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Short-time dynamics in active systems: the Vicsek model: We study the short-time dynamics (STD) of the Vicsek model with vector noise.
The study of STD has proved to be very useful in the determination of the
critical point, critical exponents, and spinodal points in equilibrium phase
transitions. Here we aim to test its applicability in active systems. We find
that, despite the essential non-equilibrium characteristics of the VM (absence
of detailed balance, activity), the STD presents qualitatively the same
phenomenology as in equilibrium systems. From the STD one can distinguish
whether the transition is continuous or discontinuous (which we have checked
also computing the Binder cumulant). When the transition is continuous, one can
determine the critical point and the critical exponents. | cond-mat |
Ostwald ripening of aqueous microbubble solutions: Bubble solutions are of growing interest because of various technological
applications in surface cleaning, water treatment, and agriculture. However,
their physicochemical properties such as the stability and interfacial charge
of bubbles are not fully understood yet. In this study, the kinetics of radii
in aqueous microbubble solutions are experimentally investigated, and the
results are discussed in the context of Ostwald ripening. The obtained
distributions of bubble radii scaled by mean radius and total number were found
to be time-independent during the observation period. Image analysis of radii
kinetics revealed that the average growth and shrinkage speed of each bubble is
governed by diffusion-limited Ostwald ripening, and the kinetic coefficient
calculated using the available physicochemical constants in literature
quantitatively agrees with the experimental data. Furthermore, the cube of mean
radius and mean volume exhibit a linear time evolution in agreement with the
Lifshitz-Slezov-Wagner (LSW) theory. The coefficients are slightly larger than
those predicted using the LSW theory, which can be qualitatively explained by
the effect of finite volume fraction. Finally, the slow down and pinning of
radius in the shrinkage dynamics of small microbubbles are discussed in detail. | cond-mat |
Impact of AC Magnetic Field on Decoherence of Quantum Dot based Single
Spin Qubit System: Quantum dot-based spin qubits are resilient towards charge noise and are
affected by magnetic noise only. However, environmental interaction leads to
decoherence in these qubit systems. The external control parameters are
directly related to the magnitude of decoherence. This in turn limits the range
of values of those parameters for which operations can be done with high
fidelity. In this work, using a model of quantum dot spin qubit system, we
investigate the impact of varying ac magnetic fields on suppression of
decoherence. We report an increment in the usable range of static magnetic
field value using our technique. | cond-mat |
Active and Passive Transport of Cargo in a Corrugated Channel: A Lattice
Model Study: Inside cells, cargos such as vesicles and organelles are transported by
molecular motors to their correct locations via active motion on cytoskeletal
tracks and passive, Brownian diffusion. During the transportation of cargos,
motor-cargo complexes (MCC) navigate the confining and crowded environment of
the cytoskeletal network and other macromolecules. Motivated by this, we study
a minimal two-state model of motor-driven cargo transport in confinement and
predict transport properties that can be tested in experiments. We assume that
the motion of the MCC is directly affected by the entropic barrier due to
confinement if it is in the passive, unbound state, but not in the active,
bound state where it moves with a constant bound velocity. We construct a
lattice model based on a Fokker Planck description of the two-state system,
study it using a kinetic Monte Carlo method and compare our numerical results
with analytical expressions for a mean field limit. We find that the effect of
confinement strongly depends on the bound velocity and the binding kinetics of
the MCC. Confinement effectively reduces the effective diffusivity and average
velocity, except when it results in an enhanced average binding rate and
thereby leads to a larger average velocity than when unconfined. | cond-mat |
Supersolidity from defect-condensation in the extended boson Hubbard
model: We study the ground state phase diagram of the hard-core extended boson
Hubbard model on the square lattice with both nearest- (nn) and
next-nearest-neighbor (nnn) hopping and repulsion, using Gutzwiller mean field
theory and quantum Monte Carlo simulations. We observe the formation of
supersolid states with checkerboard, striped, and quarter-filled crystal
structures, when the system is doped away from commensurate fillings. In the
striped supersolid phase, a strong anisotropy in the superfluid density is
obtained from the simulations; however, the transverse component remains
finite, indicating a true two-dimensional superflow. We find that upon doping,
the striped supersolid transitions directly into the supersolid with
quarter-filled crystal structure, via a first-order stripe melting transition. | cond-mat |
High pressure layered structure of carbon disulfide: Solid CS$_{2}$ is superficially similar to CO$_{2}$, with the same $Cmca$
molecular crystal structure at low pressures, which has suggested similar
phases also at high pressures. We carried out an extensive first principles
evolutionary search in order to identify the zero temperature lowest enthalpy
structures of CS$_{2}$ for increasing pressure up to 200\,GPa. Surprisingly,
the molecular $Cmca$ phase does not evolve into $\beta$-cristobalite as in
CO$_{2}$, but transforms instead into phases HP2 and HP1, both recently
described in high pressure SiS$_{2}$. HP1 in particular, with a wide stability
range, is a layered $P2_{1}/c$ structure characterized by pairs of edge-sharing
tetrahedra, and theoretically more robust than all other CS$_{2}$ phases
discussed so far. Its predicted Raman spectrum and pair correlation function
agree with experiment better than those of $\beta$-cristobalite, and further
differences are predicted between their respective IR spectra. The band gap of
HP1-CS$_{2}$ is calculated to close under pressure yielding an insulator-metal
transition near 50 GPa in agreement with experimental observations. However,
the metallic density of states remains modest above this pressure, suggesting a
different origin for the reported superconductivity. | cond-mat |
Double-dot charge transport in Si single electron/hole transistors: We studied transport through ultra-small Si quantum dot transistors
fabricated from silicon-on-insulator wafers. At high temperatures, 4K<T<100K,
the devices show single-electron or single-hole transport through the
lithographically defined dot. At T<4K, current through the devices is
characterized by multidot transport. From the analysis of the transport in
samples with double-dot characteristics, we conclude that extra dots are formed
inside the thermally grown gate oxide which surrounds the lithographically
defined dot. | cond-mat |
Collective phenomena in granular and atmospheric electrification: In clouds of suspended particles (grains, droplets, spheres, crystals, etc.),
collisions electrify the particles and the clouds, producing large electric
potential differences over large scales. This is seen most spectacularly in the
atmosphere as lighting in thunderstorms, thundersnow, dust storms, and volcanic
ash plumes where multi-million-volt potential differences over scales of
kilometers can be produced, but it is a general phenomenon in granular systems
as a whole. The electrification process is not well understood, especially for
electrification of insulating particles of the same material. To investigate
the relative importances of particle properties (material, size, etc.) and
collective phenomena (behaviors of systems at large scales not easily predicted
from local dynamics) in granular and atmospheric electrification, we used a
table-top experiment that mechanically shakes particles inside a cell where we
measure the macroscopic electric field between the electrically conducting end
plates. The measured electric fields are a result of capacitive coupling and
direct charge transfer between the particles and the plates. Using a diverse
range of mono-material particle sets (plastics, ceramic, glass, and metals), we
found that all our particle materials electrify and show similar dynamics with
long time-scale temporal variation and an electric field amplitude that depends
on the particle quantity in a complex way. These results suggest that while
particle properties do matter like previous investigations have shown,
macroscopic electrification of solids is relatively material agnostic and large
scale collective phenomena play a major role. | cond-mat |
Tunable spin and valley excitations of correlated insulators in
$Γ$-valley moiré bands: Moir\'e superlattices formed from transition metal dichalcogenides (TMDs)
have been shown to support a variety of quantum electronic phases that are
highly tunable using applied electromagnetic fields. While the valley character
of the low-energy states dramatically affects optoelectronic properties in the
constituent TMDs, this degree of freedom has yet to be fully explored in
moir\'e systems. Here, we establish twisted double bilayer WSe$_2$ as an
experimental platform to study electronic correlations within $\Gamma$-valley
moir\'e bands. Through a combination of local and global electronic
compressibility measurements, we identify charge-ordered phases at multiple
integer and fractional moir\'e band fillings $\nu$. By measuring the magnetic
field dependence of their energy gaps and the chemical potential upon doping,
we reveal spin-polarized ground states with novel spin polaron quasiparticle
excitations. In addition, an applied displacement field allows us to realize a
new mechanism of metal-insulator transition at $\nu = -1$ driven by tuning
between $\Gamma$- and $K$-valley moir\'e bands. Together, our results
demonstrate control over both the spin and valley character of the correlated
ground and excited states in this system. | cond-mat |
Spin-to-Orbital Angular Momentum Conversion in Semiconductor
Microcavities: We experimentally demonstrate a technique for the generation of optical beams
carrying orbital angular momentum using a planar semiconductor microcavity.
Despite being isotropic systems, the transverse electric - transverse magnetic
(TE-TM) polarization splitting featured by semiconductor microcavities allows
for the conversion of the circular polarization of an incoming laser beam into
the orbital angular momentum of the transmitted light field. The process
implies the formation of topological entities, a pair of optical half-vortices,
in the intracavity field. | cond-mat |
Magnetoelectric coupling in multiferroic CFO/BCTSn core shell nanofibers
elaborated by co-axial electrospinning method: Multiferroic CoFe2O4-Ba0.95Ca0.05Ti0.89Sn0.11O3 core-shell nanofibers
(CFO@BCTSn NFs) were synthesized by a sol-gel co-axial electrospinning
technique. The scanning electron microscope and transmission electron
microscope were used to check nanofibers' core-shell structure/configuration.
X-ray diffraction and a high-resolution transmission electron microscope were
used to confirm the spinel structure of CFO and the perovskite structure of
BCTSn. The magnetic character of the resultant CFO@BCTSn NFs was determined by
SQUID magnetometry. The piezoelectricity was verified using piezo-response
force microscopy, which revealed an entirely covered ferroelectric shell
outline, in accordance with SEM and TEM observations. The magnetoelectric (ME)
coefficient was measured as a function of the applied external DC magnetic
field. The maximum ME coefficient obtained for the CFO@BCTSn NFs was 346 mV
cm-1 Oe-1. The high magnetoelectric coupling suggests that CFO@BCTSn NFs could
be a promising candidate for magnetic field sensor and magnetoelectric device
applications. | cond-mat |
Bayesian Inference in the Scaling Analysis of Critical Phenomena: To determine the universality class of critical phenomena, we propose a
method of statistical inference in the scaling analysis of critical phenomena.
The method is based on Bayesian statistics, most specifically, the Gaussian
process regression. It assumes only the smoothness of a scaling function, and
it does not need a form. We demonstrate this method for the finite-size scaling
analysis of the Ising models on square and triangular lattices. Near the
critical point, the method is comparable in accuracy to the least-square
method. In addition, it works well for data to which we cannot apply the
least-square method with a polynomial of low degree. By comparing the data on
triangular lattices with the scaling function inferred from the data on square
lattices, we confirm the universality of the finite-size scaling function of
the two-dimensional Ising model. | cond-mat |
Ferromagnetic metallic Sr-rich Ln$_{1/2}$A$_{1/2}$CoO$_3$ cobaltites
with spontaneous spin rotation: The Pr$_{0.50}$Sr$_{0.5}$0CoO$_3$ perovskite exhibits unique
magnetostructural properties among the rest of ferromagnetic/metallic
Ln$_{0.50}$Sr$_{0.50}$CoO$_3$ compounds. The sudden orthorhombic-tetragonal
(Imma $\to$ I4/mcm) structural transition produces an unusual magnetic behavior
versus temperature and external magnetic fields. In particular, the symmetry
change is responsible for a spontaneous spin rotation in this metallic oxide.
We have studied half-doped Ln$_{0.50}$(Sr$_{1-x}$A$_x$)$_{0.50}$CoO$_3$
cobaltites varying the ionic radius rA of A-site cations (divalent cations and
lanthanides) in order to complete the T-rA phase diagram. The influence of the
structural distortion and the A-cations size for the occurrence of a
spontaneous spin reorientation in the metallic state has been investigated. As
the reorientation of the magnetization is driven by the temperature induced
collapse of the orthorhombic distortion, a careful investigation of the
structural symmetry is presented varying the structural distortion of the
Sr-rich half-doped cobaltites by means of both compositional and temperature
changes. The region in the phase diagram of these perovskites where the phase
of magnetic symmetry Fm'm'm replaces that of Im'm'a symmetry was determined in
this family of ferromagnetic/metallic cobaltites. In that region the
magnetization direction has rotated 45 degrees within the a-b plane with
respect to the second. | cond-mat |
Interplay of Hidden Orbital Order and Superconductivity in CeCoIn5: Visualizing atomic-orbital degrees of freedom is a frontier challenge in
scanned microscopy. Some types of orbital order are virtually imperceptible to
normal scattering techniques because they do not reduce the overall crystal
lattice symmetry. A good example is dxz/dyz ({\pi},{\pi}) orbital order in
tetragonal lattices. For enhanced detectability, here we consider the
quasiparticle scattering interference (QPI) signature of such ({\pi},{\pi})
orbital order in both normal and superconducting phases. The theory reveals
that sublattice-specific QPI signatures generated by the orbital order should
emerge strongly in the superconducting phase. Sublattice-resolved QPI
visualization in superconducting CeCoIn5 then reveals two orthogonal QPI
patterns at lattice-substitutional impurity atoms. We analyze the energy
dependence of these two orthogonal QPI patterns and find the intensity peaked
near E=0, as predicted when such ({\pi}) orbital order is inte,{\pi}rtwined
with d-wave superconductivity. Sublattice-resolved superconductive QPI
techniques thus represent a new approach for study of hidden orbital order. | cond-mat |
Pair Tunneling in Semiconductor Quantum Dots: We propose here a model for the pair tunneling states observed by Ashoori and
co-workers (Phys. Rev. Lett. {\bf 68}, 3088 (1992)) in GaAs quantum dots. We
show that while GaAs is a weakly-polar semiconductor, coupling to optical
phonons is sufficiently strong to mediate a negative-U pairing state. The
physical potential in which the two electrons are bound can be composed of a Si
impurity and a parabolic well that originates from the potential created by the
$\delta-$dopants in the backing layer of the dot. Such a pair state breaks up
at moderate magnetic field strengths ($\approx$ 2 T), as is seen
experimentally, and is unstable when the confining radius of the dot is smaller
than $\approx 400$\AA. | cond-mat |
Mpemba effect in molecular gases under nonlinear drag: We look into the Mpemba effect---the initially hotter sample cools
sooner---in a molecular gas with nonlinear viscous drag. Specifically, the gas
particles interact among them via elastic collisions and also with a background
fluid at equilibrium. Thus, within the framework of kinetic theory, our gas is
described by an Enskog--Fokker--Planck equation. The analysis is carried out in
the first Sonine approximation, in which the evolution of the temperature is
coupled to that of the excess kurtosis. This coupling leads to the emergence of
the Mpemba effect, which is observed in an early stage of the relaxation and
when the initial temperatures of the two samples are close enough. This allows
for the development of a simple theory, linearizing the temperature evolution
around a reference temperature---namely the initial temperature closer to the
asymptotic equilibrium value. The linear theory provides a semiquantitative
description of the effect, including expressions for the crossover time and the
maximum temperature difference. We also discuss the limitations of our
linearized theory. | cond-mat |
Critical Casimir torques and forces acting on needles in two spatial
dimensions: We investigate the universal orientation-dependent interactions between
non-spherical colloidal particles immersed in a critical solvent by studying
the instructive paradigm of a needle embedded in bounded two-dimensional Ising
models at bulk criticality. For a needle in an Ising strip the interaction on
mesoscopic scales depends on the width of the strip and the length, position,
and orientation of the needle. By lattice Monte Carlo simulations we evaluate
the free energy difference between needle configurations being parallel and
perpendicular to the strip. We concentrate on small but nonetheless mesoscopic
needle lengths for which analytic predictions are available for comparison. All
combinations of boundary conditions for the needles and boundaries are
considered which belong to either the "normal" or the "ordinary" surface
universality class, i.e., which induce local order or disorder, respectively.
We also derive exact results for needles of arbitrary mesoscopic length, in
particular for needles embedded in a half plane and oriented perpendicular to
the corresponding boundary as well as for needles embedded at the center line
of a symmetric strip with parallel orientation. | cond-mat |
Plastic Instabilities in Charged Granular Systems: Competition between
Elasticity and Electrostatics: Electrostatic theory preserves charges, but allows dipolar excitations.
Elasticity theory preserves dipoles, but allows quadrupolar (Eshelby like)
plastic events. Charged amorphous granular systems are interesting in their own
right; here we focus on their plastic instabilities and examine their
mechanical response to external strain and to external electric field, to
expose the competition between elasticity and electrostatics. In this paper a
generic model is offered, its mechanical instabilities are examined and a
theoretical analysis is presented. Plastic instabilities are discussed as
saddle-node bifurcations that can be fully understood in terms of eigenvalues
and eigenfunctions of the relevant Hessian matrix. This system exhibits moduli
that describe how electric polarization and stress are influenced by strain and
electric field. Theoretical expression for these moduli are offered and
compared to the measurements in numerical simulations. | cond-mat |
Interaction of chiral rafts in self-assembled colloidal membranes: Colloidal membranes are monolayer assemblies of rodlike particles that
capture the long-wavelength properties of lipid bilayer membranes on the
colloidal scale. Recent experiments on colloidal membranes formed by chiral
rodlike viruses showed that introducing a second species of virus with
different length and opposite chirality leads to the formation of rafts ---
micron-sized domains of one virus species floating in a background of the other
viruses [Sharma et al., Nature 513, 77 (2014)]. In this article we study the
interaction of such rafts using liquid crystal elasticity theory. By
numerically minimizing the director elastic free energy, we predict the tilt
angle profile for both a single raft and two rafts in a background membrane,
and the interaction between two rafts as a function of their separation. We
find that the chiral penetration depth in the background membrane sets the
scale for the range of the interaction. We compare our results with the
experimental data and find good agreement for the strength and range of the
interaction. Unlike the experiments, however, we do not observe a complete
collapse of the data when rescaled by the tilt angle at the raft edge. | cond-mat |
Absence of evidence of spin transport through amorphous
Y$_3$Fe$_5$O$_{12}$: Long-distance transport of spin information in insulators without long-range
magnetic order has been recently reported. Here, we perform a complete
characterization of amorphous Y$_3$Fe$_5$O$_{12}$ (a-YIG) films grown on top of
SiO$_2$. We confirm a clear amorphous structure and paramagnetic behavior of
our a-YIG films, with semiconducting behavior resistivity that strongly decays
with increasing temperature. The non-local transport measurements show a signal
which is not compatible with spin transport and can be attributed to the drop
of the a-YIG resistivity caused by Joule heating. Our results emphasize that
exploring spin transport in amorphous materials requires careful procedures in
order to exclude the charge contribution from the spin transport signals. | cond-mat |
Structure and oxidation kinetics of the Si(100)-SiO2 interface: We present first-principles calculations of the structural and electronic
properties of Si(001)-SiO2 interfaces. We first arrive at reasonable structures
for the c-Si/a-SiO2 interface via a Monte-Carlo simulated annealing applied to
an empirical interatomic potential, and then relax these structures using
first-principles calculations within the framework of density-functional
theory. We find a transition region at the interface, having a thickness on the
order of 20\AA, in which there is some oxygen deficiency and a corresponding
presence of sub-oxide Si species (mostly Si^+2 and Si^+3). Distributions of
bond lengths and bond angles, and the nature of the electronic states at the
interface, are investigated and discussed. The behavior of atomic oxygen in
a-SiO2 is also investigated. The peroxyl linkage configuration is found to be
lower in energy than interstitial or threefold configurations. Based on these
results, we suggest a possible mechanism for oxygen diffusion in a-SiO2 that
may be relevant to the oxidation process. | cond-mat |
Bright solitons in ultracold atoms: We review old and recent experimental and theoretical results on bright
solitons in Bose-Einstein condensates made of alkali-metal atoms and under
external optical confinement. First we deduce the three-dimensional
Gross-Pitaevskii equation (3D GPE) from the Dirac-Frenkel action of interacting
identical bosons within a time-dependent Hartree approximation. Then we discuss
the dimensional reduction of the GPE from 3D to 1D, deriving the 1D GPE and
also the 1D nonpolynomial Schr\"odinger equation (1D NPSE). Finally, we analyze
the bright solition solutions of both 1D GPE and 1D NPSE and compare these
theoretical predictions with the available experimental data. | cond-mat |
Heat release by controlled continuous-time Markov jump processes: We derive the equations governing the protocols minimizing the heat released
by a continuous-time Markov jump process on a one-dimensional countable state
space during a transition between assigned initial and final probability
distributions in a finite time horizon. In particular, we identify the
hypotheses on the transition rates under which the optimal control strategy and
the probability distribution of the Markov jump problem obey a system of
differential equations of Hamilton-Bellman-Jacobi-type. As the state-space mesh
tends to zero, these equations converge to those satisfied by the diffusion
process minimizing the heat released in the Langevin formulation of the same
problem. We also show that in full analogy with the continuum case, heat
minimization is equivalent to entropy production minimization. Thus, our
results may be interpreted as a refined version of the second law of
thermodynamics. | cond-mat |
Nanoparticles actively fragment armored droplets: Understanding the complexity of fragmentation processes is essential for
regulating intercellular communication in mechanistic biology and developing
novel bottom-up approaches in a large range of multiphase flow processes. In
this context, self-fragmentation proceeds without any external mechanical
energy input allowing one to create efficiently micro- and nanodroplets. Here
we examine self-fragmentation in emulsion nanodroplets stabilized by solid
particles with different surface features. Mesoscopic modelling and accelerated
dynamics simulations allow us to overcome the limitations of atomistic
simulations and offer detailed insight into the interplay between the evolution
of the droplet shape and the particle finite-sized effects at the interface. We
show that finite-sized nanoparticles play an active role in the necking
breakup, behaving like nano-scale razors, and affect strongly the thermodynamic
properties of the system. The role played by the particles during
self-fragmentation might be of relevance to multifunctional biomaterial design
and tuning of signaling pathways in mechanistic biology. | cond-mat |
Strongly coupled modes in a weakly driven micromechanical resonator: We demonstrate strong coupling between the flexural vibration modes of a
clamped-clamped micromechanical resonator vibrating at low amplitudes. This
coupling enables the direct measurement of the frequency response via
amplitude- and phase modulation schemes using the fundamental mode as a
mechanical detector. In the linear regime, a frequency shift of
$\mathrm{0.8\,Hz}$ is observed for a mode with a line width of
$\mathrm{5.8\,Hz}$ in vacuum. The measured response is well-described by the
analytical model based on the Euler-Bernoulli beam including tension.
Calculations predict an upper limit for the room-temperature Q-factor of
$\mathrm{4.5\times10^5}$ for our top-down fabricated micromechanical beam
resonators. | cond-mat |
Low-lying energy levels of a one-dimensional weakly interacting Bose gas
under zero boundary conditions: We diagonalize the second-quantized Hamiltonian of a one-dimensional Bose gas
with a nonpoint repulsive interatomic potential and zero boundary conditions.
At weak coupling the solutions for the ground-state energy $E_{0}$ and the
dispersion law $E(k)$ coincide with the Bogoliubov solutions for a periodic
system. In this case, the single-particle density matrix $F_{1}(x,x^{\prime})$
at $T=0$ is close to the solution for a periodic system and, at $T>0$, is
significantly different from it. We also obtain that the wave function $\langle
\hat{\psi}(x,t) \rangle$ of the effective condensate is close to a constant
$\sqrt{N_{0}/L}$ inside the system and vanishes on the boundaries (here,
$N_{0}$ is the number of atoms in the effective condensate, and $L$ is the size
of the system). We find the criterion of applicability of the method, according
to which the method works for a finite system at very low temperature and with
a weak coupling (a weak interaction or a large concentration). | cond-mat |
Touching is believing: interrogating organometal halide perovskite solar
cells at the nanoscale via scanning probe microscopy: Halide perovskite solar cells based on CH3NH3PbI3 and related materials have
emerged as the most exciting development in the next generation photovoltaic
technologies, yet the microscopic phenomena involving photo-carriers, ionic
defects, spontaneous polarization, and molecular vibration and rotation
interacting with numerous grains, grain boundaries, and interfaces are still
inadequately understood. In fact, there is still need for an effective method
to interrogate the local photovoltaic properties of halide perovskite solar
cells that can be directly traced to their microstructures on one hand and
linked to their device performance on the other hand. In this perspective, we
propose that scanning probe microscopy techniques have great potential to
realize such promises at the nanoscale, and highlight some of the recent
progresses and challenges along this line of investigation toward local probing
of photocurrent, work function, ionic activities, polarization switching, and
chemical degradation. We also emphasize the importance of multi-modality
imaging, in-operando scanning, big data analysis, and multidisciplinary
collaboration for further studies toward fully understanding of these complex
systems. | cond-mat |
Structural defects responsible for strain glassy transition in
Ni$_{50+x}$Ti$_{50-x}$: The strain glassy phase is produced by doping a small percentage of impurity
in a martensitic alloy. Its ground state is conceived to consist of martensitic
nano domains spatially separated from each other by a defect phase. The present
study, by probing the local structure around the Ni and Ti in martensitic and
strain glassy compositions of Ni$_{50+x}$Ti$_{50-x}$, for the first time,
identifies the defect phase that is responsible for inhibiting the long range
ordering of the elastic strain vector leading to the formation of the strain
glassy phase. | cond-mat |
Magnetic properties of bismuth-cobalt oxides doped by erbium: We synthesized bismuth - cobalt oxide doped by erbium with general formula
Bi3-xErxCoO3-y. Compound has structure of delta-form bismuth oxide. Magnetic
properties of the compound were measured by Faraday's method using quartz
scales in the temperature range of 80-500 K. The magnetic susceptibility and
effective magnetic moment were calculated. | cond-mat |
Active colloid with externally induced periodic bipolar motility and its
cooperative motion: Active matter physics has been developed with various types of self-propelled
particles, including those with polar and bipolar motility and beyond. However,
the bipolar motions experimentally realized so far have been either random
along the axis or periodic at intrinsic frequencies. Here we report another
kind of bipolar active particles, whose periodic bipolar self-propulsion is set
externally at a controllable frequency. We used Quincke rollers -- dielectric
particles suspended in a conducting liquid driven by an electric field -- under
an AC electric field instead of the usually used DC field. Reciprocating motion
of a single particle at the external frequency was observed experimentally and
characterized theoretically as stable periodic motion. Experimentally, we
observed not only the reciprocating motion but also non-trivial active Brownian
particle (ABP)-like persistent motion in a long time scale. This resulted in a
Lorentzian spectrum around zero frequency, which is not accounted for by a
simple extension of the conventional model of Quincke rollers to the AC field.
It was found that ABP-like motion can be reproduced by considering the
top-bottom asymmetry in the experimental system. Moreover, we found a
rotational diffusion coefficient much larger than the thermal one, as also
reported in previous experiments, which may have resulted from roughness of the
electrode surface. We also found self-organized formation of small clusters,
such as doublets and triplets, and characterized cooperative motion of
particles therein. The AC Quincke rollers reported here may serve as a model
experimental system of bipolar active matter, which appears to deserve further
investigations. | cond-mat |
Magneto-Stark and Zeeman effect as origin of second harmonic generation
of excitons in Cu$_2$O: We report on the experimental and theoretical investigation of
magnetic-field-induced second harmonic generation (SHG) and two-photon
absorption (TPA) of excited exciton states ($n \geq 3$) of the yellow series in
Cu$_2$O. In this centrosymmetric material, SHG can occur due to constructive
interplay of electric dipole and electric quadrupole/magnetic dipole
transitions for light propagating along the low-symmetry directions [111] or
[112]. By application of a magnetic field in Voigt configuration, SHG gets also
allowed for excitation along the [110]-axis and even the high-symmetry cubic
direction [001]. Combining a symmetry analysis and a microscopic theory, we
uncover the two key contributions to the magnetic-field-induced SHG: the Zeeman
effect and the magneto-Stark effect. We demonstrate systematic dependencies of
the SHG intensity on the linear polarization angles of the ingoing fundamental
laser and the outgoing SHG beam. In general, the resulting contour plots in
combination with a symmetry analysis allow one to determine uniquely the
character of involved transitions. Moreover, we can separate in magnetic field
the Zeeman and the magneto-Stark effect through appropriate choice of the
experimental geometry and polarization configuration. We present a microscopic
theory of the second harmonic generation of excitons in a centrosymmetric cubic
semiconductor taking into account the symmetry and the band structure of
cuprous oxide. Based on the developed microscopic theory we identify the main
contributions to the second-order nonlinear susceptibility of $S$-, $P$- and
$D$-excitons. We analyze the redistribution of SHG intensities between the
excitonic states both in the absence and presence of the magnetic field and
show good agreement with the experimental data. With increasing exciton
principal quantum number the magneto-Stark effect overpowers the influence of
the Zeeman effect. | cond-mat |
Bright solitary waves of atomic Bose-Einstein condensates under rotation: We analyse the rotation of bright solitary waves formed of atomic
Bose-Einstein condensates with attractive atomic interactions. By employing a
variational technique and assuming an irrotational quadrupolar flow field, we
map out the variational solutions in the rotating frame. In particular, we show
that rotation has a considerable stabilising effect on the system,
significantly raising the critical threshold for collapse of the bright
solitary waves. | cond-mat |
Non-perturbative $J_{pd}$ model and ferromagnetism in dilute magnets: We calculate magnetic couplings in the $J_{pd}$ model for dilute magnets, in
order both to identify the relevant parameters which control ferromagnetism and
also to bridge the gap between first principle calculations and model
approaches. The magnetic exchange interactions are calculated
non-perturbatively and disorder in the configuration of impurities is treated
exacly, allowing us to test the validity of effective medium theories.
Results differ qualitatively from those of weak coupling. In contrast to mean
field theory, increasing $J_{pd}$ may not favor high Curie temperatures: $T_C$
scales primarily with the bandwidth. High temperature ferromagnetism at small
dilutions is associated with resonant structure in the p-band. Comparison to
diluted magnetic semiconductors indicate that Ga(Mn)As has such a resonant
structure and thus this material is already close to optimality. | cond-mat |
Topological states in normal and superconducting $p$-wave chains: We study a two-band model of fermions in a 1d chain with an antisymmetric
hybridization that breaks inversion symmetry. We find that for certain values
of its parameters, the $sp$-chain maps formally into a $p$-wave superconducting
chain, the archetypical 1d system exhibiting Majorana fermions. The
eigenspectra, including the existence of zero energy modes in the topological
phase, agree for both models. The end states too share several similarities in
both models, such as the behavior of the localization length, the non-trivial
topological index and robustness to disorder. However, we show by mapping the
$s$- and $p$- fermions to two copies of Majoranas, that the excitations in the
ends of a finite $sp$ chain are indeed conventional fermions though endowed
with protected topological properties. Our results are obtained by a scattering
approach in a semi-infinite chain with an edge defect treated within the
$T$-matrix approximation. We augment the analytical results with exact
numerical diagonalization that allow us to extend our results to arbitrary
parameters and also to disordered systems. | cond-mat |
Suppressed compressibility of quantum Hall effect edge states in
epitaxial graphene on SiC: We determine conditions for the formation of compressible stripes near the
quantum Hall effect (QHE) edges of top-gated epitaxial graphene on
Si-terminated SiC (G/SiC) and compare those to graphene exfoliated onto
insulating substrate in the field-effect-transistor (GraFET) geometry.
For G/SiC, a large density of localised surface states on SiC just underneath
graphene layer and charge transfer between them lead both to doping of graphene
and to screening of potential profile near its edge. This suppresses formation
of compressible stripes near QHE edges in graphene, making them much narrower
than the corresponding compressible stripes in GraFETs. | cond-mat |
The many-electron ground-state determines uniquely the potential in
Spin-Density-Functional Theory for non-collinear magnetism: Since Spin Density Functional Theory was first proposed, but also recently,
examples were constructed to show that a spin-potential may share its ground
state with other spin-potentials. In fact, for collinear magnetic fields and
systems with fixed magnetization, the mapping between potentials and ground
states is invertible, provided the magnetization is not saturated and that
spin-potentials are determined within a spin-constant. We complete the proof
that the mapping is invertible also for non-collinear magnetic fields and
systems with more than one electron. We then discuss the non-collinear exchange
and correlation energy functional and suggest improvements. | cond-mat |
Substrate-Independent Catalyst-Free Synthesis of High-Purity Bi2Se3
Nanostructures: We describe a catalyst-free vapor-solid synthesis of bismuth selenide
(Bi2Se3) nanostructures at ambient pressure with hydrogen as a carrier gas. The
nanostructures were synthesized on glass, silicon and mica substrates and the
method yields a variety of nanostructures: nanowires, nanoribbons,
nanoplatelets and nanoflakes. The materials analysis shows high chemical purity
in all cases, without sacrificing the crystalline structure of Bi2Se3.
Low-temperature measurements of the nanostructures indicate contributions from
the surface states with a tunable carrier density. Samples synthesized on
flexible mica substrates show no significant change in resistance upon bending,
indicating robustness of as-grown Bi2Se3 nanostructures and their suitability
for device applications. | cond-mat |
Electron-phonon coupling and superconductivity in LiB$_{1+x}$C$_{1-x}$: By means of the first-principles density-functional theory calculation and
Wannier interpolation, electron-phonon coupling and superconductivity are
systematically explored for boron-doped LiBC (i.e. LiB$_{1+x}$C$_{1-x}$), with
$x$ between 0.1 and 0.9. Hole doping introduced by boron atoms is treated
through virtual-crystal approximation. For the investigated doping
concentrations, our calculations show the optimal doping concentration
corresponds to 0.8. By solving the anisotropic Eliashberg equations, we find
that LiB$_{1.8}$C$_{0.2}$ is a two-gap superconductor, whose superconducting
transition temperature, T$_c$, may exceed the experimentally observed value of
MgB$_2$. Similar to MgB$_2$, the two-dimensional bond-stretching $E_{2g}$
phonon modes along $\Gamma$-$A$ line have the largest contribution to
electron-phonon coupling. More importantly, we find that the first two acoustic
phonon modes $B_1$ and $A_1$ around the midpoint of $K$-$\Gamma$ line play a
vital role for the rise of T$_c$ in LiB$_{1.8}$C$_{0.2}$. The origin of strong
couplings in $B_1$ and $A_1$ modes can be attributed to enhanced
electron-phonon coupling matrix elements and softened phonons. It is revealed
that all these phonon modes couple strongly with $\sigma$-bonding electronic
states. | cond-mat |
Microwave properties of superconducting $MgB_{2}$: Measurements of the $10GHz$ microwave surface resistance of dense $MgB_{2}$
wire and pellet are reported. Significant improvements are observed in the wire
with reduction of porosity. The data lie substantially above the theoretical
estimates for a pure BCS s-wave superconductor . However the $R_{s}(20K)$ of
the wire is an order of magnitude lower than that of polycrystal
$YBa_{2}Cu_{3}O_{6.95}$ and matches with single crystal
$YBa_{2}Cu_{3}O_{6.95}.$The results show promise for the use of $MgB_{2}$ in
microwave applications. | cond-mat |
Diagrammatic Monte Carlo study of the Fröhlich polaron dispersion in
2D and 3D: We present results for the solution of the large polaron Fr\"ohlich
Hamiltonian in 3-dimensions (3D) and 2-dimensions (2D) obtained via the
Diagrammatic Monte Carlo (DMC) method. Our implementation is based on the
approach by Mishchenko [A.S. Mishchenko et al., Phys. Rev. B 62, 6317 (2000)].
Polaron ground state energies and effective polaron masses are successfully
benchmarked with data obtained using Feynman's path integral formalism. By
comparing 3D and 2D data, we verify the analytically exact scaling relations
for energies and effective masses from 3D$\to$2D, which provides a stringent
test for the quality of DMC predictions. The accuracy of our results is further
proven by providing values for the exactly known coefficients in weak- and
strong coupling expansions. Moreover, we compute polaron dispersion curves
which are validated with analytically known lower and upper limits in the small
coupling regime and verify the first order expansion results for larger
couplings, thus disproving previous critiques on the apparent incompatibility
of DMC with analytical results and furnishing useful reference for a wide range
of coupling strengths. | cond-mat |
Unexpected upper critical dimension for spin glass models in a field
predicted by the loop expansion around the Bethe solution at zero temperature: The spin-glass transition in a field in finite dimension is analyzed directly
at zero temperature using a perturbative loop expansion around the Bethe
lattice solution. The loop expansion is generated by the $M$-layer construction
whose first diagrams are evaluated numerically and analytically. The
generalized Ginzburg criterion reveals that the upper critical dimension below
which mean-field theory fails is $D_U \le 8$, at variance with the classical
result $D_U = 6$ yielded by finite-temperature replica field theory. Our
expansion around the Bethe lattice has two crucial differences with respect to
the classical one. The finite connectivity $z$ of the lattice is directly
included from the beginning in the Bethe lattice, while in the classical
computation the finite connectivity is obtained through an expansion in $1/z$.
Moreover, if one is interested in the zero temperature ($T = 0$) transition,
one can directly expand around the $T = 0$ Bethe transition. The expansion
directly at $T = 0$ is not possible in the classical framework because the
fully connected spin glass does not have a transition at $T = 0$, being in the
broken phase for any value of the external field. | cond-mat |
Epitaxy of Advanced Nanowire Quantum Devices: Semiconductor nanowires provide an ideal platform for various low-dimensional
quantum devices. In particular, topological phases of matter hosting
non-Abelian quasi-particles can emerge when a semiconductor nanowire with
strong spin-orbit coupling is brought in contact with a superconductor. To
fully exploit the potential of non-Abelian anyons for topological quantum
computing, they need to be exchanged in a well-controlled braiding operation.
Essential hardware for braiding is a network of single-crystalline nanowires
coupled to superconducting islands. Here, we demonstrate a technique for
generic bottom-up synthesis of complex quantum devices with a special focus on
nanowire networks having a predefined number of superconducting islands.
Structural analysis confirms the high crystalline quality of the nanowire
junctions, as well as an epitaxial superconductor-semiconductor interface.
Quantum transport measurements of nanowire "hashtags" reveal Aharonov-Bohm and
weak-antilocalization effects, indicating a phase coherent system with strong
spin-orbit coupling. In addition, a proximity-induced hard superconducting gap
is demonstrated in these hybrid superconductor-semiconductor nanowires,
highlighting the successful materials development necessary for a first
braiding experiment. Our approach opens new avenues for the realization of
epitaxial 3-dimensional quantum device architectures. | cond-mat |
Diffusion of Pt dimers on Pt(111): We report the results of a density-functional study of the diffusion of Pt
dimers on the (111) surface of Pt. The calculated activation energy of 0.37 eV
is in {\em exact} agreement with the recent experiment of Kyuno {\em et al.}
\protect{[}Surf. Sci. {\bf 397}, 191 (1998)\protect{]}. Our calculations
establish that the dimers are mobile at temperatures of interest for adatom
diffusion, and thus contribute to mass transport. They also indicate that the
diffusion path for dimers consists of a sequence of one-atom and (concerted)
two-atom jumps. | cond-mat |
Incommensurability and edge states in the one-dimensional S=1
bilinear-biquadratic model: Commensurate-incommensurate change on the one-dimensional S=1
bilinear-biquadratic model (${\cal H}(\alpha)=\sum_i \{{\bf S}_i\cdot {\bf
S}_{i+1} +\alpha ({\bf S}_i\cdot{\bf S}_{i+1})^2\}$) is examined. The gapped
Haldane phase has two subphases (the commensurate Haldane subphase and the
incommensurate Haldane subphase) and the commensurate-incommensurate change
point (the Affleck-Kennedy-Lieb-Tasaki point, $\alpha=1/3$). There have been
two different analytical predictions about the static structure factor in the
neighborhood of this point. By using the S{\o}rensen-Affleck prescription,
these static structure factors are related to the Green functions, and also to
the energy gap behaviors. Numerical calculations support one of the
predictions. Accordingly, the commensurate-incommensurate change is recognized
as a motion of a pair of poles in the complex plane. | cond-mat |
Experimental observation of a large ac-spin Hall effect: In spinelectronics the spin degree of freedom is used to transmit and store
information. Ideally this occurs without net charge currents in order to avoid
energy dissipation due to Joule heating. To this end the ability to create pure
spin currents i.e.without net charge transfer is essential. Spin pumping is the
most popular approach to generate pure spin currents in metals, semiconductors,
graphene, and even organic materials. When the magnetization vector in a
ferromagnet (FM) - normal metal (NM) junction is excited the spin pumping
effect leads to the injection of pure spin currents in the normal metal. The
polarization of this spin current is time dependent and contains a very small
dc component. The dc-component of the injected spin current has been intensely
studied in recent years and has given rise to controversial discussions
concerning the magnitude the spin Hall angle which is a material dependent
measure of the efficiency of spin to charge conversion . However in contrast to
the rather well understood dc component the two orders of magnitude larger ac
component has escaped experimental detection so far. Here we show that the
large ac component of the spin currents can be detected very efficiently using
the inverse spin Hall effect (ISHE). The observed ac-ISHE voltages are one
order of magnitude larger than the conventional dc-ISHE measured on the same
device. The spectral shape, angular dependence, power scaling behavior and
absolute magnitude of the signals are in line with spin pumping and ISHE
effects. Our results demonstrate that FM-NM junctions are very efficient
sources of pure spin currents in the GHz frequency range and we believe that
our result will stimulate the emerging field of ac spintronics. | cond-mat |
Correlation of the angular dependence of spin-transfer torque and giant
magnetoresistance in the limit of diffusive transport in spin valves: Angular variation of giant magnetoresistance and spin-transfer torque in
metallic spin-valve heterostructures is analyzed theoretically in the limit of
diffusive transport. It is shown that the spin-transfer torque in asymmetric
spin valves can vanish in non-collinear magnetic configurations, and such a
non-standard behavior of the torque is generally associated with a
non-monotonic angular dependence of the giant magnetoresistance, with a global
minimum at a non-collinear magnetic configuration. | cond-mat |
A new approach to the inverse problem for current mapping in thin-film
superconductors: A novel mathematical approach has been developed to complete the inversion of
the Biot-Savart law in one- and two-dimensional cases from measurements of the
perpendicular component of the magnetic field using the well-developed
Magneto-Optical Imaging technique. Our approach, especially in the 2D case, is
provided in great detail to allow a straightforward implementation as opposed
to those found in the literature. Our new approach also refines our previous
results for the 1D case [Johansen et al., Phys. Rev. B 54, 16264 (1996)], and
streamlines the method developed by Jooss et al. [Physica C 299, 215 (1998)]
deemed as the most accurate if compared to that of Roth et al. [J. Appl. Phys.
65, 361 (1989)]. We also verify and streamline the iterative technique, which
was developed following Laviano et al. [Supercond. Sci. Technol. 16, 71 (2002)]
to account for in-plane magnetic fields caused by the bending of the applied
magnetic field due to the demagnetising effect. After testing on
magneto-optical images of a high quality YBa2Cu3O7 superconducting thin film,
we show that the procedure employed is effective. | cond-mat |
Symmetry breaking orbital anisotropy on detwinned Ba(Fe1-xCox)2As2 above
the spin density wave transition: Nematicity, defined as broken rotational symmetry, has recently been observed
in competing phases proximate to the superconducting phase in the cuprate high
temperature superconductors. Similarly, the new iron-based high temperature
superconductors exhibit a tetragonal to orthorhombic structural transition
(i.e. a broken C4 symmetry) that either precedes or is coincident with a
collinear spin density wave (SDW) transition in undoped parent compounds, and
superconductivity arises when both transitions are suppressed via doping.
Evidence for strong in-plane anisotropy in the SDW state in this family of
compounds has been reported by neutron scattering, scanning tunneling
microscopy, and transport measurements. Here we present an angle resolved
photoemission spectroscopy study of detwinned single crystals of a
representative family of electron-doped iron-arsenide superconductors,
Ba(Fe1-xCox)2As2 in the underdoped region. The crystals were detwinned via
application of in-plane uniaxial stress, enabling measurements of single domain
electronic structure in the orthorhombic state. At low temperatures, our
results clearly demonstrate an in-plane electronic anisotropy characterized by
a large energy splitting of two orthogonal bands with dominant dxz and dyz
character, which is consistent with anisotropy observed by other probes. For
compositions x>0, for which the structural transition (TS) precedes the
magnetic transition (TSDW), an anisotropic splitting is observed to develop
above TSDW, indicating that it is specifically associated with TS. For
unstressed crystals, the band splitting is observed close to TS, whereas for
stressed crystals the splitting is observed to considerably higher
temperatures, revealing the presence of a surprisingly large in-plane nematic
susceptibility in the electronic structure. | cond-mat |
Spatial Confinement Affects the Heterogeneity and Interactions Between
Shoaling Fish: Living objects are able to consume chemical energy and process information
independently from others. However, living objects can coordinate to form
ordered groups such as schools of fish. This work considers these complex
groups as living materials and presents imaging-based experiments of laboratory
schools of fish to understand how this non-equilibrium activity affects the
mechanical properties of a group. We use spatial confinement to control the
motion and structure of fish within quasi-2D shoals of fish. Using image
analysis techniques, we make quantitative observations of the structures, their
spatial heterogeneity, and their temporal fluctuations. Furthermore, we utilize
Monte Carlo simulations to replicate the experimentally observed area
distribution patterns which provide insight into the effective interactions
between fish and confirm the presence of a confinement-based behavioral
preference transition. In addition, unlike in short-range interacting systems,
here structural heterogeneity and dynamic activities are positively correlated
as a result of complex interplay between spatial arrangement and behavioral
dynamics in fish collectives. | cond-mat |
Paraelectric and ferroelectric order in two-state dipolar fluids: Monte Carlo simulations are used to examine cooperative creation of polar
state in fluids of two-state particles with nonzero dipole in the excited
state. With lowering temperature such systems undergo a second order transition
from nonpolar to polar, paraelectric phase. The transition is accompanied by a
dielectric anomaly of polarization susceptibility increasing by three orders of
magnitude. The paraelectric phase is then followed by formation of a nematic
ferroelectric which further freezes into an fcc ferroelectric crystal by a
first order transition. A mean-field model of phase transitions is discussed. | cond-mat |
^{75}As NMR study of the growth of paramagnetic-metal domains due to
electron doping near the superconducting phase in LaFeAsO_{1-x}F_{x}: We studied the electric and magnetic behavior near the phase boundary between
antiferromagnetic (AF) and superconducting (SC) phases for a prototype of
high-T_c pnictides LaFeAsO_{1-x}F_{x} by using nuclear magnetic resonance, and
found that paramagnetic-metal (PM) domains segregate from AF domains. PM
domains grow in size with increasing electron doping level and are accompanied
by the onset of superconductivity, and thus application of pressure or
increasing the doping level causes superconductivity. The existence of PM
domains cannot be explained by the existing paradigm that focuses only on the
relationship between superconductivity and antiferromagnetism. Based on orbital
fluctuation theory, the existence of PM domains is evidence of the
ferroquadrupole state. | cond-mat |
Collective modes in the anisotropic unitary Fermi gas and the inclusion
of a backflow term: We study the collective modes of the confined unitary Fermi gas under
anisotropic harmonic confinement as a function of the number of atoms. We use
the equations of extended superfluid hydrodynamics, which take into account a
dispersive von Weizsacker-like term in the equaton of state. We also discuss
the inclusion of a backflow term in the extended superfluid Lagrangian and the
effects of this anomalous term on sound waves and Beliaev damping of phonons. | cond-mat |
Fermionization and Hubbard Models: We introduce a transformation which allows the fermionization of operators of
any one-dimensional spin-chain. This fermionization procedure is independent of
any eventual integrable structure and is compatible with it. We illustrate this
method on various integrable and non-integrable chains, and deduce some general
results. In particular, we fermionize XXC spin-chains and study their
symmetries. Fermionic realizations of certain Lie algebras and superalgebras
appear naturally as symmetries of some models. We also fermionize recently
obtained Hubbard models, and obtain for the first time multispecies analogues
of the Hubbard model, in their fermionic form. We comment on the conflict
between symmetry enhancement and integrability of these models. Finally, the
fermionic versions of the non integrable spin-1 and spin-3/2 Heisenberg chains
are obtained. | cond-mat |
Role of oxygen-oxygen hopping in the three-band copper-oxide model:
quasiparticle weight, metal insulator and magnetic phase boundaries, gap
values and optical conductivity: We investigate the effect of oxygen-oxygen hopping on the three-band
copper-oxide model relevant to high-$T_c$ cuprates, finding that the physics is
changed only slightly as the oxygen-oxygen hopping is varied. The location of
the metal-insulator phase boundary in the plane of interaction strength and
charge transfer energy shifts by $\sim 0.5$eV or less along the charge transfer
axis, the quasiparticle weight has approximately the same magnitude and doping
dependence and the qualitative characteristics of the electron-doped and
hole-doped sides of the phase diagram do not change. The results confirm the
identification of La$_2$CuO$_4$ as a material with intermediate correlation
strength. However, the magnetic phase boundary as well as higher-energy
features of the optical spectrum are found to depend on the magnitude of the
oxygen-oxygen hopping. We compare our results to previously published one-band
and three-band model calculations. | cond-mat |
Machine Learning Inter-Atomic Potentials Generation Driven by Active
Learning: A Case Study for Amorphous and Liquid Hafnium dioxide: We propose a novel active learning scheme for automatically sampling a
minimum number of uncorrelated configurations for fitting the Gaussian
Approximation Potential (GAP). Our active learning scheme consists of an
unsupervised machine learning (ML) scheme coupled to Bayesian optimization
technique that evaluates the GAP model. We apply this scheme to a Hafnium
dioxide (HfO2) dataset generated from a melt-quench ab initio molecular
dynamics (AIMD) protocol. Our results show that the active learning scheme,
with no prior knowledge of the dataset is able to extract a configuration that
reaches the required energy fit tolerance. Further, molecular dynamics (MD)
simulations performed using this active learned GAP model on 6144-atom systems
of amorphous and liquid state elucidate the structural properties of HfO2 with
near ab initio precision and quench rates (i.e. 1.0 K/ps) not accessible via
AIMD. The melt and amorphous x-ray structural factors generated from our
simulation are in good agreement with experiment. Additionally, the calculated
diffusion constants are in good agreement with previous ab initio studies. | cond-mat |
First-principles calculation of the thermoelectric figure of merit for
[2,2]paracyclophane-based single-molecule junctions: Here we present a theoretical study of the thermoelectric transport through
{[}2,2{]}para\-cyclo\-phane-based single-molecule junctions. Combining
electronic and vibrational structures, obtained from density functional theory
(DFT), with nonequilibrium Green's function techniques, allows us to treat both
electronic and phononic transport properties at a first-principles level. For
the electronic part, we include an approximate self-energy correction, based on
the DFT+$\Sigma$ approach. This enables us to make a reliable prediction of all
linear response transport coefficients entering the thermoelectric figure of
merit $ZT$. Paracyclophane derivatives offer a great flexibility in tuning
their chemical properties by attaching different functional groups. We show
that, for the specific molecule, the functional groups mainly influence the
thermopower, allowing to tune its sign and absolute value. We predict that the
functionalization of the bare paracyclophane leads to a largely enhanced
electronic contribution $Z_{\mathrm{el}}T$ to the figure of merit.
Nevertheless, the high phononic contribution to the thermal conductance
strongly suppresses $ZT$. Our work demonstrates the importance to include the
phonon thermal conductance for any realistic estimate of the $ZT$ for
off-resonant molecular transport junctions. In addition, it shows the
possibility of a chemical tuning of the thermoelectric properties for a series
of available molecules, leading to equally performing hole- and
electron-conducting junctions based on the same molecular framework. | cond-mat |
Origin and magnitude of 'designer' spin-orbit interaction in graphene on
semiconducting transition metal dichalcogenides: We use a combination of experimental techniques to demonstrate a general
occurrence of spin-orbit interaction (SOI) in graphene on transition metal
dichalcogenide (TMD) substrates. Our measurements indicate that SOI is
ultra-strong and extremely robust, despite it being merely
interfacially-induced, with neither graphene nor the TMD substrates changing
their structure. This is found to be the case irrespective of the TMD material
used, of the transport regime, of the carrier type in the graphene band, and of
the thickness of the graphene multilayer. Specifically, we perform weak
antilocalization measurements as the simplest and most general diagnostic of
SOI, and show that the spin relaxation time is very short in all cases
regardless of the elastic scattering time. Such a short spin-relaxation time
strongly suggests that the SOI originates from a modification of graphene band
structure. We confirmed this expectation by measuring a gate-dependent beating,
and a corresponding frequency splitting, in the low-field Shubnikov-de Haas
magneto-resistance oscillations in high quality bilayer graphene on WSe$_2$.
These measurements provide an unambiguous diagnostic of a SOI-induced splitting
in the electronic band structure, and their analysis allows us to determine the
SOI coupling constants for the Rashba term and the so-called spin-valley
coupling term, i.e., the terms that were recently predicted theoretically for
interface-induced SOI in graphene. The magnitude of the SOI splitting is found
to be on the order of 10 meV, more than 100 times greater than the SOI
intrinsic to graphene. Both the band character of the interfacially induced
SOI, as well as its robustness and large magnitude make graphene-on-TMD a
promising system to realize and explore a variety of spin-dependent transport
phenomena, such as, in particular, spin-Hall and valley-Hall topological
insulating states. | cond-mat |
Raman scattering of graphene based systems in high magnetic fields: We review the different results obtained in the last decade in the field of
Raman scattering of graphene based systems, with an applied magnetic field.
Electronic properties of graphene based systems with an applied magnetic field
will first be described. The phonon response in magnetic field, the
magneto-phono resonance, will then be introduced and described in different
systems, including graphene, multilayer graphene and bulk graphite. Electronic
Raman scattering is then be discussed in the context of Landau level
spectroscopy, of electron phonon interaction and of electron-electron
interaction. | cond-mat |
Structural and optical properties of high quality zinc-blende/wurtzite
GaAs hetero-nanowires: The structural and optical properties of 3 different kinds of GaAs nanowires
with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are
presented. A variety of shorter and longer segments of zinc-blende or wurtzite
crystal phases are observed by transmission electron microscopy in the
nanowires. Sharp photoluminescence lines are observed with emission energies
tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is
increased. The downward shift of the emission peaks can be understood by
carrier confinement at the interfaces, in quantum wells and in random short
period superlattices existent in these nanowires, assuming a staggered
band-offset between wurtzite and zinc-blende GaAs. The latter is confirmed also
by time resolved measurements. The extremely local nature of these optical
transitions is evidenced also by cathodoluminescence measurements. Raman
spectroscopy on single wires shows different strain conditions, depending on
the wurtzite content which affects also the band alignments. Finally, the
occurrence of the two crystallographic phases is discussed in thermodynamic
terms. | cond-mat |
Stationary distributions of propelled particles as a system with
quenched disorder: This article is the exploration of the viewpoint within which propelled
particles in a steady-state are regarded as a system with quenched disorder.
The analogy is exact when the rate of the drift orientation vanishes and the
linear potential, representing the drift, becomes part of an external
potential, resulting in the effective potential $u_{eff}$. The stationary
distribution is then calculated as a disorder-averaged quantity by considering
all contributing drift orientations. To extend this viewpoint to the case when
a drift orientation evolves in time, we reformulate the relevant Fokker-Planck
equation as a self-consistent relation. One interesting aspect of this
formulation is that it is represented in terms of the Boltzmann factor
$e^{-\beta u_{eff}}$. In the case of a run-and-tumble model, the formulation
reveals an effective interaction between particles. | cond-mat |
A Unified Theory of Consequences of Spontaneous Emission in a $Λ$
System: In a $\Lambda$ system with two nearly degenerate ground states and one
excited state in an atom or quantum dot, spontaneous radiative decay can lead
to a range of phenomena, including electron-photon entanglement, spontaneously
generated coherence, and two-pathway decay. We show that a treatment of the
radiative decay as a quantum evolution of a single physical system composed of
a three-level electron subsystem and photons leads to a range of consequences
depending on the electron-photon interaction and the measurement. Different
treatments of the emitted photon channel the electron-photon system into a
variety of final states. The theory is not restricted to the three-level
system. | cond-mat |
On superstatistical multiplicative-noise processes: In this manuscript we analyse the long-term probability density function of
non-stationary dynamical processes which are enclosed inward the Feller class
of processes with time varying exponents for multiplicative noise. The update
in the value of the exponent occurs in the same conditions presented by Beck
and Cohen for superstatistics. Moreover, we are able to provide a dynamical
scenario for the emergence of a generalisation of the Weibull distribution
previously introduced. | cond-mat |
Infrared phonon spectrum of the tetragonal helimagnet
Ba$_2$CuGe$_2$O$_7$: The lattice dynamics of Ba$_2$CuGe$_2$O$_7$, a compound which develops
Dzyaloshinsky-Moriya (DM) helical magnetism below $T_N$ = 3.2 K, has been
studied by measuring the infrared reflectivity of a single crystal with the
radiation polarized both in the $ab$ plane and along the $c$ axis of its
tetragonal cell, from 7 K to 300 K. In this compound, where the unit cell has
no inversion symmetry, fourteen $E$ phonon modes of the $ab$ plane, out of the
eighteen predicted, and all the ten $B_2$ modes of the $c$ axis, have been
observed. They have been assigned to the atomic motions by a comparison with
shell-model calculations, which provided vibrational frequencies in good
agreement with the experiment, while most calculated intensities turned to be
much lower than the experimental values. This discrepancy has been tentatively
explained by assuming strong electron-phonon interactions, a hypothesis
supported by the failure of the $f$- sum rule if restricted to the phonon
region. Indeed, we observe a remarkable increase in the oscillator strengths at
$T$'s low but higher than $T_N$, which suggests that the dielectric constant of
Ba$_2$CuGe$_2$O$_7$ may increase at those temperatures. | cond-mat |
Monte Carlo study of the magnetic properties of the 3D Hubbard model: We investigate numerically the magnetic properties of the 3D Isotropic and
Anisotropic Hubbard model at half-filling. The behavior of the transition
temperature as a function of the anisotropic hopping parameter is qualitatively
described. In the Isotropic model we measure the scaling properties of the
susceptibility finding agreement with the magnetic critical exponents of the 3D
Heisenberg model. We also describe several particularities concerning the
implementation of our simulation in a cluster of personal computers. | cond-mat |
Nitrogen incorporated Zinc oxide thin film for efficient ethanol
detection: Zinc oxide which is a n-type semiconducting metal oxide (SMO) has been a
promising material for detecting ethanol vapor. However, pure ZnO based ethanol
sensors often suffer from high working temperature, cross sensitivity towards
methanol and poor stability against humidity. Doping ZnO with various metal
ions has been widely explored as a proficient approach to improve its ethanol
sensing properties, while anionic dopants have been rarely considered. Here in
we demonstrate the effect of nitrogen doping on the ethanol sensing
characteristics of ZnO thin films. Nitrogen doped ZnO (N-ZnO) thin films have
been synthesized following sol-gel technique with urea as nitrogen precursor.
Ethanol sensing characteristics of the N-ZnO thin film has been com-pared with
pure ZnO sensor over a wide range of temperature and relative humidity
conditions. The N-ZnO sensor exhibits significantly large ethanol sensing
response at a lower operating temperature (~99 % at 225 {\deg}C vs ~81 % at 250
{\deg}C for ZnO), faster response time (12 s vs 33 s for ZnO), long term
stability, improved resilience against humidity and selectivity towards ethanol
over methanol and acetone. The experimental observations have been supplemented
by estimating the adsorption energies of ethanol on ZnO and N-ZnO surface using
density functional theory (DFT) calculations. We discuss that the microscopic
origin of improved ethanol sensing of N-ZnO is related to the facile adsorption
of ethanol molecules on the oxide surface which is promoted by modification of
electronic properties of ZnO by the nitrogen dopant atoms. | cond-mat |
Elemental substitution tuned magneto elastoviscous behavior of nanoscale
ferrite MFe2O4 M = Mn, Fe, Co, Ni based complex fluids: The present article reports the governing influence of substituting the M2
site in nanoscale MFe2O4 spinel ferrites by different magnetic metals
Fe,Mn,Co,Ni on magnetorheological and magneto elastoviscous behaviors of the
corresponding magnetorheological fluids MRFs. Different doped MFe2O4
nanoparticles have been synthesized using the polyol assisted hydrothermal
method. Detailed steady and oscillatory shear rheology have been performed on
the MRFs to determine the magneto-viscoelastic responses. The MRFs exhibit
shear thinning behavior and augmented yield characteristics under influence of
magnetic field. The steady state magnetoviscous behaviors are scaled against
the governing Mason number and self similar response from all the MRFs have
been noted. The MRFs conform to an extended Bingham plastic model under field
effect. Transient magnetoviscous responses show distinct hysteresis behaviors
when the MRFs are exposed to time varying magnetic fields. Oscillatory shear
studies using frequency and strain amplitude sweeps exhibit predominant solid
like behaviors under field environment. However, the relaxation behaviors and
strain amplitude sweep tests of the MRFs reveal that while the fluids show
solid like behaviors under field effect, they cannot be termed as typical
elastic fluids. Comparisons show that the MnFe2O4 MRFs have superior yield
performance among all. However, in case of dynamic and oscillatory systems,
CoFe2O4 MRFs show the best performance. The viscoelastic responses of the MRFs
are noted to correspond to a three element viscoelastic model. The study may
find importance in design and development strategies of nano MRFs for different
applications. | cond-mat |
Single-layer $1T'$-MoS$_2$ under electron irradiation from $ab$ $initio$
molecular dynamics: Irradiation with high-energy particles has recently emerged as an effective
tool for tailoring the properties of two-dimensional transition metal
dichalcogenides. In order to carry out an atomically-precise manipulation of
the lattice, a detailed understanding of the beam-induced events occurring at
the atomic scale is necessary. Here, we investigate the response of
$1T'$-MoS$_2$ to the electron irradiation by $ab$ $initio$ molecular dynamics
means. Our simulations suggest that an electron beam with energy smaller than
75 keV does not result in any knock-on damage. The displacement threshold
energies are different for the two nonequivalent sulfur atoms in $1T'$-MoS$_2$
and strongly depend on whether the top or bottom chalcogen layer is considered.
As a result, a careful tuning of the beam energy can promote the formation of
ordered defects in the sample. We further discuss the effect of the electron
irradiation in the neighborhood of a defective site, the mobility of the sulfur
vacancies created and their tendency to aggregate. Overall, our work provides
useful guidelines for the imaging and the defect engineering of $1T'$-MoS$_2$
using electron microscopy. | cond-mat |
First-Principles Constitutive Equation for Suspension Rheology: We provide a detailed derivation of a recently developed first-principles
approach to calculating averages in systems of interacting, spherical Brownian
particles under time-dependent flow. Although we restrict ourselves to flows
which are both homogeneous and incompressible, the time-dependence and geometry
(e.g. shear, extension) are arbitrary. The approximations formulated within
mode-coupling theory are particularly suited to dense colloidal suspensions and
capture the slow relaxation arising from particle interactions and the
resulting glass transition to an amorphous solid. The delicate interplay
between slow structural relaxation and time-dependent external flow in
colloidal suspensions may thus be studied within a fully tensorial theory. | cond-mat |
Tensor-entanglement renormalization group approach to topological phases: The tensor-entanglement renormalization group approach is applied to
Hamiltonians that realize a class of topologically ordered states -- string-net
condensed states. We analyze phase transitions between phases with and without
string-net condensation. These phase transitions change topological order
without changing any symmetries. This demonstrates that the tensor-entanglement
renormalization group approach can be used to study the phase diagram of a
quantum system with topologically ordered phases. | cond-mat |
Specific Heat of (Ca1-xSrx)3Ru2O7 Single Crystals: We have measured the specific heat of crystals of (Ca1-xSrx)3Ru2O7 using ac-
and relaxation-time calorimetry. Special emphasis was placed on the
characterization of the Neel (TN=56 K) and structural (Tc = 48 K) phase
transitions in the pure, x=0 material. While the latter is believed to be first
order, detailed measurements under different experimental conditions suggest
that all the latent heat (with L ~ 0.3 R) is being captured in a broadened peak
in the effective heat capacity. The specific heat has a mean-field-like step at
TN, but its magntitude (Delta cP ~ R) is too large to be associated with a
conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic
transition, while its entropy is too small to be associated with full ordering
of localized spins. The TN transition broadens with Sr substitution while its
magnitude decreases slowly. On the other hand, the entropy change associated
with the Tc transition decreases rapidly with Sr substitution and is not
observable for our x=0.58 sample. | cond-mat |
Observation of ultraslow hole dynamics in the 3D topological insulator
Bi2Se3 coated with a thin MgF2 layer using multiphoton pumped UV-Vis
transient absorption spectroscopy: Individual relaxation dynamics of electrons and holes in optically pumped
semiconductors is rarely observed due to their overlap. Here we report the
individual dynamics of long-lived (~200 mks) holes observed at room temperature
in a 10 nm thick film of the 3D topological insulator (TI) Bi2Se3 coated with a
10 nm thick MgF2 layer using transient absorption spectroscopy in the UV-Vis
region. The ultraslow hole dynamics was observed by applying multiphoton
resonant pumping of massless Dirac fermions and bound valence electrons in
Bi2Se3 at a certain wavelength sufficient for their photoemission and
subsequent trapping at the Bi2Se3/MgF2 interface. The emerging deficit of
electrons in the film makes it impossible for the remaining holes to recombine,
thus causing their ultraslow dynamics measured at a specific probing
wavelength. We also found an extremely long rise time (~600 ps) for this
ultraslow optical response, which is due to the large spin-orbit coupling (SOC)
splitting at the valence band maximum and the resulting intervalley scattering
between the splitting components. The ultraslow hole dynamics in Bi2Se3 due to
the presence of the Bi2Se3/MgF2 interface is nevertheless much faster than the
known ultraslow electron dynamics at the Si/SiO2 interface, also induced by
multiphoton excitation in Si. The observed dynamics of long-lived holes is
gradually suppressed with decreasing Bi2Se3 film thickness for the 2D TI Bi2Se3
(film thickness 5, 4, and 2 nm) due to the loss of resonance conditions for
multiphoton photoemission caused by the gap opening at the Dirac surface state
nodes. This behavior indicates that the dynamics of massive Dirac fermions
predominantly determines the relaxation of photoexcited carriers for both the
2D topologically nontrivial and 2D topologically trivial insulator phases. | cond-mat |
Numerical studies of the fractional quantum Hall effect in systems with
tunable interactions: The discovery of the fractional quantum Hall effect in GaAs-based
semiconductor devices has lead to new advances in condensed matter physics, in
particular the possibility for exotic, topological phases of matter that
possess fractional, and even non-Abelian, statistics of quasiparticles. One of
the main limitations of the experimental systems based on GaAs has been the
lack of tunability of the effective interactions between two-dimensional
electrons, which made it difficult to stabilize some of the more fragile
states, or induce phase transitions in a controlled manner. Here we review the
recent studies that have explored the effects of tunability of the interactions
offered by alternative two-dimensional systems, characterized by non-trivial
Berry phases and including graphene, bilayer graphene and topological
insulators. The tunability in these systems is achieved via external fields
that change the mass gap, or by screening via dielectric plate in the vicinity
of the device. Our study points to a number of different ways to manipulate the
effective interactions, and engineer phase transitions between quantum Hall
liquids and compressible states in a controlled manner. | cond-mat |
Fourier Transform Scanning Tunneling Spectroscopy: the possibility to
obtain constant energy maps and the band dispersion using a local measurement: We present here an overview of the Fourier Transform Scanning Tunneling
spectroscopy technique (FT-STS). This technique allows one to probe the
electronic properties of a two-dimensional system by analyzing the standing
waves formed in the vicinity of defects. We review both the experimental and
theoretical aspects of this approach, basing our analysis on some of our
previous results, as well as on other results described in the literature. We
explain how the topology of the constant energy maps can be deduced from the FT
of dI/dV map images which exhibit standing waves patterns. We show that not
only the position of the features observed in the FT maps, but also their shape
can be explained using different theoretical models of different levels of
approximation. Thus, starting with the classical and well known expression of
the Lindhard susceptibility which describes the screening of electron in a free
electron gas, we show that from the momentum dependence of the susceptibility
we can deduce the topology of the constant energy maps in a joint density of
states approximation (JDOS). We describe how some of the specific features
predicted by the JDOS are (or are not) observed experimentally in the FT maps.
The role of the phase factors which are neglected in the rough JDOS
approximation is described using the stationary phase conditions. We present
also the technique of the T-matrix approximation, which takes into account
accurately these phase factors. This technique has been successfully applied to
normal metals, as well as to systems with more complicated constant energy
contours. We present results recently obtained on graphene systems which
demonstrate the power of this technique, and the usefulness of local
measurements for determining the band structure, the map of the Fermi energy
and the constant-energy maps. | cond-mat |
Electronic structure and magnetic interactions in LiV2O4: We present results of all-electron electronic structure calculations for the
recently discovered d electron heavy fermion compound LiV_2O_4. The augmented
spherical wave calculations are based on density functional theory within the
local density approximation. The electronic properties near the Fermi energy
originate almost exclusively from V 3d t_{2g} states, which fall into two
equally occupied subbands: While sigma-type metal-metal bonding leads to rather
broad bands, small pi-type p-d overlap causes a narrow peak at E_F. Without the
geometric frustration inherent in the crystal structure, spin-polarized
calculations reveal an antiferromagnetic ground state and ferromagnetic order
at slightly higher energy. Since direct d-d exchange interaction plays only a
minor role, ordering of the localized vanadium moments can be attributed
exclusively to a rather weak superexchange interaction. With the magnetic order
suppressed by the geometric frustration, the remaining spin fluctuations
suggest an explanation of the low temperature behaviour of the specific heat. | cond-mat |
Effective thermodynamics for a marginal observer: Thermodynamics is usually formulated on the presumption that the observer has
complete information about the system he/she deals with: no parasitic current,
exact evaluation of the forces that drive the system. For example, the
acclaimed Fluctuation Relation (FR), relating the probability of time-forward
and time-reversed trajectories, assumes that the measurable transitions suffice
to characterize the process as Markovian (in our case, a continuous-time jump
process). However, most often the observer only measures a marginal current. We
show that he/she will nonetheless produce an effective description that does
not dispense with the fundamentals of thermodynamics, including the FR and the
2nd law. Our results stand on the mathematical construction of a hidden time
reversal of the dynamics, and on the physical requirement that the observed
current only accounts for a single transition in the configuration space of the
system. We employ a simple abstract example to illustrate our results and to
discuss the feasibility of generalizations. | cond-mat |
Spin-asymmetric Josephson effect: The Josephson effect is a manifestation of the macroscopic phase coherence of
superconductors and superfluids. We propose that with ultracold Fermi gases one
can realise a spin-asymmetric Josephson effect in which the two spin components
of a Cooper pair are driven asymmetrically - corresponding to driving a
Josephson junction of two superconductors with different voltages V_\uparrow
and V_\downarrow for spin up and down electrons, respectively. We predict that
the spin up and down components oscillate at the same frequency but with
different amplitudes. Our results reveal that the standard description of the
Josephson effect in terms of bosonic pair tunnelling is insufficient. We
provide an intuitive interpretation of the Josephson effect as interference in
Rabi oscillations of pairs and single particles, the latter causing the
asymmetry. | cond-mat |
Dynamics of two trapped Brownian particles: shear-induced
cross-correlations: The dynamics of two Brownian particles trapped by two neighboring harmonic
potentials in a linear shear flow is investigated. The positional correlation
functions in this system are calculated analytically and analyzed as a function
of the shear rate and the trap distance. Shear-induced cross-correlations
between particle fluctuations along orthogonal directions in the shear plane
are found. They are linear in the shear rate, asymmetric in time, and occur for
one particle as well as between both particles. Moreover, the shear rate enters
as a quadratic correction to the well-known correlations of random
displacements along parallel spatial directions. The correlation functions
depend on the orientation of the connection vector between the potential minima
with respect to the flow direction. As a consequence, the inter-particle
cross-correlations between orthogonal fluctuations can have zero, one or two
local extrema as a function of time. Possible experiments for detecting these
predicted correlations are described. | cond-mat |
Accurate Prediction of Bonding Properties by A Machine Learning-based
Model using Isolated States Before Bonding: Given the strong dependence of material structure and properties on the
length and strength of constituent bonds and the fact that surface adsorption
and chemical reactions are initiated by the formation of bonds between two
systems, bonding parameters are of key importance for material design and
industrial processes. In this study, a machine learning (ML)-based model is
used to accurately predict bonding properties from information pertaining to
isolated systems before bonding. This model employs the density of states (DOS)
before bond formation as the ML descriptor and accurately predicts binding
energy, bond distance, covalent electron amount, and Fermi energy even when
only 20% of the whole dataset is used for training. The results show that the
DOS of isolated systems before bonding is a powerful descriptor for the
accurate prediction of bonding and adsorption properties. | cond-mat |
Dielectric and structural studies of ferroelectric phase evolution in
dipole pair substituted barium titanate ceramics: Ba{[Gax,Tax]Ti(1-2x)}O3 ceramics with x equal to 0, 0.0025, 0.005, 0.01,
0.025 and 0.05 have been prepared by conventional solid-state reaction.
Structural and dielectric characterization have been performed to investigate
the effect of dipole-pair substitution concentration on the macroscopic
dielectric properties. Ba{[Gax,Tax]Ti(1-2x)}O3 evolves from a classic
ferroelectric to a diffuse phase transition (DPT) as x increases.
Ba{[Gax,Tax]Ti(1-2x)}O3 for x > or = 0.01 possesses diffuseness parameters
comparable to Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) and recently reported
(Ba0.97Pr0.03)(Ti0.9425Ce0.05)O3 (BPTC), yet it lacks the frequency and
temperature dependence of Tm necessary to be a strictly defined relaxor
ferroelectric. Additionally, Ba{[Ga0.05,Ta0.05]Ti0.9}O3 possesses a relative
permittivity, {\epsilon}r, of 700+/-16% and dissipation factor less than 0.05
at 10 kHz within the temperature range [-75{\deg}C, 120{\deg}C]. In comparison
to BaTiO3, Ba{[Gax,Tax]Ti(1-2x)}O3 possesses enhanced electrical resistivity at
and above room temperature. In-situ XRD, including Rietveld refinement, have
been performed to determine the lattice parameter, coefficient of thermal
expansion and phase transition temperature (Tc) of each composition within the
temperature range [RT, 1000{\deg}C], thus linking the dielectric properties
with the materials structure. These studies have been corroborated by
temperature dependent Raman spectroscopy to compare the Tc determined by
electrical and structural characterization. The properties of
Ba{[Gax,Tax]Ti(1-2x)}O3 are discussed in context with available models that
describe donor and acceptor dopants spatially separated in the parent matrix,
inter-relating lattice parameter, Curie temperature, and other material
properties | cond-mat |
Optically driven rotation of exciton-polariton condensates: The rotational response of quantum condensed fluids is strikingly distinct
from rotating classical fluids, especially notable for the excitation and
ordering of quantized vortex ensembles. Although widely studied in conservative
systems, the dynamics of rotating open-dissipative superfluids such as
exciton-polariton condensates remain largely unexplored, as it requires
high-frequency rotation whilst avoiding resonantly driving the condensate. We
create a rotating polariton condensate at GHz frequencies by off-resonantly
pumping with a rotating optical stirrer composed of the time-dependent
interference of two frequency-offset, structured laser modes. Acquisition of
angular momentum exceeding the critical $1\hbar$/particle is directly measured,
accompanied by the deterministic nucleation and capture of quantized vortices
with a handedness controlled by the pump rotation direction. The demonstration
of controlled optical rotation of a spontaneously formed polariton condensate
enables new opportunities for the study of open-dissipative superfluidity,
ordering of non-Hermitian quantized vortex matter, and topological states in a
highly non-linear, photonic platform. | cond-mat |
Strongly Correlated Chern Insulators in Magic-Angle Twisted Bilayer
Graphene: Interactions among electrons and the topology of their energy bands can
create novel quantum phases of matter. Most topological electronic phases
appear in systems with weak electron-electron interactions. The instances where
topological phases emerge only as a result of strong interactions are rare, and
mostly limited to those realized in the presence of intense magnetic fields.
The discovery of flat electronic bands with topological character in
magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity
to search for new strongly correlated topological phases. Here we introduce a
novel local spectroscopic technique using a scanning tunneling microscope (STM)
to detect a sequence of topological insulators in MATBG with Chern numbers C =
$\pm$ 1, $\pm$ 2, $\pm$ 3, which form near $\nu$ = $\pm$ 3, $\pm$ 2, $\pm$ 1
electrons per moir\'e unit cell respectively, and are stabilized by the
application of modest magnetic fields. One of the phases detected here (C = +1)
has been previously observed when the sublattice symmetry of MATBG was
intentionally broken by hexagonal boron nitride (hBN) substrates, with
interactions playing a secondary role. We demonstrate that strong
electron-electron interactions alone can produce not only the previously
observed phase, but also new and unexpected Chern insulating phases in MATBG.
The full sequence of phases we observed can be understood by postulating that
strong correlations favor breaking time-reversal symmetry to form Chern
insulators that are stabilized by weak magnetic fields. Our findings illustrate
that many-body correlations can create topological phases in moir\'e systems
beyond those anticipated from weakly interacting models. | cond-mat |
Phase transition of Two-timescale Two-temperature Spin-lattice Gas Model: We study phase transition of a nonequilibrium statistical-mechanical model,
in which two degrees of freedom with different time scales separated from each
other touch to their own heat bath. A general condition for finding anomalous
negative latent heat recently discovered is derived a from thermodynamic
argument. As a specific example, phase diagram of a spin-lattice gas model is
studied based on a mean-field analysis with replica method. While
configurational variables are spin and particle in this model, it is found that
the negative latent heat appears in a parameter region of the model,
irrespective of the order of their time scale. Qualitative differences in the
phase diagram are also discussed. | cond-mat |
Adiabatic magnon spectra with and without constraining field: Benchmark
against an exact magnon spectrum: The spectrum of magnon excitations in magnetic materials can be obtained
exactly from the transverse dynamic magnetic susceptibility, which is however
in practice numerically expensive. Many ab initio approaches therefore consider
instead the adiabatic magnon spectrum, which assumes a separation of time
scales of magnons and electronic excitations. There exist two alternative
implementations for adiabatic magnon spectra: one based on the magnetic force
theorem (MFT) and the other with a constraining field that enforces static
non-collinear spin configurations. We benchmark both implementations against
the exact magnon spectrum of an exactly solvable mean-field model. While both
adiabatic methods are equally valid in the low magnon energy and strong Stoner
coupling limits, we find that the constraining field method performs better
than the MFT in both the cases of strong Stoner coupling and high magnon
energies,while the MFT performs better for combined weak coupling and low
magnon energies. | cond-mat |
Observation of the In-plane Anomalous Hall Effect induced by Octupole in
Magnetization Space: The Anomalous Hall Effect (AHE) manifests as a transverse voltage
proportional to magnetization in ferromagnetic materials under the application
of a charge current, being an indispensable tool for probing magnetism,
especially in nanoscale devices. However, the AHE primarily sensitizes to
out-of-plane magnetization, thereby hindering its capacity to discern the
in-plane magnetization, a characteristic prevalent in ferromagnetic films. Here
we challenge this conventional understanding by demonstrating the in-plane
magnetization-induced AHE in iron and nickel, two ubiquitous ferromagnets. This
observation of the in-plane AHE is remarkable as it contradicts existing
theories that forbid such phenomena in cubic crystal systems. We trace the
origin of this unanticipated phenomenon to a hitherto unconsidered octupole of
the anomalous Hall conductivity in the magnetization space, a mechanism we
propose could enable the detection of in-plane AHE in a wide range of
ferromagnetic materials. This work realizes the in-plane AHE in common
ferromagnets by exploiting the anomalous Hall conductivity octupole, revealing
a new physical origin of the AHE and promising to revolutionize the design of
magnetic devices and sensors. | cond-mat |
Solving the Poisson-Boltzmann Equation to Obtain Interaction Energies
Between Confined, Like-charged Cylinders: We numerically solve the non-linear Poisson-Boltzmann equation for two
cylinders confined by two parallel charged plates. The repulsive electrical
double layer component of the cylinder pair potential is substantially reduced
by confinement between like-charged plates. While the effective cylinder
surface charge is increased by the confinement, the effective interaction
screening length is reduced, this effect being dominant so that the repulsive
confined cylinder-cylinder interaction potential is reduced. | cond-mat |
Time irreversibility from symplectic non-squeezing: The issue of how time reversible microscopic dynamics gives rise to
macroscopic irreversible processes has been a recurrent issue in Physics since
the time of Boltzmann whose ideas shaped, and essentially resolved, such an
apparent contradiction. Following Boltzmann's spirit and ideas, but employing
Gibbs's approach, we advance the view that macroscopic irreversibility of
Hamiltonian systems of many degrees of freedom can be also seen as a result of
the symplectic non-squeezing theorem. | cond-mat |
Analysis of Magnetization Loss on a Twisted Superconducting Strip in a
Constantly Ramped Magnetic Field: Magnetization loss on a twisted superconducting (SC) tape in a ramped
magnetic field is theoretically investigated through the use of a power law for
the electric field--current density characteristics and a sheet current
approximation. First, the Maxwell equation in a helicoidal coordinate system is
derived to model a twisted SC tape, taking account of the response to the
perpendicular field component in the steady state. We show that a loosely
twisted tape can be viewed as the sum of a portion of tilted flat tapes of
infinite length by examining the perpendicular field distribution on a twisted
tape. The analytic formulae for both magnetization and loss power in the tilted
flat tape approximation are verified based on the analytic solution of the
reduced Maxwell equation in the loosely twisted tape limit of $L_{\rm
p}\rightarrow \infty$ with the twist pitch length $L_{\rm p}$. These analytic
formulae show that both magnetization and loss power decrease by a factor of
$B(1+1/2n,1/2)/\pi$ (where $B$ is the beta function) for an arbitrary power of
SC nonlinear resistivity $n$, compared with those in a flat tape of infinite
length. Finally, the effect of the field-angle dependence of the critical
current density $J_{\rm c}$ on the loss power is investigated, and we
demonstrate that it is possible to obtain an approximate estimate of the loss
power value via $J_{\rm c}$ in an applied magnetic field perpendicular to the
tape surface (i.e., parallel to the $c$ axis). | cond-mat |
Controlling the dynamic range of a Josephson parametric amplifier: One of the central challenges in the development of parametric amplifiers is
the control of the dynamic range relative to its gain and bandwidth, which
typically limits quantum limited amplification to signals which contain only a
few photons per inverse bandwidth. Here, we discuss the control of the dynamic
range of Josephson parametric amplifiers by using Josephson junction arrays. We
discuss gain, bandwidth, noise, and dynamic range properties of both a
transmission line and a lumped element based parametric amplifier. Based on
these investigations we derive useful design criteria, which may find broad
application in the development of practical parametric amplifiers. | cond-mat |
Prediction of High Transition Temperatures in Ceramic Superconductors: The prediction of transition temperatures can be regarded in several ways,
either as an exacting test of theory, or as a tool for identifying theoretical
rules for defining new homology models. Popular "first principle" methods for
predicting transition temperatures in conventional crystalline superconductors
have failed for cuprate HTSC, as have parameterized models based on CuO2 planes
(with or without apical oxygen). Following a path suggested by Bayesian
probability, we find that the glassy, self-organized dopant network percolative
model is so successful that it defines a new homology class appropriate to
ceramic superconductors. The reasons for this success are discussed, and a
critical comparison is made with previous theories. The predictions are
successful for all ceramics, including new non-cuprates based on FeAs in place
of CuO2. | cond-mat |
Origin of giant magnetoresistance in layered nodal-line semimetal
TaNiTe5 nanoflakes: Layered transition metal chalcogenides have stimulated a wide research
interest due to its many exotic physical properties. In this paper, we studied
the magnetotransport properties of the exfoliated TaNiTe5, a recently
discovered Dirac nodal-line semimetal. A giant positive magnetoresistance (MR)
is observed when the current is parallel to the crystallographic c axis, while
it is strongly diminished when the current flows along the a axis. The observed
giant MR is gradually suppressed either on reducing the thickness of nanoflake
or on increasing temperature. By performing MR measurement in tilted magnetic
fields, the interlayer coupling is found to be weakened both by reducing the
thickness and by increasing temperature. We propose a mechanism of
electron-electron interaction-assisted interlayer transport as a origin of the
giant MR. The mechanism is likely to provide a explanation for the giant MR in
other layered materials. | cond-mat |
Quantum criticality of magnetic catalysis in two-dimensional correlated
Dirac fermions: We study quantum criticality of the magnetic field induced charge density
wave (CDW) order in correlated spinless Dirac fermions on the $\pi$-flux square
lattice at zero temperature as a prototypical example of the magnetic
catalysis, by using the infinite density matrix renormalization group. It is
found that the CDW order parameter $M(B)$ exhibits an anomalous magnetic field
$(B)$ scaling behavior characteristic of the $(2+1)$-dimensional chiral Ising
universality class near the quantum critical point, which leads to a strong
enhancement of $M(B)$ compared with a mean field result. We also establish a
global phase diagram in the interaction-magnetic field plane for the fermionic
quantum criticality. | cond-mat |
Composite Fermions and the Energy Gap in the Fractional Quantum Hall
Effect: The energy gaps for the fractional quantum Hall effect at filling fractions
1/3, 1/5, and 1/7 have been calculated by variational Monte Carlo using Jain's
composite fermion wave functions before and after projection onto the lowest
Landau level. Before projection there is a contribution to the energy gaps from
the first excited Landau level. After projection this contribution vanishes,
the quasielectron charge becomes more localized, and the Coulomb energy
contribution increases. The projected gaps agree well with previous
calculations, lending support to the composite fermion theory. | cond-mat |
Diluted antiferromagnets in a field seem to be in a different
universality class than the random-field Ising model: We perform large-scale Monte Carlo simulations using the Machta-Newman-Chayes
algorithms to study the critical behavior of both the diluted antiferromagnet
in a field with 30% dilution and the random-field Ising model with Gaussian
random fields for different field strengths. Analytical calculations by Cardy
[Phys. Rev. B 29, 505 (1984)] predict that both models map onto each other and
share the same universality class in the limit of vanishing fields. However, a
detailed finite-size scaling analysis of both the Binder cumulant and the
two-point finite-size correlation length suggests that even in the limit of
small fields, where the mapping is expected to work, both models are not in the
same universality class. Therefore, care should be taken when interpreting
(experimental) data for diluted antiferromagnets in a field using the
random-field Ising model. Based on our numerical data, we present analytical
expressions for the phase boundaries of both models. | cond-mat |
Nonlocal elasticity near jamming: We demonstrate that the elasticity of jammed solids is nonlocal. By forcing
frictionless soft sphere packings at varying wavelength, we directly access
their transverse and longitudinal compliances without resorting to curve
fitting. The observed wavelength dependence of the compliances is incompatible
with classical (local) elasticity, and hence quantifies the amplitude of
nonlocal effects. Three distinct length scales, two of which diverge, control
the amplitude of both nonlocal effects and fluctuations about the mean
response. Our results identify new, more accurate constitutive relations for
weakly jammed solids, including emulsions, foams, and granulates. | cond-mat |
The magnetic field induced phase separation in a model of a
superconductor with local electron pairing: We have studied the extended Hubbard model with pair hopping in the atomic
limit for arbitrary electron density and chemical potential and focus on
paramagnetic effects of the external magnetic field. The Hamiltonian considered
consists of (i) the effective on-site interaction U and (ii) the intersite
charge exchange interactions I, determining the hopping of electron pairs
between nearest-neighbour sites. The phase diagrams and thermodynamic
properties of this model have been determined within the variational approach
(VA), which treats the on-site interaction term exactly and the intersite
interactions within the mean-field approximation. Our investigation of the
general case shows that the system can exhibit not only the homogeneous phases:
superconducting (SS) and nonordered (NO), but also the phase separated states
(PS: SS-NO). Depending on the values of interaction parameters, the PS state
can occur in higher fields than the SS phase (field-induced PS). Some ground
state results beyond the VA are also presented. | cond-mat |
The effects of interstitials clustering on the configurational entropy
of bcc solid solutions: This work presents a simple model for describing the interstitials behavior
in solid solutions enlarging the current random interstitial atoms paradigm. A
general and parameter-free analytical expression to compute the configurational
entropy, valid for any tetrahedral or octahedral interstitial solutions and
suitable for the treatment of interstitials clustering, is deduced for that
purpose. The effect of interstitials clustering on the configurational entropy
is shown by applying the methodology to the Nb-H and bcc Zr-H solid solutions.
The model for Nb-H presented in this work, based on the existence of H pairs in
the \alpha phase and double pairs in the \alpha 'phase, provides the basis to
explain the unsolved controversies in this system. The unusual shape of the
partial configurational entropy measured in bcc Zr-H can be accurately
described if a small amount of H clusters are included in the solution. | cond-mat |
Self-organised quantum dots in marginally twisted MoSe$_2$/WSe$_2$ and
MoS$_2$/WS$_2$ bilayers: Moir\'e superlattices in twistronic heterostructures are a powerful tool for
materials engineering. In marginally twisted (small misalignment angle,
$\theta$) bilayers of nearly lattice-matched two-dimensional (2D) crystals
moir\'e patterns take the form of domains of commensurate stacking, separated
by a network of domain walls (NoDW) with strain hot spots at the NoDW nodes.
Here, we show that, for type-II transition metal dichalcogenide bilayers
MoX$_2$/WX$_2$ (X=S, Se), the hydrostatic strain component in these hot spots
creates quantum dots for electrons and holes. We investigate the electron/hole
states bound by such objects, discussing their manifestations via the
intralayer intraband infrared transitions. The electron/hole confinement, which
is strongest for $\theta<0.5^{\circ}$, leads to a red-shift of their
recombination line producing single photon emitters (SPE) broadly tuneable
around 1\,eV by misalignment angle. These self-organised dots can form in
bilayers with both aligned and inverted MoX$_2$ and WX$_2$ unit cells, emitting
photons with different polarizations. We also find that the hot spots of strain
reduce the intralayer MoX$_2$ A-exciton energy, enabling selective population
of the quantum dot states. | cond-mat |
Magnetotransport Properties of Antiferromagnetic YBa_2Cu_3O_6.25 Single
Crystals: In-plane and out-of-plane magnetoresistivities (MR) of antiferromagnetic
YBa_2Cu_3O_6.25 single crystals were measured in magnetic fields H applied
along the (ab) plane. In-plane MR is a superposition of two components: The
first component is strongly in-plane anisotropic, changing sign from negative
when H is parallel to the electrical current I to positive when H is
perpendicular to I. The second component is positive, quadratic in H, and
isotropic in the (ab)-plane. The out-of-plane MR displays a fourfold symmetry
upon in-plane rotation of the magnetic field, with maxima along the easy axes
of antiferromagnetic spin ordering and minima along unfavorable directions of
spin orientation (45 degrees from the Cu-O-Cu bonds). | cond-mat |
Charge renormalization and other exact coupling corrections in the
dipolar effective interaction in an electrolyte near a dielectric wall: The aim of the paper is to study the renormalizations of the charge and of
the screening length that appear in the large-distance behavior of the
effective pairwise interaction between two charges in a dilute electrolyte
solution, both along a dielectric wall and in the bulk. The electrolyte is
described by the primitive model in the framework of classical statistical
mechanics and the electrostatic response of the wall is characterized by its
dielectric constant. | cond-mat |
Large area photoelectrodes based on hybrids of CNT fibres and ALD grown
TiO2: Hybridisation is a powerful strategy towards the next generation of
multifunctional materials for environmental and sustainable energy
applications. Here, we report a new inorganic nanocarbon hybrid material
prepared with atomically controlled deposition of a monocrystalline TiO2 layer
that conformally coats a macroscopic carbon nanotube (CNT) fiber. Through X-ray
diffraction, Raman spectroscopy and photoemission spectroscopy we detect the
formation of a covalent Ti-O-C bond at the TiO2/CNT interface and a residual
strain of approximately 0.7-2 \%, which is tensile in TiO2 and compressive in
the CNT. It arises after deposition of the amorphous oxide onto the CNT surface
previously functionalized by the oxygen plasma used in ALD. These features are
observed in samples of different TiO2 thickness, in the range from 10 to 80 nm.
Ultraviolet photoemission spectroscopy on a 20 nm-thick TiO2 coated sample
gives a work function of 4.27 eV, between that of TiO2 (4.23 eV) and the CNT
fiber (4.41 eV), and the presence of new interband gap states that shift the
valence band maximum to 1.05 eV below the Fermi level. Photoelectrochemical
measurements demonstrate electron transfer from TiO2 to the CNT fiber network
under UV irradiation. Electrochemical impedance spectroscopy measurements
reveal a low resistance for charge transfer and transport, as well as a large
capacitance. Our results point to the fact that these hybrids, in which each
phase has nanometric thickness and the current collector is integrated into the
material, are very different from conventional electrodes and can provide a
number of superior properties. | cond-mat |
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