaevskii equation which can be
used phenomenologically to describe irreversible effects in a Bose-Einstein
condensate. Our approach involves the derivation of a simplified quantum
kinetic theory, in which all processes are treated locally. It is shown that
this kinetic theory can then be transformed into a number of phase-space
representations, of which the Wigner function description, although
approximate, is shown to be the most advantageous. In this description, the
quantum kinetic master equation takes the form of a GPE with noise and damping
added according to a well-defined prescription--an equation we call the
stochastic GPE. From this, a very simplified description we call the
phenomenological growth equation can be derived. We use this equation to study
i) the nucleation and growth of vortex lattices, and ii) nonlinear losses in a
hydrogen condensate, which it is shown can lead to a curious instability
phenomenon. | cond-mat |
Surface plasmons for doped graphene: Within the Dirac model for the electronic excitations of graphene, we
calculate the full polarization tensor with finite mass and chemical potential.
It has, besides the (00)-component, a second form factor, which must be
accounted for. We obtain explicit formulas for both form factors and for the
reflection coefficients. Using these, we discuss the regions in the
momentum-frequency plane where plasmons may exist and give numeric solutions
for the plasmon dispersion relations. It turns out that plasmons exist for
both, TE and TM polarizations over the whole range of the ratio of mass to
chemical potential, except for zero chemical potential, where only a TE plasmon
exists. | cond-mat |
Dynamics of fully coupled rotators with unimodal and bimodal frequency
distribution: We analyze the synchronization transition of a globally coupled network of N
phase oscillators with inertia (rotators) whose natural frequencies are
unimodally or bimodally distributed. In the unimodal case, the system exhibits
a discontinuous hysteretic transition from an incoherent to a partially
synchronized (PS) state. For sufficiently large inertia, the system reveals the
coexistence of a PS state and of a standing wave (SW) solution. In the bimodal
case, the hysteretic synchronization transition involves several states.
Namely, the system becomes coherent passing through traveling waves (TWs), SWs
and finally arriving to a PS regime. The transition to the PS state from the SW
occurs always at the same coupling, independently of the system size, while its
value increases linearly with the inertia. On the other hand the critical
coupling required to observe TWs and SWs increases with N suggesting that in
the thermodynamic limit the transition from incoherence to PS will occur
without any intermediate states. Finally a linear stability analysis reveals
that the system is hysteretic not only at the level of macroscopic indicators,
but also microscopically as verified by measuring the maximal Lyapunov
exponent. | cond-mat |
Resistive and rectifying effects of pulling gold atoms at thiol-gold
nano-contacts: We investigate, by means of first-principles calculations, structural and
transport properties of junctions made of symmetric dithiolated molecules
placed between Au electrodes. As the electrodes are pulled apart, we find that
it becomes energetically favorable that Au atoms migrate to positions between
the electrode surface and thiol terminations, with junction structures
alternating between symmetric and asymmetric. As a result, the calculated
$\emph{IV}$ curves alternate between rectifying and non-rectifying behaviors as
the electrodes are pulled apart, which is consistent with recent experimental
results. | cond-mat |
Thin Film Growth of Heavy Fermion Chiral Magnet YbNi3Al9: We grew thin films of a heavy fermion chiral magnet YbNi$_3$Al$_9$ by using
molecular beam epitaxy. They were grown on $c$-plane sapphire substrates under
ultra-high vacuum while maintaining a deposition rate at a stoichiometric ratio
among Yb, Ni, and Al. The resulting thin films contain epitaxial grains with a
$c$ axis parallel to the substrate surface: The YbNi$_3$Al$_9$ $c$ axis is
parallel to the sapphire $b$ or $a$ axis. The temperature dependence of the
resistivity exhibits a typical feature of a dense Kondo system with a broad
shoulder structure at $\sim$40\,K, as well as a kink as a signature of the
chiral helimagnetic ordering at 3.6\,K. These features are consistent with
those previously observed in bulk samples. The shift in the kink associated
with the field-induced phase transition is found in the magnetoresistance
curves under a magnetic field applied in the direction perpendicular to the
$c$-axis. The magnetic phase diagram well reproduces that for the bulk
crystals, implying that the chiral soliton lattice phase arises under magnetic
fields, even in thin films. | cond-mat |
Quantum Confined Stark Effect in Wide Parabolic Quantum Wells: We show how to compute the optical functions of Wide Parabolic Quantum Wells
(WPQWs) exposed to uniform electric F applied in the growth direction, in the
excitonic energy region. The effect of the coherence between the electron-hole
pair and the electromagnetic field of the propagating wave including the
electron-hole screened Coulomb potential is adopted, and the valence band
structure is taken into account in the cylindrical approximation. The role of
the interaction potential and of the applied electric field, which mix the
energy states according to different quantum numbers and create symmetry
forbidden transitions, is stressed. We use the Real Density Matrix Approach
(RDMA) and an effective e-h potential, which enable to derive analytical
expressions for the WPQWs electrooptical functions. Choosing the
susceptibility, we performed numerical calculations appropriate to a
GaAs/GaAlAs WPQWs. We have obtained a red shift of the absorption maxima
(Quantum Confined Stark Effect), asymmetric upon the change of the direction of
the applied field (F -> -F), parabolic for the ground state and strongly
dependent on the confinement parameters (the QWs sizes), changes in the
oscillator strengths, and new peaks related to the states with different parity
for electron and hole. | cond-mat |
Lanthanide molecular nanomagnets as probabilistic bits: Over the decades, the spin dynamics of a large set of lanthanide complexes
have been explored. Lanthanide-based molecular nanomagnets are bistable spin
systems, generally conceptualized as classical bits, but many lanthanide
complexes have also been presented as candidate quantum bits (qubits). Here we
offer a third alternative and model them as probabilistic bits (p-bits), where
their stochastic behavior constitutes a computational resource instead of a
limitation. We present a modelling tool for molecular spin p-bits, we
demonstrate its capability to simulate bulk magnetic relaxation data and ac
experiments and to simulate a minimal p-bit network under realistic conditions.
Finally, we go back to a recent systematic data gathering and screen the best
lanthanide complexes for p-bit behavior, lay out the performance of the
different lanthanide ions and chemical families and offer some chemical design
considerations. | cond-mat |
Magnetic-Field-Induced 4f-Octupole in CeB6 Probed by Resonant X-ray
Diffraction: CeB6, a typical Gamma_8-quartet system, exhibits a mysterious
antiferroquadrupolar ordered phase in magnetic fields, which is considered as
originating from the T_{xyz}-type magnetic octupole moment induced by the
field. By resonant x-ray diffraction in magnetic fields, we have verified that
the T_{xyz}-type octupole is indeed induced in the 4f-orbital of Ce with a
propagation vector (1/2, 1/2, 1/2), thereby supporting the theory. We observed
an asymmetric field dependence of the intensity for an electric quadrupole (E2)
resonance when the field was reversed, and extracted a field dependence of the
octupole by utilizing the interference with an electric dipole (E1) resonance.
The result is in good agreement with that of the NMR-line splitting, which
reflects the transferred hyperfine field at the Boron nucleus from the
anisotropic spin distribution of Ce with an O_{xy}-type quadrupole. The
field-reversal method used in the present study opens up the possibility of
being widely applied to other multipole ordering systems such as NpO2,
Ce_{x}La_{1-x}B_{6}, SmRu_{4}P_{12}, and so on. | cond-mat |
The type II Weyl semimetals at low temperatures: chiral anomaly, elastic
deformations, zero sound: We consider the properties of the type II Weyl semimetals at low temperatures
basing on the particular tight - binding model. In the presence of electric
field directed along the line connecting the Weyl points of opposite chirality
the occupied states flow along this axis giving rise to the creation of
electron - hole pairs. The electrons belong to a vicinity of one of the two
type II Weyl points while the holes belong to the vicinity of the other. This
process may be considered as the manifestation of the chiral anomaly that
exists without any external magnetic field. It may be observed experimentally
through the measurement of conductivity. Next, we consider the modification of
the theory in the presence of elastic deformations. In the domain of the
considered model, where it describes the type I Weyl semimetals the elastic
deformations lead to the appearance of emergent gravity. In the domain of the
type II Weyl semimetals the form of the Fermi surface is changed due to the
elastic deformations, and its fluctuations represent the special modes of the
zero sound. We find that there is one - to one correspondence between them and
the sound waves of the elasticity theory. Next, we discuss the influence of the
elastic deformations on the conductivity. The particularly interesting case is
when our model describes the intermediate state between the type I and the type
II Weyl semimetal. Then without the elastic deformations there are the Fermi
lines instead of the Fermi points/Fermi surface, while the DC conductivity
vanishes. However, even small elastic deformations may lead to the appearance
of large conductivity. | cond-mat |
Ultra-low-energy computing paradigm using giant spin Hall devices: Spin Hall effect converts charge current to spin current, which can exert
spin-torque to switch the magnetization of a nanomagnet. Recently, it is shown
that the ratio of spin current to charge current using spin Hall effect can be
made more than unity by using the areal geometry judiciously, unlike the case
of conventional spin-transfer-torque switching of nanomagnets. This can enable
energy-efficient means to write a bit of information in nanomagnets. Here, we
study the energy dissipation in such spin Hall devices. By solving stochastic
Landau-Lifshitz-Gilbert equation of magnetization dynamics in the presence of
room temperature thermal fluctuations, we show a methodology to simultaneously
reduce switching delay, its variance and energy dissipation, while lateral
dimensions of the spin Hall devices are scaled down. | cond-mat |
Magnetization controlled by crystallization in soft magnetic
Fe-Si-B-P-Cu alloys: Soft magnetic materials have low coercive fields and high permeability.
Recently, nanocrystalline alloys obtained using annealing amorphous alloys have
attracted much interest since nanocrystalline alloys with small grain sizes of
tens of nanometers exhibit low coercive fields comparable to that of amorphous
alloys. Since nanocrystalline soft magnetic materials attain remarkable soft
magnetic properties by controlling the grain size, the crystal grains'
microstructure has a substantial influence on the soft magnetic properties. In
this research, we examined the magnetic properties of Fe-Si-B-P-Cu
nanocrystalline soft magnetic alloys obtained by annealing amorphous alloys.
During crystallization, the observation findings reveal the correlation between
the generated microstructures and soft magnetic properties. | cond-mat |
Cluster mean-field study of the parity conserving phase transition: The phase transition of the even offspringed branching and annihilating
random walk is studied by N-cluster mean-field approximations on
one-dimensional lattices. By allowing to reach zero branching rate a phase
transition can be seen for any N <= 12.The coherent anomaly extrapolations
applied for the series of approximations results in $\nu_{\perp}=1.85(3)$ and
$\beta=0.96(2)$. | cond-mat |
Non-local quantum fluctuations and fermionic superfluidity in the
imbalanced attractive Hubbard model: We study fermionic superfluidity in strongly anisotropic optical lattices
with attractive interactions utilizing the cluster DMFT method, and focusing in
particular on the role of non-local quantum fluctuations. We show that
non-local quantum fluctuations impact the BCS superfluid transition
dramatically. Moreover, we show that exotic superfluid states with delicate
order parameter structure, such as the Fulde-Ferrell-Larkin-Ovchinnikov phase
driven by spin population imbalance, can emerge even in the presence of such
strong fluctuations. | cond-mat |
Analytically solvable model of an electronic Mach-Zehnder interferometer: We consider a class of models of non-equilibrium electronic Mach-Zehnder
interferometers built on integer quantum Hall edges states. The models are
characterized by the electron-electron interaction being restricted to the
inner part of the interferometer and transmission coefficients of the quantum
quantum point contacts, defining the interferometer, which may take arbitrary
values from zero to one. We establish an exact solution of these models in
terms of single-particle quantities --- determinants and resolvents of Fredholm
integral operators. In the general situation, the results can be obtained
numerically. In the case of strong charging interaction, the operators acquire
the block Toeplitz form. Analyzing the corresponding Riemann-Hilbert problem,
we reduce the result to certain singular single-channel determinants (which are
a generalization of Toeplitz determinants with Fisher-Hartwig singularities),
and obtain an analytic result for the interference current (and, in particular,
for the visibility of Aharonov-Bohm oscillations). Our results, which are in
good agreement with experimental observations, show an intimate connection
between the observed "lobe" structure in the visibility of Aharonov-Bohm
oscillations and multiple branches in the asymptotics of singular integral
determinants. | cond-mat |
Ferroelastic twin wall mediated ferro-flexoelectricity and bulk
photovoltaic effect in SrTiO$_3$: Ferroelastic twin walls in nonpolar materials can give rise to a spontaneous
polarization due to symmetry breaking. Nevertheless, the bi-stable polarity of
twin walls and its reversal have not yet been demonstrated. Here, we report
that the polarity of SrTiO$_3$ twin walls can be switched by ultra-low strain
gradient. Using first-principles-based machine-learning potential, we
demonstrate that the twin walls can be deterministically rotated and realigned
in specific directions under strain gradient, which breaks the inversion
symmetry of a sequence of walls and leads to a macroscopic polarization. The
system can maintain polarity even after the strain gradient is removed. As a
result, the polarization of twin walls can exhibit ferroelectric-like
hysteresis loop upon cyclic bending, namely ferro-flexoelectricity. Finally, we
propose a scheme to experimentally detect the polarity of twin wall by
measuring the bulk photovoltaic responses. Our findings suggest a
twin-wall-mediated ferro-flexoelectricity in SrTiO$_3$, which could be
potentially exploited as functional elements in nano-electronic devices design. | cond-mat |
Computing solubility and thermodynamics properties of H2O2 in water: Hydrogen peroxide plays a key role in many environmental and industrial
chemical processes. We performed classical Molecular Dynamics and Continuous
Fractional Component Monte Carlo simulations to calculate thermodynamic
properties of H2O2 in aqueous solutions. The quality of the available force
fields for H2O2 developed by Orabi & English, and by Cordeiro was
systematically evaluated. To assess which water force field is suitable for
predicting properties of H2O2 in aqueous solutions, four water force fields
were used, namely the TIP3P, TIP4P/2005, TIP5P-E, and a modified TIP3P force
field. While the computed densities of pure H2O2 in the temperature range of
253-353 K using the force field by Orabi & English are in excellent agreement
with experimental results, the densities using the force field by Cordeiro are
underestimated by 3%. The TIP4P/2005 force field in combination with the H2O2
force field developed by Orabi & English can predict the densities of H2O2
aqueous solution for the whole range of H2O2 mole fractions in very good
agreement with experimental results. The TIP4P/2005 force field in combination
with either of the H2O2 force fields can predict the viscosities of H2O2
aqueous solutions for the whole range of H2O2 mole fractions in good agreement
with experimental results. The diffusion coefficients for H2O2 and water
molecules using the TIP4P/2005 force field with either of the H2O2 force fields
are almost constant for the whole range of H2O2 mole fractions. The Cordeiro
force field for H2O2 in combination with either of the water force fields can
predict the Henry coefficients of H2O2 in water in better agreement with
experimental values than the force field by Orabi & English. | cond-mat |
On the mechanical modeling of the extreme softening/stiffening response
of axially loaded tensegrity prisms: We study the geometrically nonlinear behavior of uniformly compressed
tensegrity prisms, through fully elastic and rigid--elastic models. The
presented models predict a variety of mechanical behaviors in the regime of
large displacements, including an extreme stiffening-type response, already
known in the literature, and a newly discovered, extreme softening behavior.
The latter may lead to a snap buckling event producing an axial collapse of the
structure. The switching from one mechanical regime to another depends on the
aspect ratio of the structure, the magnitude of the applied prestress, and the
material properties of the constituent elements. We discuss potential acoustic
applications of such behaviors, which are related to the design and manufacture
of tensegrity lattices and innovative phononic crystals. | cond-mat |
Reduction in energy dissipation rate with increased effective applied
field: Dynamics of the response of type-II superconductors to a time-varying
magnetic field can exhibit a rate-independent or rate-dependent hysteresis. An
energy dissipation rate in a superconductor placed in a time-varying magnetic
field depends on its wave form and type of hysteresis, which depends on
temperature. The same wave form may reduce the energy dissipation rate in the
case of true hysteresis, while it may increase the energy dissipation rate in
the case of dynamic hysteresis compared with an energy dissipation rate in a
pure sinusoidal field. We present experimental data which confirm the energy
dissipation rate calculated using the critical state theory for the case of
rate-independent hysteresis and limiting behavior in a normal state for the
case of rate-dependent hysteresis. | cond-mat |
BCS-BEC Crossover and Pairing Fluctuations in a Two Band
Superfluid/Superconductor: A T Matrix Approach: We investigate pairing fluctuation effects in a two band fermionic system,
where a shallow band in the Bardeen--Cooper--Schrieffer--Bose--Einstein
condensation (BCS-BEC) crossover regime is coupled with a weakly interacting
deep band. Within a diagrammatic $T$ matrix approach, we~report how
thermodynamic quantities such as the critical temperature, chemical potential,
{and~momentum distributions} undergo the crossover from the BCS to BEC regime
by tuning the intraband coupling in the shallow band. We also generalize the
definition of Tan's contact to a two band system and report the two contacts
for different pair-exchange couplings. The present results are compared with
those obtained by the simpler Nozi\`eres--Schmitt--Rink approximation. We
confirm a pronounced enhancement of the critical temperature due to the
multiband configuration, as well as to the pair-exchange coupling. | cond-mat |
Phase diagram of geometric d-wave superconductor Josephson junctions: We show that a constriction-type Josephson junction realized by an epitactic
thin film of a d-wave superconductor with an appropriate boundary geometry
exhibits intrinsic phase differences between 0 and pi depending on geometric
parameters and temperature. Based on microscopic Eilenberger theory, we provide
a general derivation of the relation between the change of the free energy of
the junction and the current-phase relation. From the change of the free
energy, we calculate phase diagrams and discuss transitions driven by geometric
parameters and temperature. | cond-mat |
Time-reversal symmetry breaking state in dirty three-band superconductor: I study the effects of disorder on the superconductivity of a three-band
model with repulsive interband pairing. Such a model can support several
possible superconducting order parameters, including a complex time-reversal
symmetry breaking (TRSB) state. Impurity scattering suppresses the critical
temperature of all these states, but the complex state survives, and remains a
part of the phase diagram of the model even in the presence of moderate amount
of disorder. This means that the TRSB states could be experimentally accessible
in multiband materials like iron pnictides and chalcogenides. | cond-mat |
Exact weak-coupling radius of the Holstein polaron in one, two, and
three dimensions: We apply weak-coupling perturbation theory to the Holstein molecular crystal
model in order to compute an electron-phonon correlation function
characterizing the shape and size of the polaron lattice distortion in one,
two, and three dimensions. This correlation function is computed exactly to
leading order in the electron-phonon coupling constant, permitting a complete
description of correlations in any dimension for both isotropic and arbitrarily
anisotropic cases. Using this exact result, the width of the polaron is
characterized along arbitrary directions. The width of the polaron thus
determined disagrees in every dimension with some well-known characterizations
of polarons, signalling in particular the breakdown of the adiabatic
approximation and the characterizations of self-trapping associated with it. | cond-mat |
Evidence of exchange-striction and charge disproportionation in the
magneto-electric material Ni3TeO6: The chiral magneto-electric compound Ni3TeO6 is investigated through
temperature-dependent synchrotron-based powder x-ray diffraction and x-ray
absorption spectroscopy between 15 to 300 K. Our work provides direct evidence
for the exchange-striction in the material around the concomitant onset point
of collinear antiferromagnetic and magneto-electric phases. The x-ray
absorption near edge spectra and x-ray photoelectron spectra show that the
sample consists of both Ni2+ and Ni3+ ions in the lattice. The ionic state of
Ni is found to be quite robust, and it is largely independent of the
preparation route. Additionally, the minority Te4+ state is found to coexist
with the majority Te6+ state, which may arise from the charge
disproportionation between Ni and Te ions (Ni2+ + Te6+ --> Ni3+ + Te4+). The
observed mixed valency of Ni is also confirmed by the total paramagnetic moment
per Ni atom in the system. This mixed valency in the metal ions and the
exchange-striction may be attributed to the observed magneto-electric effect in
the system. | cond-mat |
SiNx:Tb3+--Yb3+, an efficient down-conversion layer compatible with a
silicon solar cell process: SiN x : Tb 3+-Yb 3+, an efficient down-conversion layer compatible with
silicon solar cell process Abstract Tb 3+-Yb 3+ co-doped SiN x down-conversion
layers compatible with silicon Photovoltaic Technology were prepared by
reactive magnetron co-sputtering. Efficient sensitization of Tb 3+ ions through
a SiN x host matrix and cooperative energy transfer between Tb 3+ and Yb 3+
ions were evidenced as driving mechanisms of the down-conversion process. In
this paper, the film composition and microstructure are investigated alongside
their optical properties, with the aim of maximizing the rare earth ions
incorporation and emission efficiency. An optimized layer achieving the highest
Yb 3+ emission intensity was obtained by reactive magnetron co-sputtering in a
nitride rich atmosphere for 1.2 W/cm${}^2$ and 0.15 W/cm${}^2$ power density
applied on the Tb and Yb targets, respectively. It was determined that
depositing at 200 {\textdegree}C and annealing at 850 {\textdegree}C leads to
comparable Yb 3+ emission intensity than depositing at 500 {\textdegree}C and
annealing at 600 {\textdegree}C, which is promising for applications toward
silicon solar cells. | cond-mat |
Generation of optical potentials for ultracold atoms using a
superluminescent diode: We report on the realization and characterisation of optical potentials for
ultracold atoms using a superluminescent diode. The light emitted by this class
of diodes is characterised by high spatial coherence but low temporal
coherence. On the one hand, this implies that it follows Gaussian propagation
similar to lasers, allowing for high intensities and well-collimated beams. On
the other, it significantly reduces those interference effects that lead to
severe distortions in imaging. By using a high-resolution optical setup, we
produce patterned optical potentials with a digital micromirror device and
demonstrate that the quality of the patterns produced by our superluminescent
diode is consistently and substantially higher than those produced by our
laser. We show that the resulting optical potentials can be used to arrange the
atoms in arbitrary structures and manipulate them dynamically. Our results can
open new opportunities in the fields of quantum simulations and atomtronics. | cond-mat |
Structure and diffusion time scales of disordered clusters: The eigenvalue spectra of the transition probability matrix for random walks
traversing critically disordered clusters in three different types of
percolation problems show that the random walker sees a developing Euclidean
signature for short time scales as the local, full-coordination constraint is
iteratively applied. | cond-mat |
Resonance Plasmon Linewidth Oscillations in Spheroidal Metallic
Nanoparticle Embedded in a Dielectric Matrix: The kinetic approach is applied to calculate oscillations of a surface
plasmon linewidth in a spheroidal metal nanoparticle embedded in any dielectric
media. The principal attention is focused on the case, when the free electron
path is much greater than the particle size.
The linewidth of the plasmon resonance as a function of the particle radius,
shape, dielectric constant of the surrounding medium, and the light frequency
is studied in detail. It is found that the resonance plasmon linewidth
oscillates with increasing both the particle size and the dielectric constant
of surrounding medium.
The main attention is paid to the electron surface-scattering contribution to
the plasmon decay.
All calculations the plasmon resonance linewidth are illustrated by the
example of the Na nanoparticles with different radii.
The results obtained in the kinetic approach are compared with the known ones
from other models.
The role of the radiative damping is discussed as well. | cond-mat |
Emergence of one-dimensional wires of free carriers in
transition-metal-dichalcogenide nanostructures: We highlight the emergence of metallic states in two-dimensional
transition-metal-dichalcogenide nanostructures -nanoribbons, islands, and
inversion domain boundaries- as a widespread and universal phenomenon driven by
the polar discontinuities occurring at their edges or boundaries. We show that
such metallic states form one-dimensional wires of electrons or holes, with a
free charge density that increases with the system size, up to complete
screening of the polarization charge, and can also be controlled by the
specific edge or boundary configurations, e.g. through chemisorption of
hydrogen or sulfur atoms at the edges. For triangular islands, local polar
discontinuities occur even in the absence of a total dipole moment for the
island and lead to an accumulation of free carriers close to the edges,
providing a consistent explanation of previous experimental observations. To
further stress the universal character of these mechanisms, we show that polar
discontinuities give rise to metallic states also at inversion domain
boundaries. These findings underscore the potential of engineering
transition-metal-dichalcogenide nanostructures for manifold applications in
nano- and opto-electronics, spintronics, catalysis, and solar-energy
harvesting. | cond-mat |
Spectral partitions on infinite graphs: Statistical models on infinite graphs may exhibit inhomogeneous thermodynamic
behaviour at macroscopic scales. This phenomenon is of geometrical origin and
may be properly described in terms of spectral partitions into subgraphs with
well defined spectral dimensions and spectral weights. These subgraphs are
shown to be thermodynamically homogeneous and effectively decoupled. | cond-mat |
Double-$Q$ spin chirality stripes in the anomalous Hall antiferromagnet
CoNb$_3$S$_6$: The metallic antiferromagnet CoNb$_3$S$_6$ exhibits a giant anomalous Hall
effect (AHE) that cannot be explained by a collinear N\'eel order on
intercalated Co ions. Thus, a noncoplanar structure is expected. We carried out
resonant elastic x-ray scattering (REXS) to reexamine the magnetic structure of
CoNb$_3$S$_6$ and found a double-$Q$ ($2Q$) order with a $(\frac{1}{2}00)$
commensurate component and a long-wavelength modulation. Circular dichroism and
linear polarization analysis reveal that the commensurate components on the two
Co sites are noncollinear and the modulation is helical. The resulting magnetic
structure has a staggered scalar spin chirality forming a stripe pattern in
real space. Furthermore, we found that the helical modulation wavevector
exhibits a sample dependence and develops a low-symmetry domain structure. We
propose that quenched-in lattice strain controls the helical domain structure,
accounting for much of the sample dependence. These results provide insight
into the mechanism of the AHE in CoNb$_3$S$_6$ and identifies potential routes
for controlling the Hall response and realizing other unconventional electronic
phenomena in metallic antiferromagnets. | cond-mat |
Imaging the itinerant-to-localized transmutation of electrons across the
metal-to-insulator transition in V$_2$O$_3$: In solids, strong repulsion between electrons can inhibit their movement and
result in a "Mott" metal-to-insulator transition (MIT), a fundamental
phenomenon whose understanding has remained a challenge for over 50 years. A
key issue is how the wave-like itinerant electrons change into a localized-like
state due to increased interactions. However, observing the MIT in terms of the
energy- and momentum-resolved electronic structure of the system, the only
direct way to probe both itinerant and localized states, has been elusive. Here
we show, using angle-resolved photoemission spectroscopy (ARPES), that in
V$_2$O$_3$ the temperature-induced MIT is characterized by the progressive
disappearance of its itinerant conduction band, without any change in its
energy-momentum dispersion, and the simultaneous shift to larger binding
energies of a quasi-localized state initially located near the Fermi level. | cond-mat |
Hybrid surface waves in two-dimensional Rashba-Dresselhaus materials: We address the electromagnetic properties of two-dimensional electron gas
confined by a dielectric environment in the presence of both Rashba and
Dresselhaus spin-orbit interactions. It is demonstrated that off-diagonal
components of the conductivity tensor resulting from a delicate interplay
between Rashba and Dresselhaus couplings lead to the hybridization of
transverse electric and transverse magnetic surface electromagnetic modes
localized at the interface. We show that the characteristics of these hybrid
surface waves can be controlled by additional intense external off-resonant
coherent pumping. | cond-mat |
Effect of external electric field on the charge density waves in one
dimensional Hubbard superlattices: We have studied the ground state of the one dimensional Hubbard superlattice
structures with different unit cell sizes in the presence of electric field.
Self consistent Hartree-Fock approximation calculation is done in the weak to
intermediate interaction regime. Studying the charge gap at the Fermi level and
the charge density structure factor, we get an idea how the charge modulation
on the superlattice is governed by the competition between the electronic
correlation and the external electric field. | cond-mat |
AC Conductance in Dense Array of the Ge$_{0.7}$Si$_{0.3}$ Quantum Dots
in Si: Complex AC-conductance, $\sigma^{AC}$, in the systems with dense
Ge$_{0.7}$Si$_{0.3}$ quantum dot (QD) arrays in Si has been determined from
simultaneous measurements of attenuation, $\Delta\Gamma=\Gamma(H)-\Gamma(0)$,
and velocity, $\Delta V /V=(V(H)-V(0)) / V(0)$, of surface acoustic waves (SAW)
with frequencies $f$ = 30-300 MHz as functions of transverse magnetic field $H
\leq$ 18 T in the temperature range $T$ = 1-20 K. It has been shown that in the
sample with dopant (B) concentration 8.2$ \times 10^{11}$ cm$^{-2}$ at
temperatures $T \leq$4 K the AC conductivity is dominated by hopping between
states localized in different QDs. The observed power-law temperature
dependence, $\sigma_1(H=0)\propto T^{2.4}$, and weak frequency dependence,
$\sigma_1(H=0)\propto \omega^0$, of the AC conductivity are consistent with
predictions of the two-site model for AC hopping conductivity for the case of
$\omega \tau_0 \gg $1, where $\omega=2\pi f$ is the SAW angular frequency and
$\tau_0$ is the typical population relaxation time. At $T >$ 7 K the AC
conductivity is due to thermal activation of the carriers (holes) to the
mobility edge. In intermediate temperature region 4$ < T<$ 7 K, where AC
conductivity is due to a combination of hops between QDs and diffusion on the
mobility edge, one succeeded to separate both contributions. Temperature
dependence of hopping contribution to the conductivity above $T^*\sim$ 4.5 K
saturates, evidencing crossover to the regime where $\omega \tau_0 < $1. From
crossover condition, $\omega \tau_0(T^*)$ = 1, the typical value, $\tau_0$, of
the relaxation time has been determined. | cond-mat |
Critical Phenomena of Ferromagnetic Transition in Double-Exchange
Systems: Critical phenomena of ferromagnetic transition at finite temperatures are
studied in double-exchange systems. In order to investigate strong interplay
between charge and spin degrees of freedom, Monte Carlo technique is applied to
include fluctuations in a controlled and unbiased manner. By using finite-size
scaling analysis, critical exponents and transition temperature are estimated
for a model with Ising spin symmetry in two dimensions. The obtained exponents
are far distinct from the mean-field values, but consistent with those of spin
models with short-range exchange interactions. The universality class of this
transition belongs to that of short-range interaction with the same spin
symmetry. We also discuss the case for three dimensions. The results are
compared with experimental results in perovskite manganites which show colossal
magnetoresistance. | cond-mat |
From many-body oscillations to thermalization in an isolated spinor gas: The dynamics of a many-body system can take many forms, from a purely
reversible evolution to fast thermalization. Here we show experimentally and
numerically that an assembly of spin 1 atoms all in the same spatial mode
allows one to explore this wide variety of behaviors. When the system can be
described by a Bogoliubov analysis, the relevant energy spectrum is linear and
leads to undamped oscillations of many-body observables. Outside this regime,
the non-linearity of the spectrum leads to irreversibity, characterized by a
universal behavior. When the integrability of the Hamiltonian is broken, a
chaotic dynamics emerges and leads to thermalization, in agreement with the
Eigenstate Thermalization Hypothesis paradigm. | cond-mat |
Mechanism of Polarization Fatigue in BiFeO3: the Role of Schottky
Barrier: By using piezoelectric force microscopy and scanning Kelvin probe microscopy,
we have investigated the domain evolution and space charge distribution in
planar BiFeO3 capacitors with different electrodes. It is observed that charge
injection at the film/electrode interface leads to domain pinning and
polarization fatigue in BiFeO3. Furthermore, the Schottky barrier at the
interface is crucial for the charge injection process. Lowering the Schottky
barrier by using low work function metals as the electrodes can also improve
the fatigue property of the device, similar to what oxide electrodes can
achieve. | cond-mat |
Interface effects on titanium growth on graphene: Poor quality interfaces between metal and graphene cause non-linearity and
impair the carrier mobility in graphene devices. Here, we use aberration
corrected scanning transmission electron microscopy to observe hexagonally
close-packed Ti nano-islands grown on atomically clean graphene, and establish
a 30{\deg} epitaxial relationship between the lattices. Due to the strong
binding of Ti on graphene, at the limit of a monolayer, the Ti lattice constant
is mediated by the graphene epitaxy, and compared to bulk Ti, is strained by
ca. 3.7% to a value of 0.306(3) nm. The resulting interfacial strain is
slightly greater than what has been predicted by density functional theory
calculations. Our early growth stage investigations also reveal that, in
contrast to widespread assumptions, Ti does not fully wet graphene but grows
initially in clusters with a thickness of 1-2 layers. Raman spectroscopy
implies charge transfer between the Ti islands and graphene substrate. | cond-mat |
Structure and energetics of carbon-related defects in SiC
(0001)/SiO$_{\rm 2}$ systems revealed by first-principles calculations:
Defects in SiC, SiO$_{\rm 2}$, and just at their interface: We report first-principles calculations that reveal the atomic forms,
stability, and energy levels of carbon-related defects in SiC (0001)/SiO$_{\rm
2}$ systems. We clarify the stable position (SiC side, SiO$_{\rm 2}$ side, or
just at the SiC/SiO$_{\rm 2}$ interface) of defects depending on the oxidation
environment. Under an O-rich condition, the di-carbon antisite ((C$_{\rm
2}$)$_{\rm Si}$) in the SiC side is stable and critical for $n$-channel
MOSFETs, whereas the di-carbon defect (Si-C-C-Si) at the interface becomes
critical under an O-poor condition. Our results suggest that the oxidation of
SiC under a high-temperature O-poor condition is favorable in reducing the
defects, in consistent with recent experimental reports. | cond-mat |
Elucidating the initial steps in α-uranium hydriding using
first-principles calculations: Hydrogen embrittlement of uranium, which arises due to the formation of a
structurally weak pyrophoric hydride, poses a major safety risk in material
applications. Previous experiments have shown that hydriding begins on top or
near the surface (i.e., subsurface) of a-uranium. However, the fundamental
molecular-level mechanism of this process remains unknown. In this work,
starting from pristine {\alpha}-U bulk and surfaces, we present a systematic
investigation of possible mechanisms for formation of the metal hydride.
Specifically, we address this problem by examining the individual steps of
hydrogen embrittlement, including surface adsorption, subsurface absorption,
and the inter-layer diffusion of atomic hydrogen. Furthermore, by examining
these processes across different facets, we highlight the importance of both
(1) hydrogen monolayer coverage and (2) applied tensile strain on hydriding
kinetics. Taken together, by studying previously overlooked phenomena, this
study provides foundational insights into the initial steps of this overall
complex process. We anticipate that this work will guide near-term future
development of multiscale kinetic models for uranium hydriding and
subsequently, identify potential strategies to mitigate this undesired process. | cond-mat |
Spin mixing between subbands and extraordinary Landau levels shift in
wide HgTe quantum wells: We present both the experimental and theoretical investigation of a
non-trivial electron Landau levels shift in magnetic field in wide ~20 nm HgTe
quantum wells: Landau levels split under magnetic fields but become degenerate
again when magnetic field increases. We reproduced this behavior qualitatively
within an isotropic 6-band Kane model, then using semiclassical calculations we
showed this behavior is due to the mixing of the conduction band with total
spin 3/2 with the next well subband with spin 1/2 which reduces the average
vertical spin from 3/2 to around 1. This change of the average spin changes the
Berry phase explaining the evolution of Landau levels under magnetic field. | cond-mat |
Machine Learning Percolation Model: Recent advances in machine learning have become increasingly popular in the
applications of phase transitions and critical phenomena. By machine learning
approaches, we try to identify the physical characteristics in the
two-dimensional percolation model. To achieve this, we adopt Monte Carlo
simulation to generate dataset at first, and then we employ several approaches
to analyze the dataset. Four kinds of convolutional neural networks (CNNs), one
variational autoencoder (VAE), one convolutional VAE (cVAE), one principal
component analysis (PCA), and one $k$-means are used for identifying order
parameter, the permeability, and the critical transition point. The former
three kinds of CNNs can simulate the two order parameters and the permeability
with high accuracy, and good extrapolating performance. The former two kinds of
CNNs have high anti-noise ability. To validate the robustness of the former
three kinds of CNNs, we also use the VAE and the cVAE to generate new
percolating configurations to add perturbations into the raw configurations. We
find that there is no difference by using the raw or the perturbed
configurations to identify the physical characteristics, under the prerequisite
of corresponding labels. In the case of lacking labels, we use unsupervised
learning to detect the physical characteristics. The PCA, a classical
unsupervised learning, performs well when identifying the permeability but
fails to deduce order parameter. Hence, we apply the fourth kinds of CNNs with
different preset thresholds, and identify a new order parameter and the
critical transition point. Our findings indicate that the effectiveness of
machine learning still needs to be evaluated in the applications of phase
transitions and critical phenomena. | cond-mat |
Unusual eigenvalue spectrum and relaxation in the Lévy
Ornstein-Uhlenbeck process: We consider the rates of relaxation of a particle in a harmonic well, subject
to L\'evy noise characterized by its L\'evy index $\mu$. Using the propagator
for this L\'evy Ornstein-Uhlenbeck process (LOUP), we show that the eigenvalue
spectrum of the associated Fokker-Planck operator has the form $(n+m\mu)\nu$
where $\nu$ is the force constant characterizing the well, and
$n,m\in\mathbb{N}$. If $\mu$ is irrational, the eigenvalues are all
non-degenerate, but rational $\mu$ can lead to degeneracy. The maximum
degeneracy is shown to be two. The left eigenfunctions of the fractional
Fokker-Planck operator are very simple while the right eigenfunctions may be
obtained from the lowest eigenfunction by a combination of two different
step-up operators. Further, we find that the acceptable eigenfunctions should
have the asymptotic behavior $|x|^{-n_1+n_2\;\mu}$ as $|x| \rightarrow \infty$,
with $n_1$ and $n_2$ being positive integers, though this condition alone is
not enough to identify them uniquely. We also assert that the rates of
relaxation of LOUP are determined by the eigenvalues of the associated
fractional Fokker-Planck operator and do not depend on the initial state if the
moments of the initial distribution are all finite. If the initial distribution
has fat tails, for which the higher moments diverge, one would have
non-spectral relaxation, as pointed out by Toenjes et. al (Physical Review
Letters, 110, 150602 (2013)). | cond-mat |
A droplet near the critical point: the divergence of Tolman's length: Application of "complete scaling" [Kim et al., Phys. Rev. E 67, 061506
(2003); Anisimov and Wang, Phys. Rev. Lett. 97, 25703 (2006)] to the
interfacial behavior of fluids shows that Tolman's length, a curvature
correction to the surface tension, diverges at the critical point of fluids
much more strongly than is commonly believed. The amplitude of the divergence
depends on the degree of asymmetry in fluid phase coexistence. A new universal
amplitude ratio, which involves this asymmetry, is introduced. In highly
asymmetric fluids and fluid mixtures the Tolman length may become large enough
near criticality to be detected in precise experiments with microcapillaries
and in simulations. | cond-mat |
The dynamics of azurite Cu$_3$(CO$_3$)$_2$(OH)$_2$ in a magnetic field
as determined by neutron scattering: Azurite, a natural mineral made up of CuO chains, is also an intriguing
spin-1/2 quantum magnet. There has been much debate as to the 1-dimensional
(1D) nature of this material by theorists and experimentalists alike. The focus
of this debate lies in the interactions between Cu-ions within the
antiferromagnetically ordered state below 1.9 K. We present high-resolution
inelastic neutron scattering data which highlight the complexity of the
magnetic ground state of azurite. The application of magnetic fields and
temperatures were used to probe the excitations revealing important information
about the dynamics in this system. From this we are able to conclude that the
1D Heisenberg antiferromagnetic spin chain model is not sufficient to describe
the dynamics in azurite. Instead additional coupling including interchain
interactions and an anisotropic staggered field are necessary to fully model
the observed excitations. | cond-mat |
Zero temperature dynamics of Ising model on a densely connected small
world network: The zero temperature quenching dynamics of the ferromagnetic Ising model on a
densely connected small world network is studied where long range bonds are
added randomly with a finite probability $p$. We find that in contrast to the
sparsely connected networks and random graph, there is no freezing and an
initial random configuration of the spins reaches the equilibrium configuration
within a very few Monte Carlo time steps in the thermodynamic limit for any $p
\ne 0$. The residual energy and the number of spins flipped at any time shows
an exponential relaxation to equilibrium. The persistence probability is also
studied and it shows a saturation within a few time steps, the saturation value
being 0.5 in the thermodynamic limit. These results are explained in the light
of the topological properties of the network which is highly clustered and has
a novel small world behaviour. | cond-mat |
Energy landscapes in random systems, driven interfaces and wetting: We discuss the zero-temperature susceptibility of elastic manifolds with
quenched randomness. It diverges with system size due to low-lying local
minima. The distribution of energy gaps is deduced to be constant in the limit
of vanishing gaps by comparing numerics with a probabilistic argument. The
typical manifold response arises from a level-crossing phenomenon and implies
that wetting in random systems begins with a discrete transition. The
associated ``jump field'' scales as $<h > \sim L^{-5/3}$ and $L^{-2.2}$ for
(1+1) and (2+1) dimensional manifolds with random bond disorder. | cond-mat |
The 5D term origin of the excited triplet in LaCoO3: We provide the proof for the 5D term origin of an excited triplet observed in
the recent electron-spin-resonance (ESR) experiments by Noguchi et al. (Phys.
Rev. B 66, 094404 (2002)). We have succeeded to fully describe experimental ESR
results both for the zero-field g-factor, of 3.35, and the splitting D of 4.90
cm-1, as well as for the magnetic field applied along different
crystallographic directions within the localized electron atomic-like approach
as originating from excitations within the lowest triplet of the 5T2g
octahedral subterm of the 5D term. In our atomic-like approach the d electrons
of the Co3+ ion in LaCoO3 form the highly-correlated atomic-like 3d6 system
with the singlet 1A1 ground state (an octahedral subterm of the 1I term) and
the excited octahedral subterm 5T2g of the 5D term. We take the ESR experiment
as confirmation of the existence of the discrete electronic structure for 3d
electron states in LaCoO3 in the meV scale.
PACS No: 76.30.Fc; 75.10.Dg : 73.30.-m
Keywords: electronic structure, crystal field, spin-orbit coupling, LaCoO3
Submitted 24.12.2002 to Phys. Rev. B. | cond-mat |
Symmetrized Liouvillian Gap in Markovian Open Quantum Systems: Markovian open quantum systems display complicated relaxation dynamics. The
spectral gap of the Liouvillian characterizes the asymptotic decay rate towards
the steady state, but it does not necessarily give a correct estimate of the
relaxation time because the crossover time to the asymptotic regime may be too
long. We here give a rigorous upper bound on the transient decay of
auto-correlation functions in the steady state by introducing the symmetrized
Liouvillian gap. The standard Liouvillian gap and the symmetrized one are
identical in an equilibrium situation but differ from each other in the absence
of the detailed balance condition. It is numerically shown that the symmetrized
Liouvillian gap always give a correct upper bound on the decay of the
auto-correlation function, but the standard Liouvillian gap does not. | cond-mat |
Weakly parametrized Jastrow ansatz for a strongly correlated Bose system: We consider the Jastrow pair-product wavefunction for the strongly correlated
Bose systems, in our case liquid helium-4. An ansatz is proposed for the pair
factors which consists of a numeric solution to a modified and parametrized
pair scattering equation. We consider a number of such simple one-variable
parametrizations. Additionally, we allow for a parametrizeable cutoff of the
pair factors and for the addition of a long-range phonon tail. This approach
results in the many-body wavefunctions that have between just one and three
parameters. Calculation of observables is carried with the Variational Monte
Carlo method. We find that such a simple parametrization is sufficient to
produce results that are comparable in quality to the best available two-body
Jastrow factors for helium. For two-parameter wavefunction, we find variational
energies of $-6.04$~K per particle for a system of one thousand particles. It
is also shown that short-range two-body correlations are reproduced in detail
by two- and three-parameter functions. | cond-mat |
Underpotential deposition of Cu on Au(111) in sulfate-containing
electrolytes: a theoretical and experimental study: We study the underpotential deposition of Cu on single-crystal Au(111)
electrodes in sulfate-containing electrolytes by a combination of computational
statistical-mechanics based lattice-gas modeling and experiments. The
experimental methods are in situ cyclic voltammetry and coulometry and ex situ
Auger electron spectroscopy and low-energy electron diffraction. The
experimentally obtained voltammetric current and charge densities and adsorbate
coverages are compared with the predictions of a two-component lattice-gas
model for the coadsorption of Cu and sulfate. This model includes effective,
lateral interactions out to fourth-nearest neighbors. Using group-theoretical
ground-state calculations and Monte Carlo simulations, we estimate effective
electrovalences and lateral adsorbate--adsorbate interactions so as to obtain
overall agreement with experiments, including both our own and those of other
groups. In agreement with earlier work, we find a mixed R3xR3 phase consisting
of 2/3 monolayer Cu and 1/3 monolayer sulfate at intermediate electrode
potentials, delimited by phase transitions at both higher and lower potentials.
Our approach provides estimates of the effective electrovalences and lateral
interaction energies, which cannot yet be calculated by first-principles
methods. | cond-mat |
High pressure x-ray study of spin-Peierls physics in the quantum spin
chain material TiOCl: The application of pressure can induce transitions between unconventional
quantum phases in correlated materials. The inorganic compound TiOCl, composed
of chains of S=1/2 Ti ions, is an ideal realization of a spin-Peierls system
with a relatively simple unit cell. At ambient pressure, it is an insulator due
to strong electronic interactions (a Mott insulator). Its resistivity shows a
sudden decrease with increasing pressure, indicating a transition to a more
metallic state which may coincide with the emergence of charge density wave
order. Therefore, high pressure studies of the structure with x-rays are
crucial in determining the ground-state physics in this quantum magnet. In
ambient pressure, TiOCl exhibits a transition to an incommensurate nearly
dimerized state at $T_{c2}=92$ K and to a commensurate dimerized state at
$T_{c1}=66$ K. Here, we discover a rich phase diagram as a function of
temperature and pressure using x-ray diffraction on a single crystal in a
diamond anvil cell down to $T=4$ K and pressures up to 14.5 GPa. Remarkably,
the magnetic interaction scale increases dramatically with increasing pressure,
as indicated by the high onset temperature of the spin-Peierls phase. At
$\sim$7 GPa, the extrapolated onset of the spin-Peierls phase occurs above
$T=300$ K, indicating a quantum singlet state exists at room temperature.
Further comparisons are made with the phase diagrams of related spin-Peierls
systems that display metallicity and superconductivity under pressure. | cond-mat |
Conversion of multilayer graphene into continuous ultrathin sp3-bonded
carbon films on metal surfaces: The conversion of multilayer graphenes into sp^3-bonded carbon films on metal
surfaces (through hydrogenation or fluorination of the outer surface of the top
graphene layer) is indicated through first-principles computations. The main
driving force for this conversion is the hybridization between carbon sp^3
orbitals and metal surface dz^2 orbitals. The induced electronic gap states in
the carbon layers are confined in a region within 0.5 nm of the metal surface.
Whether the conversion occurs depend on the fraction of hydrogenated
(fluorinated) C atoms and on the number of stacked graphene layers. In the
analysis of the Eliashberg spectral functions for the sp^3 carbon films on
diamagnetic metals, the strong covalent metal-sp^3 carbon bonds induce soft
phonon modes that predominantly contribute to large electron-phonon couplings,
suggesting the possibility of phonon-mediated superconductivity. Our results
suggest a route to experimental realization of large-area ultrathin sp^3-bonded
carbon films on metal surfaces. | cond-mat |
Persistent local order heterogeneity in the supercritical carbon dioxide: The supercritical state is currently viewed as uniform and homogeneous on the
pressure-temperature phase diagram in terms of physical properties. Here, we
study structural properties of the supercritical carbon dioxide, and discover
the existence of persistent medium-range order correlations which make
supercritical carbon dioxide non-uniform and heterogeneous on an intermediate
length scale, a result not hitherto anticipated. We report on the carbon
dioxide heterogeneity shell structure where, in the first shell, both carbon
and oxygen atoms experience gas-like type inter- actions with short range order
correlations, while within the second shell oxygen atoms essentially exhibit
liquid-like type of interactions with medium range order correlations due to
localisation of transverse-like phonon packets. We show that the local order
heterogeneity remains in the three phase-like equilibrium within very wide
temperature range. Importantly, we highlight a catalytic role of atoms inside
the nearest neighbor heterogeneity shell in providing a mechanism for diffusion
in the supercritical carbon dioxide on an intermediate length scale. Finally,
we discuss important implications for answering the intriguing question whether
Venus may have had carbon dioxide oceans and urge for an experimental detection
of this persistent local order heterogeneity. | cond-mat |
Spin texture induced by non-magnetic doping and spin dynamics in 2D
triangular lattice antiferromagnet h-Y(Mn,Al)O3: Novel effects induced by nonmagnetic impurities in frustrated magnets and
quantum spin liquid represent a highly nontrivial and interesting problem. A
theoretical proposal of extended modulated spin structures induced by doping of
such magnets, distinct from the well-known skyrmions has attracted significant
interest. Here, we demonstrate that nonmagnetic impurities can produce such
extended spin structures in h-YMnO3, a triangular antiferromagnet with
noncollinear magnetic order. Using inelastic neutron scattering (INS), we
measured the full dynamical structure factor in Al-doped h-YMnO3 and confirmed
the presence of magnon damping with a clear momentum dependence. Our
theoretical calculations can reproduce the key features of the INS data,
supporting the formation of the proposed spin textures. As such, our study
provides the first experimental confirmation of the impurity-induced spin
textures. It offers new insights and understanding of the impurity effects in a
broad class of noncollinear magnetic systems. | cond-mat |
Atomic structure analysis of nanocrystalline Boehmite AlO(OH): Nanocrystalline n-AlO(OH) (Disperal P2(R) of Sasol) was investigated by means
of the atomic pair distribution function (PDF). The PDF is derived from powder
diffraction data, an ideally resolved PDF is obtained from a synchrotron source
which provides a large maximal scattering vector length Qmax > 300 nm-1. Here,
however, we were able to reveal atomic structure details of the about 4 nm
particles from in-house diffraction data (Qmax = 130 nm-1): PDF least squares
model refinements show that n-AlO(OH) is of the layered Boehmite structure type
(oC16, Cmcm). But the structure is uniformly distorted in domains of ca. 2 nm
size within the nano particles. The hydrogen bonds between the layers are
widened up significantly by +13 pm, accounting for the higher reactivity when
compared to microcrystalline Boehmite. Our results from only one
"nanocrystallographic" experiment are consistent with a trend which was found
via the Rietveld technique on a series of AlO(OH) of different crystallite size
(Bokhimi et al., 2001). In addition, the PDF contains information on structural
distortion as a function of (coherence) domain size within the nano particles. | cond-mat |
Preferential out-of-plane conduction and quasi-one-dimensional
electronic states in layered 1T-TaS2: Layered transition metal dichalcogenides (TMDs) are commonly classified as
quasi-two-dimensional materials, meaning that their electronic structure
closely resembles that of an individual layer, which results in resistivity
anisotropies reaching thousands. Here, we show that this rule does not hold for
1T-TaS2 - a compound with the richest phase diagram among TMDs. While the onset
of charge density wave order makes the in-plane conduction non-metallic, we
reveal that the out-of-plane charge transport is metallic and the resistivity
anisotropy is close to one. We support our findings with ab-initio calculations
predicting a pronounced quasi-one-dimensional character of the electronic
structure. Consequently, we interpret the highly debated metal-insulator
transition in 1T-TaS2 as a quasi-one-dimensional instability, contrary to the
long-standing Mott localisation picture. In a broader context, these findings
are relevant for the newly born field of van der Waals heterostructures, where
tuning interlayer interactions (e.g. by twist, strain, intercalation, etc.)
leads to new emergent phenomena. | cond-mat |
Bistable Spin Currents from Quantum Dots Embedded in a Microcavity: We examine the spin current generated by quantum dots embedded in an optical
microcavity. The dots are connected to leads, which allow electrons to tunnel
into and out of the dot. The spin current is generated by spin flip transitions
induced by a quantized electromagnetic field inside the cavity with one of the
Zeeman states lying below the Fermi level of the leads and the other above. In
the limit of strong Coulomb blockade, this model is analogous to the
Jaynes-Cummings model in quantum optics. We find that the cavity field
amplitude and the spin current exhibit bistability as a function of the laser
amplitude, which is driving the cavity mode. Even in the limit of a single dot,
the spin current and the Q distribution of the cavity field have a bimodal
structure. | cond-mat |
Exact Thermodynamics and Transport in the Classical Sine-Gordon Model: We revisit the exact thermodynamic description of the classical sine-Gordon
field theory, a notorious integrable model. We found that existing results in
the literature based on the soliton-gas picture did not correctly take into
account light, but extended, solitons and thus led to incorrect results. This
issue is regularized upon requantization: we derive the correct thermodynamics
by taking the semiclassical limit of the quantum model. Our results are then
extended to transport settings by means of Generalized Hydrodynamics. | cond-mat |
3-Wave Mixing Josephson Dipole Element: Parametric conversion and amplification based on three-wave mixing are
powerful primitives for efficient quantum operations. For superconducting
qubits, such operations can be realized with a quadrupole Josephson junction
element, the Josephson Ring Modulator (JRM), which behaves as a loss-less
three-wave mixer. However, combining multiple quadrupole elements is a
difficult task so it would be advantageous to have a pure three-wave dipole
element that could be tessellated for increased power handling and/or
information throughput. Here, we present a dipole circuit element with
third-order nonlinearity, which implements three-wave mixing. Experimental
results for a non-degenerate amplifier based on the proposed pure third-order
nonlinearity are reported. | cond-mat |
Heat transport in quantum spin chains: the relevance of integrability: We investigate heat transport in various quantum spin chains, using the
projector operator technique. We find that anomalous heat transport is linked
not to the integrability of the Hamiltonian, but to whether it can be mapped to
a model of non-interacting fermions. Our results also suggest how seemingly
anomalous transport may occur at low temperatures in a much wider class of
models. | cond-mat |
Affect of film thickness on the blue photoluminescence from ZnO: Zinc oxide (ZnO) films having various thicknesses were synthesized on
sapphire substrates by thermal oxidation of Zn-metallic films in air ambient.
X-ray diffraction (XRD) spectra indicate that the resulting films possess a
polycrystalline hexagonal wurtzite structure without preferred orientation. For
films having a thickness of 200 nm, crystal grain size was observed to decrease
with increasing annealing temperature up to 600C, and then increase at higher
temperatures. Thicker films demonstrated a modest increase in grain size as
temperature increased from 300C to 1200C. The influence of film thickness on
the optical properties was investigated using room temperature
photoluminescence (PL). Specifically, PL spectra indicate four emission bands:
excitonic ultraviolet, blue, and deep-level green and yellow emission. The
strongest UV emission and narrowest full width at half maximum (0.09 eV) was
observed for films having a thickness of 200 nm and annealed at low temperature
(300C). The ratio of deep-level green emission to UV excitonic emission was
observed to decrease with decreasing annealing temperature, which is attributed
to the generation of fewer oxygen vacancies and interstitial oxygen ions in the
bulk. As film thickness decreased, we observed the emergence of blue emission
and a significant red shift (0.15 eV) in the bandgap. The emergence of blue
emission and the corresponding decrease in emission associated with bulk
defects when depletion width grows relative to the bulk suggests that the
origin of the blue emission is related to the negatively charged Zinc
interstitials found within the deletion region near the interface. | cond-mat |
Spectroscopic evidence for polaronic behaviour of the strong spin-orbit
insulator Sr$_3$Ir$_2$O$_7$: We investigate the bilayer Ruddlesden-Popper iridate Sr$_3$Ir$_2$O$_7$ by
temperature-dependent angle-resolved photoemission. We find a narrow-gap
correlated insulator, with spectral features indicative of a polaronic ground
state, strikingly similar to that observed previously for the parent compounds
of the cuprate superconductors. We additionally observe similar behaviour for
the single-layer cousin Sr$_2$IrO$_4$, indicating that strong electron-boson
coupling dominates the low-energy excitations of this exotic family of
materials, and providing a microscopic link between the insulating ground
states of the seemingly-disparate 3d cuprates and 5d iridates. | cond-mat |
Antioxidant activity and toxicity study of cerium oxide nanoparticles
stabilized with innovative functional copolymers: Oxidative stress, which is one of the main harmful mechanisms of pathologies
including is-chemic stroke, contributes to both neurons and endothelial cell
damages, leading to vascular lesions. Although many antioxidants have been
tested in preclinical studies, no treatment is currently available for stroke
patients. Since cerium oxide nanoparticles (CNPs) exhibit remarkable
antioxidant capacities, our objective is to develop an innovative coating to
enhance CNPs biocompatibility without disrupting their antioxidant capacities
or enhance their toxicity. This study reports the synthesis and
characterization of functional polymers and their impact on the enzyme-like
catalytic activity of CNPs. To study the toxicity and the antioxidant
properties of CNPs for stroke and particularly endothelial damages, in vitro
studies are conducted on a cerebral endothelial cell line (bEnd.3). Despite
their internalization in bEnd.3 cells, coated CNPs are devoid of cytotoxicity.
Microscopy studies report an intracellular localization of CNPs, more precisely
in endosomes. All CNPs reduces glutamate-induced intracellular production of
ROS in endothelial cells but one CNP significantly reduces both the production
of mitochondrial super-oxide anion and DNA oxidation. In vivo studies report a
lack of toxicity in mice. This study there-fore describes and identifies
biocompatible CNPs with interesting antioxidant properties for ischemic stroke
and related pathologies. | cond-mat |
Optimal orbitals from energy fluctuations in correlated wave functions: A quantum Monte Carlo method of determining Jastrow-Slater wave functions for
which the energy is stationary with respect to variations in the
single-particle orbitals is presented. A potential is determined by a
least-squares fitting of fluctuations in the energy with a linear combination
of one-body operators. This potential is used in a self-consistent scheme for
the orbitals whose solution ensures that the energy of the correlated wave
function is stationary with respect to variations in the orbitals. The method
is feasible for atoms, molecules, and solids and is demonstrated for the carbon
and neon atoms. | cond-mat |
Bending to kinetic energy transfer in adhesive peel front
micro-instability: We report an extensive experimental study of a detachment front dynamics
instability, appearing at microscopic scales during the peeling of adhesive
tapes. The amplitude of this instability scales with its period as
$A_{\text{mss}} \propto T_{\text{mss}}^{1/3}$, with a pre-factor evolving
slightly with the peel angle $\theta$, and increasing systematically with the
bending modulus $B$ of the tape backing. Establishing a local energy budget of
the detachment process during one period of this micro-instability, our
theoretical model shows that the elastic bending energy stored in the portion
of tape to be peeled is converted into kinetic energy, providing a quantitative
description of the experimental scaling law. | cond-mat |
Phase stabilization by electronic entropy in plutonium: (Pu) has an unusually rich phase diagram that includes seven distinct solid
state phases and an unusually large 25% collapse in volume from its delta phase
to its low temperature alpha phase via a series of structural transitions.
Despite considerable advances in our understanding of strong electronic
correlations within various structural phases of Pu and other actinides, the
thermodynamic mechanism responsible for driving the volume collapse has
continued to remain a mystery. Here we utilize the unique sensitivity of
magnetostriction measurements to unstable f electron shells to uncover the
crucial role played by electronic entropy in stabilizing delta-Pu against
volume collapse. We find that in contrast to valence fluctuating rare earths,
which typically have a single f electron shell instability whose excitations
drive the volume in a single direction in temperature and magnetic field,
delta-Pu exhibits two such instabilities whose excitations drive the volume in
opposite directions while producing an abundance of entropy at elevated
temperatures. The two instabilities imply a near degeneracy between several
different configurations of the 5f atomic shell, giving rise to a considerably
richer behavior than found in rare earth metals. We use heat capacity
measurements to establish a robust thermodynamic connection between the two
excitation energies, the atomic volume, and the previously reported excess
entropy of delta-Pu at elevated temperatures. | cond-mat |
Orbitally driven spin-singlet dimerization in $S$=1
La$_{4}$Ru$_{2}$O$_{10}$: Using x-ray absorption spectroscopy at the Ru-$L_{2,3}$ edge we reveal that
the Ru$^{4+}$ ions remain in the $S$=1 spin state across the rare 4d-orbital
ordering transition and spin-gap formation. We find using local spin density
approximation + Hubbard U (LSDA+U) band structure calculations that the crystal
fields in the low temperature phase are not strong enough to stabilize the
$S$=0 state. Instead, we identify a distinct orbital ordering with a
significant anisotropy of the antiferromagnetic exchange couplings. We conclude
that La$_{4}$Ru$_{2}$O$_{10}$ appears to be a novel material in which the
orbital physics drives the formation of spin-singlet dimers in a quasi
2-dimensional $S$=1 system. | cond-mat |
Phase diagram and critical behavior of the square-lattice Ising model
with competing nearest- and next-nearest-neighbor interactions: Using the parallel tempering algorithm and GPU accelerated techniques, we
have performed large-scale Monte Carlo simulations of the Ising model on a
square lattice with antiferromagnetic (repulsive) nearest-neighbor(NN) and
next-nearest-neighbor(NNN) interactions of the same strength and subject to a
uniform magnetic field. Both transitions from the (2x1) and row-shifted (2x2)
ordered phases to the paramagnetic phase are continuous. From our data
analysis, reentrance behavior of the (2x1) critical line and a bicritical point
which separates the two ordered phases at T=0 are confirmed. Based on the
critical exponents we obtained along the phase boundary, Suzuki's weak
universality seems to hold. | cond-mat |
Light bipolarons in a system of electrons coupled to dispersive optical
phonons: We investigate the ground state properties of the bipolaron coupled to
quantum dispersive optical phonons in the one-dimensional Holstein-Hubbard
model. We concentrate on the interplay between the phonon dispersion and the
Coulomb repulsion and their mutual effect on the bipolaron effective mass, the
binding energy, and the phase diagram. Most surprisingly, the sign of the
curvature of the optical phonon dispersion plays a decisive role on the
bipolaron binding energy in the presence of the Coulomb repulsion $U$. In
particular, when the sign of the phonon dispersion curvature matches the sign
of the electron dispersion curvature, the bipolaron remains bound in the strong
coupling limit even when $U\to \infty$ and the binding emanates from the
exchange of phonons between two electrons residing on adjacent sites. At
moderate electron-phonon coupling a light bipolaron exists up to large values
of $U$. Finally, an intuitive explanation of the role of the phonon dispersion
on the bipolaron binding energy is derived using the strong coupling limit
where the binding emanates from the exchange of phonons between two electrons
residing on adjacent sites which leads to enhanced stability of bipolarons at
elevated Coulomb repulsion. | cond-mat |
The Berry Phase Rectification Tensor and The Solar Rectification Vector: We introduce an operational definition of the Berry Phase Rectification
Tensor as the second-order change of polarization of a material in response to
an ideal short pulse of an electric field. Under time-reversal symmetry this
tensor depends exclusively on the Berry phases of the Bloch bands and not their
energy dispersions, making it an intrinsic property to each material which
contains contributions from both the inter-band shift currents and the
intra-band Berry Curvature Dipole. We also introduce the Solar Rectification
Vector as a technologically relevant figure of merit for a bulk photo-current
generation under ideal black-body radiation in analogy with the classic solar
cell model of Shockley and Queisser. We perform first-principles calculations
of the Berry Phase Rectification Tensor and the Solar Rectification Vector for
the Weyl semimetal TaAs and the insulator LiAsSe2 which features large shift
currents close to the peak of solar radiation intensity. | cond-mat |
Dynamical Simulations of Trapped Bose Gases at Finite Temperatures: In this paper, we develop a numerical procedure for investigating the
dynamics of trapped Bose gases based on the ZGN theory. The dynamical equations
used consist of a generalized Gross-Pitaevskii equation for the condensate
order parameter and a semiclassical kinetic equation for the thermal cloud. The
former is solved using a fast Fourier transform split-operator technique while
the Boltzmann equation is treated by means of N-body simulations. The two
components are coupled by mean fields as well as collisional processes that
transfer atoms between the two. This scheme has been applied to a model
equilibration problem, dipole oscillations in isotropic traps and scissors
modes in anisotropic traps. In the case of the latter, the frequencies and
damping rates of the condensate mode have been extracted from the simulations
for a wide range of temperatures. Good agreement with recent experiments has
been found. | cond-mat |
Observation of Dipole-Induced Spin Texture in an $^{87}$Rb Bose-Einstein
Condensate: We report the spin texture formation resulting from the magnetic
dipole-dipole interaction in a spin-2 $^{87}$Rb Bose-Einstein condensate. The
spinor condensate is prepared in the transversely polarized spin state and the
time evolution is observed under a magnetic field of 90 mG with a gradient of 3
mG/cm using Stern-Gerlach imaging. The experimental results are compared with
numerical simulations of the Gross-Pitaevskii equation, which reveals that the
observed spatial modulation of the longitudinal magnetization is due to the
spin precession in an effective magnetic field produced by the dipole-dipole
interaction. These results show that the dipole-dipole interaction has
considerable effects even on spinor condensates of alkali metal atoms. | cond-mat |
A Feshbach resonance in collisions between ultracold ground state
molecules: Collisional resonances are an important tool which has been used to modify
interactions in ultracold gases, for realizing novel Hamiltonians in quantum
simulations, for creating molecules from atomic gases and for controlling
chemical reactions. So far, such resonances have been observed for atom-atom
collisions, atom-molecule collisions and collisions between Feshbach molecules
which are very weakly bound. Whether such resonances exist for ultracold ground
state molecules has been debated due to the possibly high density of states
and/or rapid decay of the resonant complex. Here we report a very pronounced
and narrow (25 mG) Feshbach resonance in collisions between two ground state
NaLi molecules. This molecular Feshbach resonance has two special
characteristics. First, the collisional loss rate is enhanced by more than two
orders of magnitude above the background loss rate which is saturated at the
$p$-wave universal value, due to strong chemical reactivity. Second, the
resonance is located at a magnetic field where two open channels become nearly
degenerate. This implies the intermediate complex predominantly decays to the
second open channel. We describe the resonant loss feature using a model with
coupled modes which is analogous to a Fabry-P\'erot cavity. Our observations
prove the existence of long-lived coherent intermediate complexes even in
systems without reaction barriers and open up the possibility of coherent
control of chemical reactions. | cond-mat |
Resilience of gas-phase anharmonicity in the vibrational response of
adsorbed carbon monoxide and breakdown under electrical conditions: In surface catalysis, the adsorption of carbon monoxide on transition-metal
electrodes represents the prototype of strong chemisorption. Notwithstanding
significant changes in the molecular orbitals of adsorbed CO, spectroscopic
experiments highlight a close correlation between the adsorbate stretching
frequency and equilibrium bond length for a wide range of adsorption geometries
and substrate compositions. In this work, we study the origins of this
correlation, commonly known as Badger's rule, by deconvoluting and examining
contributions from the adsorption environment to the intramolecular potential
using first-principles calculations. Noting that intramolecular anharmonicity
is preserved upon CO chemisorption, we show that Badger's rule for adsorbed CO
can be expressed solely in terms of the tabulated Herzberg spectroscopic
constants of isolated CO. Moreover, although it had been previously established
using finite-cluster models that Badger's rule is not affected by electrical
conditions, we find here that Badger's rule breaks down when the electrified
surface is represented as a periodic slab. Examining this breakdown in terms of
anharmonic contributions from the effective surface charge reveals limitations
of conventional finite-cluster models in describing electrical conditions at
metal electrodes. | cond-mat |
Interaction-induced beats of Friedel oscillations in quantum wires: We analyze the spectrum of electron density oscillations in an interacting
one-dimensional electron system with an impurity. The system's inhomogeneity is
characterized by different values of Fermi wave vectors $k_F=k_{L/R}$ on
left/right side of the scatterer, leading to a Landauer dipole formation. We
demonstrate, that while in the noninteracting system the Friedel oscillations
possess only one periodicity related to the local $k_F$, say $k_L$ on the left
side, the interplay of the interactions and the Landauer dipole generates an
additional peak in the spectrum of density oscillations at the counterpart
$k_R$. Being only present in correlated systems, the position and shape of this
spectral feature, which in coordinate space is observable as a beating pattern
in the Friedel oscillations, reveals many important details about the nature of
interactions. Thus it has a potential to become an investigation tool in
condensed matter physics. | cond-mat |
Non-local features of the spin-orbit exciton in Kitaev materials: A comparative resonant inelastic x-ray scattering (RIXS) study of three
well-known Kitaev materials is presented: $\alpha$-Li$_2$IrO$_3$,
Na$_2$IrO$_3$, and $\alpha$-RuCl$_3$. Despite similar low-energy physics, these
materials show distinct electronic properties, such as the large difference in
the size of the charge gap. The RIXS spectra of the spin-orbit exciton for
these materials show remarkably similar three-peak features, including sharp
low energy peak (peak A) as well as transitions between $j_{\text{eff}}=1/2$
and $j_{\text{eff}}=3/2$ states. Comparison of experimental spectra with
cluster calculations reveals that the observed three-peak structure reflects
the significant role that non-local physics plays in the electronic structure
of these materials. In particular, the low-energy peak A arises from a
holon-doublon pair rather than a conventional particle-hole exciton as proposed
earlier. Our study suggests that while spin-orbit assisted Mott insulator is
still the best description for these materials, electron itinerancy cannot be
ignored when formulating low-energy Hamiltonian of these materials. | cond-mat |
Comment on the energy spectrum of Tonks-Girardeau gas: Withdrawn due to major errors. | cond-mat |
Two-channel point-contact tunneling theory of superconductors: We introduce a two-channel tunneling model to generalize the widely used BTK
theory of point-contact conductance between a normal metal contact and
superconductor. Tunneling of electrons can occur via localized surface states
or directly, resulting in a Fano resonance in the differential conductance
$G=dI/dV$. We present an analysis of $G$ within the two-channel model when
applied to soft point-contacts between normal metallic silver particles and
prototypical heavy-fermion superconductors CeCoIn$_5$ and CeRhIn$_5$ at high
pressures. In the normal state the Fano line shape of the measured $G$ is well
described by a model with two tunneling channels and a large
temperature-independent background conductance. In the superconducting state a
strongly suppressed Andreev reflection signal is explained by the presence of
the background conductance. We report Andreev signal in CeCoIn$_5$ consistent
with standard $d_{x^2-y^2}$-wave pairing, assuming an equal mixture of
tunneling into [100] and [110] crystallographic interfaces. Whereas in
CeRhIn$_5$ at 1.8 and 2.0 GPa the signal is described by a $d_{x^2-y^2}$-wave
gap with reduced nodal region, i.e., increased slope of the gap opening on the
Fermi surface. A possibility is that the shape of the high-pressure Andreev
signal is affected by the proximity of a line of quantum critical points that
extends from 1.75 to 2.3 GPa, which is not accounted for in our description of
the heavy-fermion superconductor. | cond-mat |
Fundamental differences between exciton and quantum dot duo: We present five major reasons why semiconductor exciton, that is, a
correlated electron-hole pair in a bulk, quantum well, or quantum wire, is
conceptually different from a pair in a quantum dot: (1) the origin of pair
binding, (2) the interaction with additional carriers, (3) the quantum nature
of the pair, (4) the coupling to photon, and (5) the photon-absorption
mechanism. Due to these differences, we should refrain from calling an
electron-hole pair in a quantum dot an exciton, as commonly done; we propose to
call it a duo. Within the same frame of chamber musics, we likewise propose to
call three and four carriers in a dot, a trio and a quatuor, instead of a trion
and a biexciton. | cond-mat |
Mechanical Theory of Nonequilibrium Coexistence and Motility-Induced
Phase Separation: Nonequilibrium phase transitions are routinely observed in both natural and
synthetic systems. The ubiquity of these transitions highlights the conspicuous
absence of a general theory of phase coexistence that is broadly applicable to
both nonequilibrium and equilibrium systems. Here, we present a general
mechanical theory for phase separation rooted in ideas explored nearly a
half-century ago in the study of inhomogeneous fluids. The core idea is that
the mechanical forces within the interface separating two coexisting phases
uniquely determine coexistence criteria, regardless of whether a system is in
equilibrium or not. We demonstrate the power and utility of this theory by
applying it to active Brownian particles, predicting a quantitative phase
diagram for motility-induced phase separation in both two and three dimensions.
This formulation additionally allows for the prediction of novel interfacial
phenomena, such as an increasing interface width while moving deeper into the
two-phase region, a uniquely nonequilibrium effect confirmed by computer
simulations. The self-consistent determination of bulk phase behavior and
interfacial phenomena offered by this mechanical perspective provide a concrete
path forward towards a general theory for nonequilibrium phase transitions. | cond-mat |
Auger recombination and carrier multiplication in embedded silicon and
germanium nanocrystals: For Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices, Auger
recombination (AR) and carrier multiplication (CM) lifetimes are computed
exactly in a three-dimensional real space grid using empirical pseudopotential
wave functions. Our results in support of recent experimental data offer new
predictions. We extract simple Auger constants valid for NCs. We show that both
Si and Ge NCs can benefit from photovoltaic efficiency improvement via CM due
to the fact that under an optical excitation exceeding twice the band gap
energy, the electrons gain lion's share from the total excess energy and can
cause a CM. We predict that CM becomes especially efficient for hot electrons
with an excess energy of about 1 eV above the CM threshold. | cond-mat |
Fully resolved currents from quantum transport calculations: We extract local current distributions from interatomic currents calculated
using a fully relativistic quantum mechanical scattering formalism by
interpolation onto a three-dimensional grid. The method is illustrated with
calculations for Pt$|$Ir and Pt$|$Au multilayers as well as for thin films of
Pt and Au that include temperature-dependent lattice disorder. The current flow
is studied in the "classical" and "Knudsen" limits determined by the sample
thickness relative to the mean free path $\lambda$, introducing current
streamlines to visualize the results. For periodic multilayers, our results in
the classical limit reveal that transport inside a metal can be described using
a single value of resistivity $\rho$ combined with a linear variation of $\rho$
at the interface while the Knudsen limit indicates a strong spatial dependence
of $\rho$ inside a metal and an anomalous dip of the current density at the
interface which is accentuated in a region where transient shunting persists. | cond-mat |
Melting tungsten nanoparticles: a molecular dynamics study: We report a molecular dynamics simulation of melting of tungsten (W)
nanoparticles. The modified embedded atom method (MEAM) interatomic potentials
are used to describe the interaction between tungsten atoms. The melting
temperature of unsupported tungsten nanoparticles of different sizes are found
to decrease as the size of the particles decreases. The melting temperature
obtained in the present study is approximately a decreasing function of inverse
radius, in a good agreement with the predictions of thermodynamic models. We
also observed that the melting of a W nanoparticle is preceded by the
premelting of its outer skin at a temperature lower than its melting
temperature. | cond-mat |
Magnetic-field induced triplet superconductivity in the Hubbard model on
a triangular lattice: We propose theoretically that a magnetic field can realize spin-triplet
superconductivity in repulsively interacting electron systems having strong
ferromagnetic spin fluctuations. We confirm the general idea for the
low-density Hubbard model on a triangular lattice, whose Fermi surface consists
of disconnected pieces, by calculating the pairing susceptibility in a moderate
magnetic field with the quantum Monte-Carlo method combined with the dynamical
cluster approximation. | cond-mat |
Achieving fast oxygen diffusion in perovskites by cation ordering: The oxygen-exchange behavior has been studied in half-doped manganese and
cobalt perovskite oxides. We have found that the oxygen diffusivity in
Gd_{0.5}Ba_{0.5}MnO_{3-\delta} can be enhanced by orders of magnitude by
inducing crystallographic ordering among lanthanide and alkali-earth ions in
the A-site sublattice. Transformation of a simple cubic perovskite, with
randomly occupied A-sites, into a layered crystal GdBaMn_2O_{5+x} (or
isostructural GdBaCo_2O_{5+x} for cobalt oxide) with alternating lanthanide and
alkali-earth planes reduces the oxygen bonding strength and provides
disorder-free channels for ion motion, pointing to an efficient way to design
new ionic conductors. | cond-mat |
Comparative classical and ab initio Molecular Dynamics study of molten
and glassy germanium dioxide: A Molecular Dynamics (MD) study of static and dynamic properties of molten
and glassy germanium dioxide is presented. The interactions between the atoms
are modelled by the classical pair potential proposed by Oeffner and Elliott
(OE) [Oeffner R D and Elliott S R 1998, Phys. Rev. B, 58, 14791]. We compare
our results to experiments and previous simulations. In addition, an ab initio
method, the so-called Car-Parrinello Molecular Dynamics (CPMD), is applied to
check the accuracy of the structural properties, as obtained by the classical
MD simulations with the OE potential. As in a similar study for SiO2, the
structure predicted by CPMD is only slightly softer than that resulting from
the classical MD. In contrast to earlier simulations, both the static structure
and dynamic properties are in very good agreement with pertinent experimental
data. MD simulations with the OE potential are also used to study the
relaxation dynamics. As previously found for SiO2, for high temperatures the
dynamics of molten GeO2 is compatible with a description in terms of mode
coupling theory. | cond-mat |
Reinforcement Learning of Artificial Microswimmers: The behavior of living systems is based on the experience they gained through
their interactions with the environment [1]. This experience is stored in the
complex biochemical networks of cells and organisms to provide a relationship
between a sensed situation and what to do in this situation [2-4]. An
implementation of such processes in artificial systems has been achieved
through different machine learning algorithms [5, 6]. However, for microscopic
systems such as artificial microswimmers which mimic propulsion as one of the
basic functionalities of living systems [7, 8] such adaptive behavior and
learning processes have not been implemented so far. Here we introduce machine
learning algorithms to the motion of artificial microswimmers with a hybrid
approach. We employ self-thermophoretic artificial microswimmers in a real
world environment [9, 10] which are controlled by a real-time microscopy system
to introduce reinforcement learning [11-13]. We demonstrate the solution of a
standard problem of reinforcement learning - the navigation in a grid world.
Due to the size of the microswimmer, noise introduced by Brownian motion if
found to contribute considerably to both the learning process and the actions
within a learned behavior. We extend the learning process to multiple swimmers
and sharing of information. Our work represents a first step towards the
integration of learning strategies into microsystems and provides a platform
for the study of the emergence of adaptive and collective behavior. | cond-mat |
Simple derivation of Young, Wenzel and Cassie-Baxter equations and its
interpretations: In this paper we have derived Young's, Wenzel's and Cassie-Baxter's equations
using conceptual model rather than showing rigorous derivation to help the
new-comers in this field. We then pointed out that if the substrate is
initially hydorphilic then one can modify the surface morphology and make the
substrate to become hydrophobic or superhydrophobic. But, if the substrate is
initially hydrophobic then one can only make it superhydrophobic but not
hydrophilic by modifying the surface morphology using the formalisms mentioned
in this paper | cond-mat |
Phase diagram and neutron spin resonance of superconducting
NaFe$_{1-x}$Cu$_x$As: We use transport and neutron scattering to study the electronic phase diagram
and spin excitations of NaFe$_{1-x}$Cu$_x$As single crystals. Similar to Co-
and Ni-doped NaFeAs, a bulk superconducting phase appears near $x\approx2\%$
with the suppression of stripe-type magnetic order in NaFeAs. Upon further
increasing Cu concentration the system becomes insulating, culminating in an
antiferromagnetically ordered insulating phase near $x\approx 50\%$. Using
transport measurements, we demonstrate that the resistivity in
NaFe$_{1-x}$Cu$_x$As exhibits non-Fermi-liquid behavior near $x\approx1.8\%$.
Our inelastic neutron scattering experiments reveal a single neutron spin
resonance mode exhibiting weak dispersion along $c$-axis in
NaFe$_{0.98}$Cu$_{0.02}$As. The resonance is high in energy relative to the
superconducting transition temperature $T_{\rm c}$ but weak in intensity,
likely resulting from impurity effects. These results are similar to other iron
pnictides superconductors despite the superconducting phase in
NaFe$_{1-x}$Cu$_x$As is continuously connected to an antiferromagnetically
ordered insulating phase near $x\approx 50\%$ with significant electronic
correlations. Therefore, electron correlations is an important ingredient of
superconductivity in NaFe$_{1-x}$Cu$_x$As and other iron pnictides. | cond-mat |
Fluid critical behavior at liquid-gas phase transition: Analytic method
for microscopic description: The behavior of fluids in the vicinity of the liquid-gas critical point is
studied within the cell fluid model framework. The analytic method for deriving
the equation of state of a cell fluid model in the low-temperature region
(T<Tc) is developed using the renormalization group transformation within the
collective variables approach. Mathematical description within the grand
canonical ensemble is illustrated by an example of the Morse interaction
potential possessing the Fourier transform. A specific feature of the proposed
method lies in the possibility to use exclusively microscopic characteristics
of a fluid (parameters of the interaction potential) for calculating
macroscopic quantities (pressure and other thermodynamic quantities) without
involving the hard-spheres reference system. The grand partition function,
thermodynamic potential, and equation of state of the model near the critical
point are derived taking into account the non-Gaussian (quartic) distribution
of order parameter fluctuations. A nonlinear equation, which links the density
and the chemical potential, is presented and solved. Graphs of the dependence
of the density on the chemical potential are plotted for various values of the
relative temperature. The numerical estimates of the critical-point parameters
for potassium, obtained in addition to the estimates for sodium, are given. The
calculated critical-point parameters for liquid alkali metals (sodium and
potassium) are in accord with experimental data. The coexistence curve for
sodium is plotted and compared with other authors' data in the immediate
vicinity of Tc, where theoretical and experimental researches are difficult to
carry out. The differences between the obtained results and the earlier
published results for T>Tc are discussed. | cond-mat |
Optical Response of Solid CO$_2$ as a Tool for the Determination of the
High Pressure Phase: We report first-principles calculations of the frequency dependent linear and
second-order optical properties of the two probable extended-solid phases of
CO$_2$--V, i.e. $I\bar42d$ and $P2_12_12_1$. Compared to the parent $Cmca$
phase the linear optical susceptibility of both phases is much smaller. We find
that $I\bar42d$ and $P2_12_12_1$ differ substantially in their linear optical
response in the higher energy regime. The nonlinear optical responses of the
two possible crystal structures differ by roughly a factor of five. Since the
differences in the nonlinear optical spectra are pronounced in the low energy
regime, i.e. below the band gap of diamond, measurements with the sample inside
the diamond anvil cell are feasible. We therefore suggest optical experiments
in comparison with our calculated data as a tool for the unambiguous
identification of the high pressure phase of CO$_2$. | cond-mat |
Coexistence of Multifold and Multidimensional Topological Phonons in
KMgBO$_{3}$: Topological interpretations of phonons facilitate a new platform for novel
concepts in phonon physics. Though there are ubiquitous set of reports on
topological electronic excitations, the same for phonons are extremely limited.
Here, we propose a new candidate material, KMgBO 3 , which showcase the
co-existence of several multifold and multidimensional topological phonon
excitations, which are protected by spatial and non-spatial symmetries. This
includes zero dimensional double, triple and quadratic Weyl phonon nodes, one
dimensional nodal line/loop and two dimensional doubly degenerate nodal surface
states. Nodal line/loop emerges from the spin- 12 phonon nodes, while the two
dimensional doubly degenerate nodal surface arises from a combination of two
fold screw rotational and time reversal symmetries. Application of strain
breaks the C 3 rotational symmetry, which annihilates the spin-1 double Weyl
nodes, but preserves other topological features. Interestingly, strain helps to
create two extra single Weyl nodes, which in turn preserve the total chirality.
Alloying also breaks certain symmetries, destroying most of the topological
phonon features in the present case. Thus, KMgBO 3 is a promising candidate
which hosts various Weyl points, large Fermi arcs with a very clean phonon
spectra and tunable topological phonon excitations, and hence certainly worth
for future theoretical/experimental investigation of topological phononics. | cond-mat |
Switchable coupling for superconducting qubits using double resonance in
the presence of crosstalk: Several methods have been proposed recently to achieve switchable coupling
between superconducting qubits. We discuss some of the main considerations
regarding the feasibility of implementing one of those proposals: the
double-resonance method. We analyze mainly issues related to the achievable
effective coupling strength and the effects of crosstalk on this coupling
approach. We also find a new, crosstalk-assisted coupling channel that can be
an attractive alternative when implementing the double-resonance coupling
proposal. | cond-mat |
Variational Monte Carlo study of stripes as a function of doping in the
$t-t'$ Hubbard model: We perform variational Monte Carlo simulations of the single-band Hubbard
model on the square lattice with both nearest ($t$) and next-nearest ($t'$)
neighbor hoppings. Our work investigates the consequences of increasing hole
doping on the instauration of stripes and the behavior of the superconducting
order parameter, with a discussion on how the two phenomena affect each other.
We consider two different values of the next-nearest neighbor hopping
parameter, that are appropriate for describing cuprate superconductors. We
observe that stripes are the optimal state in a wide doping range; the stripe
wavelength reduces at increasing doping, until stripes melt into a uniform
state for large values of doping. Superconducting pair-pair correlations,
indicating the presence of superconductivity, are always suppressed in the
presence of stripes. Our results suggest that the phase diagram for the
single-band Hubbard model is dominated by stripes, with superconductivity being
possible only in a narrow doping range between striped states and a
nonsuperconducting metal. | cond-mat |
Modified Double Exchange Model with Novel Spin and Orbital Coupling:
Phase Diagram of The Manganites: From a general model of the Mn oxides R_{1-x}A_{x}MnO_3, we derive an
effective Hamiltonian in the low-energy subspace using the projection operator
method, in which a novel coupling between the spin and orbital degrees of
freedom is included. A phase diagram for temperature T versus doping
concentration x is computed by means of Monte Carlo simulation. Our result is
consistent with experimental observations in the Mn oxides with relatively wide
conduction band, such as Pr_{1-x}Sr_{x} MnO_{3} and La_{1-x}Sr_{x}MnO_{3}.
According to the obtained orbital ordering, we also predict that the motion of
charge carriers transforms from three-dimensional to two-dimensional as x is
increased beyond a critical value. | cond-mat |
Non-universal scaling in a model of information transmission and herd
behavior: We present a generalized dynamical model describing the sharing of
information, and corresponding herd behavior, in a population based on the
recent model proposed by Eguiluz and Zimmermann. By introducing a
size-dependent probability for dissociation of a cluster, we show that the
exponent characterizing the distribution of cluster sizes becomes
model-dependent and non-universal. The resulting system, which provides a
simplified model of a financial market, yields power law behavior with an
easily tunable exponent. | cond-mat |
On the form of prior for constrained thermodynamic processes with
uncertainty: We consider the standard thermodynamic processes with constraints, but with
additional uncertainty about the control parameters. Motivated by inductive
reasoning, we assign prior distribution that provides a rational guess about
likely values of the uncertain parameters.The priors are derived explicitly for
both the entropy conserving and the energy conserving processes. The proposed
form is useful when the constraint equation cannot be treated analytically. The
inference is performed using spin-1/2 systems as models for heat reservoirs.
Analytical results are derived in the high temperatures limit. Comparisons are
found between the estimates of thermal quantities and the optimal values
described by extremum principles. We also seek a intuitive interpretation of
the prior and show that it becomes uniform over the quantity which is conserved
in the process. We find further points of correspondence between the inference
based approach and the thermodynamic framework. | cond-mat |
Excitonic quantum criticality: from bilayer graphene to narrow Chern
bands: We study a family of excitonic quantum phase transitions describing the
evolution of a bilayer metallic state to an inter-layer coherent state where
excitons condense. We argue that such transitions can be continuous and exhibit
a non-Fermi liquid counterflow response
${\rho_{\mathrm{counterflow}}(\omega)\sim\omega^{2/z}}$ that directly encodes
the dynamical critical exponent $z$. This physics is relevant to any system
with spin, valley, or layer degrees of freedom. We consider two contexts for
excitonic quantum criticality: (1) a weakly interacting graphene bilayer, and
(2) a system of two narrow, half-filled Chern bands at zero external magnetic
field, with total Chern number $C_{\mathrm{tot}}=0$, which may soon be
realizable in moir\'{e} fractional quantum anomalous Hall systems. The latter
system hosts a time-reversed pair of composite Fermi liquid states, and the
condensation of excitons of the composite fermions leads to an exotic exciton
insulator* state with a charge neutral Fermi surface. Our work sheds new light
on the physics of inter-layer coherence transitions in 2D materials, and it
constitutes the first unambiguous example of quantum critical transport in a
clean non-Fermi liquid metal. | cond-mat |
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