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Chain decay and rates disorder in the totally asymmetric simple
exclusion process: We theoretically study the Totally Asymmetric Exclusion Process (TASEP) with
quenched jumping rates disorder and finite lifetime chain. TASEP is widely used
to model the translation of messenger RNAs by Ribosomes in protein synthesis.
Since the exact solution of the TASEP model is analytically and computationally
intractable for biologically relevant systems parameters, the canonical
Mean-Field (MF) approaches of solving coupled non-linear differential equations
is also computational expensive for the scale of relevant biological data
analysis. In this article, we provide alternative approach to computing the MF
steady state solution via a computationally efficient system of non-linear
algebraic equations. We further outline a framework for including correlations
progressively via the exact solution of small size TASEP system. Leading order
approximation in the biologically relevant entry rate limited regime shows
remarkable agreement with the full Monte-Carlo simulation result for a wide
range of system parameter space. These results could be of importance to the
kinetic rates inference in Ribo-Seq data analysis and other related problems. | cond-mat |
rf SQUID metamaterials: An rf superconducting quantum interference device (SQUID) array in an
alternating magnetic field is investigated with respect to its effective
magnetic permeability, within the effective medium approximation. This system
acts as an inherently nonlinear magnetic metamaterial, leading to negative
magnetic response, and thus negative permeability, above the resonance
frequency of the individual SQUIDs. Moreover, the permeability exhibits
oscillatory behavior at low field intensities, allowing its tuning by a slight
change of the intensity of the applied field. | cond-mat |
Hydrodynamics of granular gases of inelastic and rough hard disks or
spheres. II. Stability analysis: Conditions for the stability under linear perturbations around the
homogeneous cooling state are studied for dilute granular gases of inelastic
and rough hard disks or spheres with constant coefficients of normal ($\alpha$)
and tangential ($\beta$) restitution. After a formally exact linear stability
analysis of the Navier--Stokes--Fourier hydrodynamic equations in terms of the
translational ($d_t$) and rotational ($d_r$) degrees of freedom, the transport
coefficients derived in the companion paper [A. Meg\'ias and A. Santos,
"Hydrodynamics of granular gases of inelastic and rough hard disks or spheres.
I. Transport coefficients," Phys. Rev. E 104, 034901 (2021)] are employed.
Known results for hard spheres [V. Garz\'o, A. Santos, and G. M. Kremer, Phys.
Rev. E 97, 052901 (2018)] are recovered by setting $d_t=d_r=3$, while novel
results for hard disks ($d_t=2$, $d_r=1$) are obtained. In the latter case, a
high-inelasticity peculiar region in the $(\alpha,\beta)$ parameter space is
found, inside which the critical wave number associated with the longitudinal
modes diverges. Comparison with event-driven molecular dynamics simulations for
dilute systems of hard disks at $\alpha=0.2$ shows that this theoretical region
of absolute instability may be an artifact of the extrapolation to high
inelasticity of the approximations made in the derivation of the transport
coefficients, although it signals a shrinking of the conditions for stability.
In the case of moderate inelasticity ($\alpha=0.7$), however, a good agreement
between the theoretical predictions and the simulation results is found. | cond-mat |
Electrically tunable multi-terminal SQUID-on-tip: We present a new nanoscale superconducting quantum interference device
(SQUID) whose interference pattern can be shifted electrically in-situ. The
device consists of a nanoscale four-terminal/four-junction SQUID fabricated at
the apex of a sharp pipette using a self-aligned three-step deposition of Pb.
In contrast to conventional two-terminal/two-junction SQUIDs that display
optimal sensitivity when flux biased to about a quarter of the flux quantum,
the additional terminals and junctions allow optimal sensitivity at arbitrary
applied flux, thus eliminating the magnetic field "blind spots". We demonstrate
spin sensitivity of 5 to 8 $\mu_B/\text{Hz}^{1/2}$ over a continuous field
range of 0 to 0.5 T, with promising applications for nanoscale scanning
magnetic imaging. | cond-mat |
Domain wall architecture in tetragonal ferroelectric thin films: Domain walls in ferroelectrics exhibit a plethora of phases and
functionalities not found in the bulk. The interplay of electrostatic,
chemical, topological, and distortive inhomogeneities at the walls can be so
complex, however, that this obstructs their technological performance. In
tetragonal ferroelectrics like PbZrxTi1-xO3, for example, the desired
functional 180{\deg} domain walls within out-of-plane-polarized c-domains are
interspersed by in-plane-polarized a-domains and the associated network of
domain walls remains challenging to analyze. Here we use a combination of STEM
and optical second harmonic generation (SHG) to determine the relation between
strain, film thickness, local electric fields and the resulting domain and
domain-wall structures across the entire thickness of a set of PZT films. We
quantify the distribution of a-domains in the c-domain matrix of the films.
Using locally applied electric fields we control the a/c distribution and
induce the technologically preferable 180{\deg} domain walls. We find that
these voltage induced walls are tilted and exhibit a mixed Ising-N\'eel type
transverse rotation of polarization across the wall with a specific nonlinear
optical response. | cond-mat |
Observation of Giant Quantized Phonon Modes in Graphene via Tunneling
Spectra: Phonons, the fundamental vibrational modes of a crystal lattice, play a
crucial role in determining electronic properties of materials through
electron-phonon interaction. However, it has proved difficult to directly probe
the phonon modes of materials in electrical measurements. Here, we report the
observation of giant quantized phonon peaks of the K and K out-of-plane phonon
in graphene monolayer in magnetic fields via tunneling spectra, which are
usually used to measure local electronic properties of materials. A
perpendicular magnetic field quantizes massless Dirac fermions in graphene into
discrete Landau levels (LLs). We demonstrate that emission or absorption of
phonons of quasiparticles in the LLs of graphene generates a new sequence of
discrete states: the quantized phonon modes. In our tunneling spectra, the
intensity of the observed phonon peaks is about 50 times larger than that of
the LLs because that the K and K out-of-plane phonon opens an inelastic
tunneling channel. We also show that it is possible to switch on off the
quantized phonon modes at nanoscale by controlling interactions between
graphene and the supporting substrate. | cond-mat |
Rheology and Contact Lifetime Distribution in Dense Granular Flows: We study the rheology and distribution of interparticle contact lifetimes for
gravity-driven, dense granular flows of non-cohesive particles down an inclined
plane using large-scale, three dimensional, granular dynamics simulations.
Rather than observing a large number of long-lived contacts as might be
expected for dense flows, brief binary collisions predominate. In the hard
particle limit, the rheology conforms to Bagnold scaling, where the shear
stress is quadratic in the strain rate. As the particles are made softer,
however, we find significant deviations from Bagnold rheology; the material
flows more like a viscous fluid. We attribute this change in the collective
rheology of the material to subtle changes in the contact lifetime distribution
involving the increasing lifetime and number of the long-lived contacts in the
softer particle systems. | cond-mat |
Fermion Parity Flips and Majorana Bound States at twist defects in
Superconducting Fractional Topological Phases: In this paper we consider a layered heterostructure of an Abelian
topologically ordered state (TO), such as a fractional Chern insulator/quantum
Hall state with an s-wave superconductor in order to explore the existence of
non-Abelian defects. In order to uncover such defects we must augment the
original TO by a $\mathbb{Z}_2$ gauge theory sector coming from the s-wave SC.
We first determine the extended TO for a wide variety of fractional quantum
Hall or fractional Chern insulator heterostructures. We prove the existence of
a general anyon permutation symmetry (AS) that exists in any fermionic Abelian
TO state in contact with an s-wave superconductor. Physically this permutation
corresponds to adding a fermion to an odd flux vortices (in units of $h/2e$) as
they travel around the associated topological (twist) defect. As such, we call
it a fermion parity flip AS. We consider twist defects which mutate anyons
according to the fermion parity flip symmetry and show that they can be
realized at domain walls between distinct gapped edges or interfaces of the TO
superconducting state. We analyze the properties of such defects and show that
fermion parity flip twist defects are always associated with Majorana zero
modes. Our formalism also reproduces known results such as
Majorana/parafermionic bound states at superconducting domain walls of
topological/Fractional Chern insulators when twist defects are constructed
based on charge conjugation symmetry. Finally, we briefly describe more exotic
twist liquid phases obtained by gauging the AS where the twist defects become
deconfined anyonic excitations. | cond-mat |
Revealing the nature of defects in quasi free standing mono-layer
graphene on SiC(0001) by means of Density Functional Theory: Quasi free standing monolayer graphene (QFMLG) grown on SiC by selective Si
evaporation from the Si-rich SiC(0001) face and H intercalation displays
irregularities in STM and AFM analysis, appearing as localized features, which
we previously identified as vacancies in the H layer coverage [Y Murata, et al.
Nano Res, in press, DOI: 10.1007/s12274-017-1697-x]. The size, shape,
brightness, location, and concentration of these features, however, are
variable, depending on the hydrogenation conditions. In order to shed light on
the nature of these features, in this work we perform a systematic Density
Functional Theory study on the structural and electronic properties of QFMLG
with defects in the H coverage arranged in different configurations including
up to 13 vacant H atoms, and show that these generate localized electronic
states with specific electronic structure. Based on the comparison of simulated
and measured STM images we are able to associate different vacancies of large
size (7-13 missing H) to the different observed features. The presence of large
vacancies is in agreement with the tendency of single H vacancies to aggregate,
as demonstrated here by DFT results. This gives some hints into the
hydrogenation process. Our work unravels the structural diversity of defects of
H coverage in QFMLG and provides operative ways to interpret the variety in the
STM images. The energy of the localized states generated by these vacancies is
tunable by means of their size and shape, suggesting applications in nano- and
opto-electronics. | cond-mat |
Stability conditions for a large anharmonic bipolaron: A large polaron is a quasiparticle that consists of a nearly free electron
interacting with the phonons of a material, whose lattice parameters are much
smaller than the polaron scale. The electron-phonon interaction also leads to
an attractive interaction between electrons, which can allow two polarons to
pair up and form a bipolaron. It has been shown that large bipolarons can form
in theory due to strong 1-electron-1-phonon coupling, but they have not been
seen in real materials because the critical value of the required
electron-phonon interaction is too large. Here, we investigate the effect of
1-electron-2-phonon coupling on the large bipolaron problem.
Starting from a generalization of the Fr\"ohlich Hamiltonian that includes
both the standard 1-electron-1-phonon interaction as well as an anharmonic
1-electron-2-phonon interaction, we use the path integral method to find a
semi-analytical upper bound for the bipolaron energy that is valid at all
values of the Fr\"ohlich coupling strength $\alpha$. We find the bipolaron
phase diagram and conditions for the bipolaron stability by comparing the
bipolaron energy to the energy of two free polarons. The critical value of the
Fr\"ohlich coupling strength $\alpha_{\text{crit}}$ is calculated as a function
of the strength of the 1-electron-2-phonon interaction. The results suggest
that large bipolaron formation is more likely in materials with significant
1-electron-2-phonon interaction as well as strong 1-electron-1-phonon
interaction, such as strontium titanate. | cond-mat |
Comments on "Competition Between Fractional Quantum Hall Liquid ...", by
G. Gervais, L. W. Engel, H. L. Stormer, D. C. Tsui, et al (cond-mat/0402169): The quantum Hall effect in ultra-high mobility GaAs/AlGaAs has been measured
and plateaus are found at many different fractions. The resistivity is
quantized as \rho =h/ie^2 where i exhibits many different values. The fractions
5/3, 8/5, 11/7, 14/9, 17/11 fit the formula, i=3p\pm 2/(2p \pm 1) and it is
claimed that 2p flux quanta are attached to the electron. The fractions 4/11,
7/11, 12/7, 13/8 and 15/11 do not fit the expression for i, even then the
authors insist that flux quanta are attached to the electron and hence
composite fermions (CF) are formed. We report that the interpretation of the
experimental data in terms of CF is incorrect. | cond-mat |
Protected edge modes without symmetry: We discuss the question of when a gapped 2D electron system without any
symmetry has a protected gapless edge mode. While it is well known that systems
with a nonzero thermal Hall conductance, $K_H \neq 0$, support such modes, here
we show that robust modes can also occur when $K_H = 0$ -- if the system has
quasiparticles with fractional statistics. We show that some types of
fractional statistics are compatible with a gapped edge, while others are
fundamentally incompatible. More generally, we give a criterion for when an
electron system with abelian statistics and $K_H = 0$ can support a gapped
edge: we show that a gapped edge is possible if and only if there exists a
subset of quasiparticle types $M$ such that (1) all the quasiparticles in $M$
have trivial mutual statistics, and (2) every quasiparticle that is not in $M$
has nontrivial mutual statistics with at least one quasiparticle in $M$. We
derive this criterion using three different approaches: a microscopic analysis
of the edge, a general argument based on braiding statistics, and finally a
conformal field theory approach that uses constraints from modular invariance.
We also discuss the analogous result for 2D boson systems. | cond-mat |
Non-Fermi Liquid Fixed Point in 2+1 Dimensions: We construct models of excitations about a Fermi surface that display
calculable deviations from Fermi liquid behavior in the low-energy limit. They
arise as a consequence of coupling to a Chern-Simons gauge field, whose
fluctations are controlled through a ${1\over{k^x}}$ interaction. The Fermi
liquid fixed point is shown to be unstable in the infrared for $x<1$, and an
infrared-stable fixed point is found in a $(1-x)$-expansion, analogous to the
$\epsilon$-expansion of critical phenomena. $x=1$ corresponds to Coulomb
interactions, and in this case we find a logarithmic approach to zero coupling.
We describe the low-energy behavior of metals in the universality class of the
new fixed point, and discuss its possible application to the compressible
$\nu={1\over2}$ quantum Hall state and to the normal state of copper-oxide
superconductors. | cond-mat |
Factors Enabling Delocalized Charge-Carriers in Pnictogen-Based Solar
Absorbers: In-depth Investigation into CuSbSe2: Inorganic semiconductors based on heavy pnictogen cations (Sb3+ and Bi3+)
have gained significant attention as potential nontoxic and stable alternatives
to lead-halide perovskites for solar cell applications. A limitation of these
novel materials, which is being increasingly commonly found, is carrier
localization, which substantially reduces mobilities and diffusion lengths.
Herein, the layered p\v{r}\'ibramite CuSbSe2 is investigated and discovered to
have delocalized free carriers, as shown through optical pump terahertz probe
spectroscopy and temperature-dependent mobility measurements. Using a
combination of theory and experiment, it is found that the underlying factors
are: 1) weak coupling to acoustic phonons due to low deformation potentials, as
lattice distortions are primarily accommodated through rigid inter-layer
movement rather than straining inter-atomic bonds, and 2) weak coupling to
optical phonons due to the ionic contributions to the dielectric constant being
low compared to electronic contributions. This work provides important insights
into how pnictogen-based semiconductors avoiding carrier localization could be
identified. | cond-mat |
Relationship between Population Dynamics and the Self-Energy in Driven
Non-Equilibrium Systems: We compare the decay rates of excited populations directly calculated within
a Keldysh formalism to the equation of motion of the population itself for a
Hubbard-Holstein model in two dimensions. While it is true that these two
approaches must give the same answer, it is common to make a number of
simplifying assumptions within the differential equation for the populations
that allows one to interpret the decay in terms of hot electrons interacting
with a phonon bath. Here we show how care must be taken to ensure an accurate
treatment of the equation of motion for the populations due to the fact that
there are identities that require cancellations of terms that naively look like
they contribute to the decay rates. In particular, the average time dependence
of the Green's functions and self-energies plays a pivotal role in determining
these decay rates. | cond-mat |
Collective excitations of the Chern-insulator states in commensurate
double moiré superlattices of twisted bilayer graphene on hexagonal boron
nitride: We study the collective excitation modes of the Chern insulator states in
magic-angle twisted bilayer graphene aligned with hexagonal boron nitride
(TBG/BN) at odd integer fillings ($\nu$) of the flat bands. For the $1 \times
1$ commensurate double moir\'{e} superlattices in TBG/BN at three twist angles
($\theta'$) between BN and graphene, self-consistent Hartree-Fock calculations
show that the electron-electron interaction and the broken $C_{2z}$ symmetry
lead to the Chern-insulator ground states with valley-spin flavor polarized HF
bands at odd $\nu$. In the active-band approximation, the HF bands in the same
flavor of TBG/BN are much more separated than those of the pristine TBG with
TBG/BN having a larger intra-flavor band gap so that the energies of the lowest
intra-flavor exciton modes of TBG/BN computed within the time-dependent HF
method are much higher than those of TBG and reach about 20 meV, and the
exciton wavefunctions of TBG/BN become less localized than those of TBG. The
inter-flavor valley-wave modes in TBG/BN have excitation energies higher than
2.5 meV which is also much larger than that of TBG, while the spin-wave modes
all have zero excitation gap. In contrast to TBG with particle-hole symmetric
excitation modes for positive and negative $\nu$, the excitation spectrums and
gaps of TBG/BN at positive $\nu$ are rather different from those at negative
$\nu$. The quantitative behavior of the excitation spectrum of TBG/BN also
varies with $\theta'$. Full HF calculations demonstrate that more HF bands
besides the two central bands can have rather large contributions from the
single-particle flat-band states, then the lowest exciton modes that determine
the optical properties of the Chern insulator states in TBG/BN are generally
the ones between the remote and flat-like bands, while the valley-wave modes
have similar energies as those in the active-band approximation. | cond-mat |
Spontaneous Charge Oscillations in Dielectric Confined Quasi-2D Systems: We report spontaneous electric field and charge oscillations in dielectric
confined Quasi-2D charged systems. A simple relationship is found for the
oscillation wave number, which is solely determined by the dielectric mismatch
and the length scale of confinement. We analytically show that the emergence of
charge/field oscillation is due to the arising of a first-order pole in the
quasi-2D Green's function. The oscillatory behavior is further validated
numerically, and its influence on collective behaviors of the confined
particles is studied via computer simulations. Interestingly, the substrate
permittivity alone can trigger spontaneous formations of lattice structures,
which may provide new insights in the study of Quasi-2D systems and the design
of future nanodevices. | cond-mat |
Self-motile colloidal particles: from directed propulsion to random walk: The motion of an artificial micro-scale swimmer that uses a chemical reaction
catalyzed on its own surface to achieve autonomous propulsion is fully
characterized experimentally. It is shown that at short times, it has a
substantial component of directed motion, with a velocity that depends on the
concentration of fuel molecules. At longer times, the motion reverts to a
random walk with a substantially enhanced diffusion coefficient. Our results
suggest strategies for designing artificial chemotactic systems. | cond-mat |
How Silicon and Boron Dopants Govern the Cryogenic Scintillation
Properties of N-type GaAs: This paper is the first report describing how the concentrations of silicon
and boron govern the cryogenic scintillation properties of n-type GaAs. It
shows that valence band holes are promptly trapped on radiative centers and
then combine radiatively with silicon donor band electrons at rates that
increase with the density of free carriers. It also presents the range of
silicon and boron concentrations needed for efficient light emission under
X-ray excitation, which along with its low band gap and apparent absence of
afterglow, make scintillating GaAs suitable for the detection of rare,
low-energy electronic excitations from interacting dark matter particles. A
total of 29 samples from four different suppliers were studied. Luminosities
and timing responses were measured for the four principal emission bands
centered at 860, 930, 1070, and 1335 nm, and for the total emissions.
Excitation pulses of 40 kVp X-rays were provided by a light-excited X-ray tube
driven by an ultra-fast laser. Scintillation emissions from 800 to 1350 nm were
measured using an InGaAs photomultiplier. Within the concentration ranges of
free carriers from 2 x 10^16/cm3 to 6 x 10^17/cm3 and boron from 1.5 x
10^18/cm3 to 6 x 10^18/cm3, nine samples have luminosities > 70 photons/keV and
two have luminosities > 110 photons/keV. Other samples in that range have lower
luminosities due to higher concentrations of non-radiative centers. The decay
times decrease by typically a factor of ten with increasing free carrier
concentrations from 10^17/cm3 to 2 x 10^18/cm3. | cond-mat |
Superfluid Turbulence in the Kelvin Wave Cascade Regime: Theoretical considerations are made of superfluid turbulence in the Kelvin
wave cascade regime at low temperatures (T < 1K) and length scales of the order
or smaller than the intervortical distance. The energy spectrum is shown to be
in accord with the Kolmogorov scaling. The vortex line decay equation is shown
to have an underlying Hamiltonian framework. Effects of spatial intermittency
(exhibited in laboratory experiments) on superfluid turbulence are incorporated
via the fractal nature of the vortex lines, for length scales of the order or
smaller than the intervortical distance. The spatial intermittency effects are
shown to enhance the vortex line density L, for a given value of intervortex
spacing L, and to provide for a mechanism commensurate with the enhanced
depolarization of vortex lines. The spatial intermittency is found to steepen
the energy spectrum in qualitative agreement with laboratory experiments and to
enhance vortex line decay. | cond-mat |
Computational Pipeline to probe NaV1.7 gain-of-functions variants in
neuropathic painful syndromes: Applications of machine learning and graph theory techniques to neuroscience
have witnessed an increased interest in the last decade due to the large data
availability and unprecedented technology developments. Their employment to
investigate the effect of mutational changes in genes encoding for proteins
modulating the membrane of excitable cells, whose biological correlates are
assessed at electrophysiological level, could provide useful predictive clues.
We apply this concept to the analysis of variants in sodium channel NaV1.7
subunit found in patients with chronic painful syndromes, by the implementation
of a dedicated computational pipeline empowering different and complementary
techniques including homology modeling, network theory, and machine learning.
By testing three templates of different origin and sequence identities, we
provide an optimal condition for its use. Our findings reveal the usefulness of
our computational pipeline in supporting the selection of candidates for cell
electrophysiology assay and with potential clinical applications. | cond-mat |
Electrophoresis of active Janus particles: We theoretically consider the dynamics of a self-propelled active Janus motor
moving in an external electric field. The external field can manipulate the
route of a Janus particle and enforce it to move towards the desired targets.
To investigate the trajectory of this active motor, we use a perturbative
scheme. At the leading orders of surface activity of the Janus particle and
also the external field, the orientational dynamics of the Janus particles
behave like a mathematical pendulum with an angular the velocity that is
sensitive to both the electric field and surface activity of the motor. | cond-mat |
Silicon in the Quantum Limit: Quantum Computing and Decoherence in
Silicon Architectures: Semiconductor architectures hold promise for quantum information processing
(QIP) applications due to their large industrial base and perceived scalability
potential. Electron spins in silicon in particular may be an excellent
architecture for QIP and also for spin electronics (spintronics) applications.
While the charge of an electron is easily manipulated by charged gates, the
spin degree of freedom is well isolated from charge fluctuations. Inherently
small spin-orbit coupling and the existence of a spin-zero Si isotope
facilitate long single spin qubit coherence times. Here we consider the
relaxation properties of localized electronic states in silicon due to donors,
quantum wells, and quantum dots, including effects due to phonons and Rashba
spin-orbit coupling. Our analysis is impeded by the complicated, many-valley
band structure of silicon and previously unaddressed physics in silicon quantum
wells. We find that electron spins in silicon and especially strained silicon
have excellent decoherence properties. Where possible we compare with
experiment to test our theories. We go beyond issues of coherence in a quantum
computer to problems of control and measurement. Precisely what makes spin
relaxation so long in semiconductor architectures makes spin measurement so
difficult. To address this, we propose a new scheme for spin readout which has
the added benefit of automatic spin initialization, a vital component of
quantum computing and quantum error correction. Our results represent important
practical milestones on the way to the design and construction of a
silicon-based quantum computer. | cond-mat |
Optical absorption of non-interacting tight-binding electrons in a
Peierls-distorted chain at half band-filling: In this first of three articles on the optical absorption of electrons in
half-filled Peierls-distorted chains we present analytical results for
non-interacting tight-binding electrons. We carefully derive explicit
expressions for the current operator, the dipole transition matrix elements,
and the optical absorption for electrons with a cosine dispersion relation of
band width $W$ and dimerization parameter $\delta$. New correction
(``$\eta$''-)terms to the current operator are identified. A broad band-to-band
transition is found in the frequency range $W\delta < \omega < W$ whose shape
is determined by the joint density of states for the upper and lower Peierls
subbands and the strong momentum dependence of the transition matrix elements. | cond-mat |
On various levels of deterministic toy models for the Richardson cascade
in turbulence: The Desnyanski-Novikov shell model is a deterministic dynamical model for
scalar velocities $v_t(n)$ defined on the one-dimensional-lattice $n=0,1,2,..$
labelling the length-scales $l_n=l_0 2^{-n}$, in order to describe the cascade
of energy from the biggest scale where it is injected by some external forcing
towards the smaller scales where it is dissipated by viscosity. We describe the
generalization of this model in two directions : (i) the
one-dimensional-lattice $n=0,1,2,..$ labelling the length-scales $l_n=l_0
2^{-n}$ is replaced by a scale-spatial tree structure of nested cells in order
to allow spatial heterogeneities between different coherent structures that are
localized in different regions of the whole volume ; (ii) the scalar velocities
$v_t(n)$ are replaced by 3D-vector velocities in order to take into account the
vorticity in the dynamical equations and to include vortex-stretching effects. | cond-mat |
Low-dimensional antiferromagnetic fluctuations in the heavy-fermion
paramagnetic ladder UTe$_2$: Inelastic-neutron-scattering measurements were performed on a single crystal
of the heavy-fermion paramagnet UTe$_2$ above its superconducting temperature.
We confirm the presence of antiferromagnetic fluctuations with the
incommensurate wavevector $\mathbf{k}_1=(0,0.57,0)$. A quasielastic signal is
found, whose momentum-transfer dependence is compatible with fluctuations of
magnetic moments $\mu\parallel\mathbf{a}$, with a sine-wave modulation of
wavevector $\mathbf{k}_1$ and in-phase moments on the nearest U atoms. Low
dimensionality of the magnetic fluctuations, consequence of the ladder
structure, is indicated by weak correlations along the direction $\mathbf{c}$.
These fluctuations saturate below the temperature $T_1^*\simeq15$~K, in
possible relation with anomalies observed in thermodynamic,
electrical-transport and nuclear-magnetic-resonance measurements. The absence
or weakness of ferromagnetic fluctuations, in our data collected at
temperatures down to 2.1 K and energy transfers from 0.6 to 7.5 meV, is
emphasized. These results constitute constraints for models of
magnetically-mediated superconductivity in UTe$_2$. | cond-mat |
Scale invariance and universality of force networks in static granular
matter: Force networks form the skeleton of static granular matter. They are the key
ingredient to mechanical properties, such as stability, elasticity and sound
transmission, which are of utmost importance for civil engineering and
industrial processing. Previous studies have focused on the global structure of
external forces (the boundary condition), and on the probability distribution
of individual contact forces. The disordered spatial structure of the force
network, however, has remained elusive so far. Here we report evidence for
scale invariance of clusters of particles that interact via relatively strong
forces. We analyzed granular packings generated by molecular dynamics
simulations mimicking real granular matter; despite the visual variation, force
networks for various values of the confining pressure and other parameters have
identical scaling exponents and scaling function, and thus determine a
universality class. Remarkably, the flat ensemble of force configurations--a
simple generalization of equilibrium statistical mechanics--belongs to the same
universality class, while some widely studied simplified models do not. | cond-mat |
Phase Diagram and Snap-Off Transition for a Twisted Party Balloon: All children enjoy inflating balloons and twisting them into different shapes
and animals. Snapping the balloon into two separate compartments is a necessary
step that bears resemblance to the pinch-off phenomenon for water droplet
detached from the faucet. In addition to testing whether balloons exhibit the
properties of self-similarity and memory effect that are often associated with
the latter event, we determine their phase diagram by experiments. It turns out
that a common party balloon does not just snap. They in fact can assume five
more shapes, i.e., straight, necking, wrinkled, helix, and supercoil, depending
on the twist angle and ratio of its length and diameter. Moreover, history also
matters due to their prominent hysteresis. One may shift the phase boundary
or/and reshuffle the phases by untwisting or lengthening the balloon at
different twist angle and initial length. Heuristic models are provided to
obtain analytic expressions for the phase boundaries. | cond-mat |
Tunneling Spin Injection into Single Layer Graphene (Supplementary
Information): We achieve tunneling spin injection from Co into single layer graphene (SLG)
using TiO2 seeded MgO barriers. A non-local magnetoresistance ({\Delta}RNL) of
130 {\Omega} is observed at room temperature, which is the largest value
observed in any material. Investigating {\Delta}RNL vs. SLG conductivity from
the transparent to the tunneling contact regimes demonstrates the contrasting
behaviors predicted by the drift-diffusion theory of spin transport.
Furthermore, tunnel barriers reduce the contact-induced spin relaxation and are
therefore important for future investigations of spin relaxation in graphene. | cond-mat |
Single crystal study of the layered heavy fermion compounds
Ce$_2$PdIn$_8$, Ce$_3$PdIn$_{11}$, Ce$_2$PtIn$_8$ and Ce$_3$PtIn$_{11}$: We report on single crystal growth and crystallographic parameters results of
Ce$_2$PdIn$_8$, Ce$_3$PdIn$_{11}$, Ce$_2$PtIn$_8$ and Ce$_3$PtIn$_{11}$. The
Pt-systems Ce$_2$PtIn$_8$ and Ce$_3$PtIn$_{11}$ are synthesized for the first
time. All these compounds are member of the Ce$_n$T$_m$In$_{3n+2m}$ (n = 1,
2,..; m = 1, 2,.. and T = transition metal) to which the extensively studied
heavy fermion superconductor CeCoIn$_5$ belongs. Single crystals have been
grown by In self-flux method. Differential scanning calorimetry studies were
used to derive optimal growth conditions. Evidently, the maximum growth
conditions for these materials should not exceed 750 $^{\circ}$C. Single
crystal x-ray data show that Ce$_2$TIn$_8$ compounds crystallize in the
tetragonal Ho$_2$CoGa$_8$ phase (space group P4/mmm) with lattice parameters a
=4.6898(3) $\AA$ and c =12.1490(8) $\AA$ for the Pt-based one (Pd: a =
4.6881(4) $\AA$ and c = 12.2031(8) \AA). The Ce$_3$TIn$_{11}$ compounds adopt
the Ce$_3$PdIn$_{11}$ structure with a = 4.6874(4) $\AA$ and c = 16.8422(12)
$\AA$ for the Pt-based one (Pd: a = 4.6896 $\AA$ and c = 16.891 \AA). Specific
heat experiments on Ce$_3$PtIn$_{11}$ and Ce$_3$PdIn$_{11}$ have revealed that
both compounds undergo two successive magnetic transitions at T$_1$ ~ 2.2 K
followed by T$_N$ ~ 2.0 K and T$_1$ ~ 1.7 K and T$_N$ ~ 1.5 K, respectively.
Additionally, both compounds exhibit enhanced Sommerfeld coefficients yielding
{\gamma}$_{Pt}$ = 0.300 J/mol K$^2$ Ce ({\gamma}$_{Pd}$ = 0.290 J/mol K$^2$
Ce), hence qualifying them as heavy fermion materials. | cond-mat |
Magnetization Plateaux in Bethe Ansatz Solvable Spin-S Ladders: We examine the properties of the Bethe Ansatz solvable two- and three-leg
spin-$S$ ladders. These models include Heisenberg rung interactions of
arbitrary strength and thus capture the physics of the spin-$S$ Heisenberg
ladders for strong rung coupling. The discrete values derived for the
magnetization plateaux are seen to fit with the general prediction based on the
Lieb-Schultz- Mattis theorem. We examine the magnetic phase diagram of the
spin-1 ladder in detail and find an extended magnetization plateau at the
fractional value $<M > = {1/2}$ in agreement with the experimental observation
for the spin-1 ladder compound BIP-TENO. | cond-mat |
Study of off-diagonal disorder using the typical medium dynamical
cluster approximation: We generalize the typical medium dynamical cluster approximation (TMDCA) and
the local Blackman, Esterling, and Berk (BEB) method for systems with
off-diagonal disorder. Using our extended formalism we perform a systematic
study of the effects of non-local disorder-induced correlations and of
off-diagonal disorder on the density of states and the mobility edge of the
Anderson localized states. We apply our method to the three-dimensional
Anderson model with configuration dependent hopping and find fast convergence
with modest cluster sizes. Our results are in good agreement with the data
obtained using exact diagonalization, and the transfer matrix and kernel
polynomial methods. | cond-mat |
Two-Particle Self-Consistent Approach to Anisotropic Superconductivity: A nonperturbative approach to anisotropic superconductivity is developed
based on the idea of two-particle self-consistent (TPSC) theory by Vilk and
Tremblay. A sum-rule which the momentum-dependent pairing susceptibility
satisfies is derived. An effective pairing interaction between quasiparticles
is determined so that the susceptibility should fulfill this exact sum-rule, in
which fluctuations belonging to different symmetries couple at finite momentum.
It is demonstrated that the mode coupling between d-wave and s-wave pairing
fluctuations leads to suppression of the d-wave fluctuation near the Mott
insulator. | cond-mat |
Chiral-symmetric Topological Origin of Nonlinear Fixed Points: Particle-hole symmetry and chiral symmetry play a pivotal role in multiple
areas of physics, yet they remain un-studied in systems with nonlinear
interactions that are beyond Kerr-type. Here, we establish these two
non-spatial symmetries in systems with strong and general nonlinear
interactions. Chiral symmetry ensures the quantization of the Berry phase of
nonlinear normal modes and categorizes the topological phases of nonlinear
dynamics. We show edge modes that serve as topologically protected fixed points
of chiral-symmetric nonlinear dynamics. Our theoretical framework paves the way
towards the topological classification of general nonlinear dynamics. | cond-mat |
Competing magnetic fluctuations and orders in a multiorbital model of
doped SrCo$_2$As$_2$: We revisit the intriguing magnetic behavior of the paradigmatic itinerant
frustrated magnet $\rm{Sr}\rm{Co}_2\rm{As}_2$, which shows strong and competing
magnetic fluctuations yet does not develop long-range magnetic order. By
calculating the static spin susceptibility $\chi(\mathbf{q})$ within a
realistic sixteen orbital Hubbard-Hund model, we determine the leading
instability to be ferromagnetic (FM). We then explore the effect of doping and
calculate the critical Hubbard interaction strength $U_c$ that is required for
the development of magnetic order. We find that $U_c$ decreases under electron
doping and with increasing Hund's coupling $J$, but increases rapidly under
hole doping. This suggests that magnetic order could possibly emerge under
electron doping but not under hole doping, which agrees with experimental
findings. We map out the leading magnetic instability as a function of doping
and Hund's coupling and find several antiferromagnetic phases in addition to
FM. We also quantify the degree of itinerant frustration in the model and
resolve the contributions of different orbitals to the magnetic susceptibility.
Finally, we discuss the dynamic spin susceptibility, $\chi(\mathbf{q},
\omega)$, at finite frequencies, where we recover the anisotropy of the peaks
at $\mathbf{Q}_\pi = (\pi, 0)$ and $(0, \pi)$ observed by inelastic neutron
scattering that is associated with the phenomenon of itinerant magnetic
frustration. By comparing results between theory and experiment, we conclude
that the essential experimental features of doped SrCo$_2$As$_2$ are well
captured by a Hubbard-Hund multiorbital model if one considers a small shift of
the chemical potential towards hole doping. | cond-mat |
Quantum weight: We introduce the concept of quantum weight as a fundamental property of
insulating states of matter that is encoded in the ground-state static
structure and measures quantum fluctuation in electrons' center of mass. We
find a sum rule that directly relates quantum weight -- a ground state property
-- with the negative-first moment of the optical conductivity above the gap
frequency. Building on this connection to optical absorption, we derive both an
upper bound and a lower bound on quantum weight in terms of electron density,
dielectric constant, and energy gap. Therefore, quantum weight constitutes a
key material parameter that can be experimentally determined from X-ray
scattering. | cond-mat |
Zeno and anti-Zeno dynamics in spin-bath models: We investigate the quantum Zeno and anti-Zeno effects in spin bath models:
the spin-boson model and a spin-fermion model. We show that the Zeno-anti-Zeno
transition is critically controlled by the system-bath coupling parameter, the
same parameter that determines spin decoherence rate. We also discuss the
crossover in a biased system, at high temperatures, and for a nonequilibrium
spin-fermion system, manifesting the counteracting roles of electrical bias,
temperature, and magnetic field on the spin decoherence rate. | cond-mat |
The nature of the short wavelength excitations in vitreous silica:
X-Rays Brillouin scattering study: The dynamical structure factor (S(Q,E)) of vitreous silica has been measured
by Inelastic X-ray Scattering varying the exchanged wavevector (Q) at fixed
exchanged energy (E) - an experimental procedure that, contrary to the usual
one at constant Q, provides spectra with much better identified inelastic
features. This allows the first direct evidence of Brillouin peaks in the
S(Q,E) of SiO_2 at energies above the Boson Peak (BP) energy, a finding that
excludes the possibility that the BP marks the transition from propagating to
localised dynamics in glasses. | cond-mat |
Electron-phonon coupling of Fe-adatom electron states on MgO/Ag(100): We study the strength of the electron-phonon interaction on Fe single adatoms
on MgO/Ag(100) based on many-body \textit{ab-initio} spin collinear
calculations. In particular, we analyze the relative importance of the
substrate and, among other results, we conclude that the interface electron
state of Ag(100) plays a prominent role in determining the electron-phonon
coupling of localized Fe electron states. The analysis of the hybridization of
the adatom with the substrate reveals qualitative differences for even or odd
coverages of MgO, affecting significantly the spectral structure and strength
of the electron-phonon coupling. Our calculations indicate that the
electron-phonon interaction is very strong for $\le$~1 layers of MgO, while it
is sharply suppressed for larger coverages, a trend that is consistent with
recent experimental findings. | cond-mat |
From Luttinger liquid to Mott insulator: the correct low-energy
description of the one-dimensional Hubbard model by an unbiased variational
approach: We show that a particular class of variational wave functions reproduces the
low-energy properties of the Hubbard model in one dimension. Our approach
generalizes to finite on-site Coulomb repulsion the fully-projected wave
function proposed by Hellberg and Mele [Phys. Rev. Lett. {\bf 67}, 2080 (1991)]
for describing the Luttinger-liquid behavior of the doped $t{-}J$ model. Within
our approach, the long-range Jastrow factor emerges from a careful minimization
of the energy, without assuming any parametric form for the long-distance tail.
Specifically, in the conducting phase of the Hubbard model at finite hole
doping, we obtain the correct power-law behavior of the correlation functions,
with the exponents predicted by the Tomonaga-Luttinger theory. By decreasing
the doping, the insulating phase is reached with a continuous change of the
small-$q$ part of the Jastrow factor. | cond-mat |
Noncommutative generalized Gibbs ensemble in isolated integrable quantum
systems: The generalized Gibbs ensemble (GGE), which involves multiple conserved
quantities other than the Hamiltonian, has served as the statistical-mechanical
description of the long-time behavior for several isolated integrable quantum
systems. The GGE may involve a noncommutative set of conserved quantities in
view of the maximum entropy principle, and show that the GGE thus generalized
(noncommutative GGE, NCGGE) gives a more qualitatively accurate description of
the long-time behaviors than that of the conventional GGE. Providing a clear
understanding of why the (NC)GGE well describes the long-time behaviors, we
construct, for noninteracting models, the exact NCGGE that describes the
long-time behaviors without an error even at finite system size. It is
noteworthy that the NCGGE involves nonlocal conserved quantities, which can be
necessary for describing long-time behaviors of local observables. We also give
some extensions of the NCGGE and demonstrate how accurately they describe the
long-time behaviors of few-body observables. | cond-mat |
Pseudofermion ferromagnetism in the Kondo lattices: a mean-field
approach: Ground state ferromagnetism of the Kondo lattices is investigated within
slave fermion approach by Coleman and Andrei within a mean-field approximation
in the effective hybridization model. Conditions for formation of both
saturated (half-metallic) and non-saturated magnetic state are obtained for
various lattices. A description in terms of universal functions which depend
only on bare electron density of states (DOS) is presented. A crucial role of
the energy dependence of the bare DOS (especially, of DOS peaks) for the
small-moment ferromagnetism formation is demonstrated. | cond-mat |
Electron refrigeration in hybrid structures with spin-split
superconductors: Electron tunneling between superconductors and normal metals has been used
for an efficient refrigeration of electrons in the latter. Such cooling is a
non-linear effect and usually requires a large voltage. Here we study the
electron cooling in heterostructures based on superconductors with a
spin-splitting field coupled to normal metals via spin-filtering barriers. The
cooling power shows a linear term in the applied voltage. This improves the
coefficient of performance of electron refrigeration in the normal metal by
shifting its optimum cooling to lower voltage, and also allows for cooling the
spin-split superconductor by reverting the sign of the voltage. We also show
how tunnel coupling spin-split superconductors with regular ones allows for a
highly efficient refrigeration of the latter. | cond-mat |
Relaxational dynamics in 3D randomly diluted Ising models: We study the purely relaxational dynamics (model A) at criticality in
three-dimensional disordered Ising systems whose static critical behaviour
belongs to the randomly diluted Ising universality class. We consider the
site-diluted and bond-diluted Ising models, and the +- J Ising model along the
paramagnetic-ferromagnetic transition line. We perform Monte Carlo simulations
at the critical point using the Metropolis algorithm and study the dynamic
behaviour in equilibrium at various values of the disorder parameter. The
results provide a robust evidence of the existence of a unique model-A dynamic
universality class which describes the relaxational critical dynamics in all
considered models. In particular, the analysis of the size-dependence of
suitably defined autocorrelation times at the critical point provides the
estimate z=2.35(2) for the universal dynamic critical exponent. We also study
the off-equilibrium relaxational dynamics following a quench from T=\infty to
T=T_c. In agreement with the field-theory scenario, the analysis of the
off-equilibrium dynamic critical behavior gives an estimate of z that is
perfectly consistent with the equilibrium estimate z=2.35(2). | cond-mat |
Quantum critical phenomena of long-range interacting bosons in a
time-dependent random potential: We study the superfluid-insulator transition of a particle-hole symmetric
system of long-range interacting bosons in a time-dependent random potential in
two dimensions, using the momentum-shell renormalization-group method. We find
a new stable fixed point with non-zero values of the parameters representing
the short- and long-range interactions and disorder when the interaction is
asymptotically logarithmic. This is contrasted to the non-random case with a
logarithmic interaction, where the transition is argued to be first-order, and
to the $1/r$ Coulomb interaction case, where either a first-order transition or
an XY-like transition is possible depending on the parameters. We propose that
our model may be relevant in studying the vortex liquid-vortex glass transition
of interacting vortex lines in point-disordered type-II superconductors. | cond-mat |
Integrable multiparametric quantum spin chains: Using Reshetikhin's construction for multiparametric quantum algebras we
obtain the associated multiparametric quantum spin chains. We show that under
certain restrictions these models can be mapped to quantum spin chains with
twisted boundary conditions. We illustrate how this general formalism applies
to construct multiparametric versions of the supersymmetric t-J and U models. | cond-mat |
Modeling Short-Range and Three-Membered Ring Structures in Lithium
Borosilicate Glasses using Machine Learning Potential: Lithium borosilicate (LBS) glass is a prototypical lithium-ion conducting
oxide glasses available for an all-solid state buttery. Nevertheless, the
atomistic modeling of LBS glass using $ab$ $initio$ (AIMD) and classical
molecular dynamics (CMD) simulations have critical limitations due to
computational cost and inaccuracy in reproducing the glass microstructures,
respectively. To overcome these difficulties, a machine-learning potential
(MLP) was examined in this work for modeling LBS glasses using DeepMD. The
glass structures obtained by this MLP possessed fourhold-coordinated boron
($^4$B) confirmed well with the experimental data and abundance of
three-membered rings. The models were energetically more stable compared with
those constructed with a functional force-field even though both the models
included reasonable $^4$B. The results confirmed MLP to be superior to model
the boron-containing glasses and address the inherent shortcomings of the AIMD
and CMD. This study also discusses some limitations of MLP for modeling
glasses. | cond-mat |
Dynamical Properties of an Antiferromagnet near the Quantum Critical
Point: Application to LaCuO_2.5: For a system of two-chain spin ladders, the ground state for weak interladder
coupling is the spin-liquid state of the isolated ladder, but is an ordered
antiferromagnet (AF) for sufficiently large interactions. We generalize the
bond-operator mean-field theory to describe both regimes, and to focus on the
transition between them. In the AF phase near the quantum critical point (QCP)
we find both spin waves and a low-lying but massive amplitude mode which is
absent in a conventional AF. The static susceptibility has the form $\chi(T) =
\chi_0 + a T^2$, with $\chi_0$ small for a system near criticality. We consider
the dynamical properties to examine novel features due to the presence of the
amplitude mode, and compute the dynamic structure factor. LaCuO$_{2.5}$ is
thought to be such an unconventional AF, whose ordered phase is located very
close to the QCP of the transition to the spin liquid. From the N\'eel
temperature we deduce the interladder coupling, the small ordered moment and
the gap in the amplitude mode. The dynamical properties unique to near-critical
AFs are expected to be observable in LaCuO$_{2.5}$. | cond-mat |
Halogen in Materials Design: Revealing the Nature of Hydrogen Bonding
and Other Non-Covalent Interactions in the Polymorphic Transformations of
Methylammonium Lead Tribromide Perovskite: Methylammonium lead tribromide perovskite (CH3NH3PbBr3, or MAPbBr3) as a
photovoltaic material has attracted a great deal of recent interest. Factors
that are important in their application in optoelectronic devices include their
fractional contribution of the composition of the materials as well as their
microscopic arrangement that is responsible for the formation of well-defined
macroscopic structures. CH3NH3PbBr3 assumes different polymorphs (orthorhombic,
tetragonal and cubic) depending on the evolution temperature of the bulk
material. Density functional theory calculations have been performed on
polymorphs of CH3NH3PbBr3 to demonstrate that the H atoms on C of the methyl
group in MA entrapped within a MAPbBr3 perovskite cage are not electronically
innocent, as is often contended. We show here that these H atoms are involved
in attractive interactions with the surrounding bromides of corner-sharing
octahedra of the CH3NH3PbBr3 cage to form Br...H(-C) hydrogen bonding
interactions. This is analogous to the way the H atoms on N of the ammonium
group in MA form Br...H(-N) hydrogen bonding interactions to stabilize the
structure of CH3NH3PbBr3. Both these hydrogen bonding interactions are shown to
persist regardless of the nature of the three polymorphic forms of CH3NH3PbBr3.
These, together with the Br...C(-N) carbon bonding, the Br...N(-C) pnictogen
bonding, and the Br...Br lump-hole type intermolecular non-covalent
interactions identified for the first time in this study, are shown to be
collectively responsible for the eventual emergence of the orthorhombic
geometry of the CH3NH3PbBr3 system. These conclusions are arrived at from a
systematic analysis of the results obtained from combined DFT, Quantum Theory
of Atoms in Molecules, and Reduced Density Gradient Non-Covalent Interaction
calculations carried out on the three temperature-dependent polymorphic
geometries of CH3NH3PbBr3. | cond-mat |
Application of the finite-temperature Lanczos method for the evaluation
of magnetocaloric properties of large magnetic molecules: We discuss the magnetocaloric properties of gadolinium containing magnetic
molecules which potentially could be used for sub-Kelvin cooling. We show that
a degeneracy of a singlet ground state could be advantageous in order to
support adiabatic processes to low temperatures and simultaneously minimize
disturbing dipolar interactions. Since the Hilbert spaces of such spin systems
assume very large dimensions we evaluate the necessary thermodynamic
observables by means of the Finite-Temperature Lanczos Method. | cond-mat |
Optical and electrical properties of Nd3+doped Na2O-ZnO-TeO2 Material: Neodymium doped Na2O-ZnO-TeO2 (NZT) glasses were prepared by the conventional
melt quenching technique. DTA and TG were used to confirmation of glass
preparation through the glass transition temperature at 447{\deg}C for the
glass system. The analysis of FTIR spectra and X-ray diffraction described the
nature of the samples were ionic and amorphous respectively. The optical
bandgap energy was estimated using absorption spectra and found to be decreased
from 2.63eV to 1.32 eV due to the increase of doping concentration. The
intensity of the emission spectra was enhanced for the higher concentration of
Nd3+ ions. The dielectric constant of the glass samples was found to be
constant for the large range of frequency (3 kHz to 1 MHz). The variation of
conductivity with the temperature of the samples had shown the Arrhenius
mechanism of conduction. | cond-mat |
Potts model: Duality, Uniformization and the Seiberg-Witten modulus: The introduction of a modulus z(K), analogous to u=<tr phi^2> in the N=2 SUSY
SU(2) gauge theory solved by Seiberg and Witten, and whose defining property is
the invariance under the symmetry and duality transformations of the effective
coupling K, reveals an intriguing correspondence between the D=2 Ising and
Potts models on the square lattice. The moduli spaces of both models, the
spaces of inequivalent effective temperatures K, correspond to a
three-punctured sphere M_3=P^1(C)\{z=+1,-1,\infty}. Furthermore, in both
models, the locus of Fisher zeroes is given by the segment joining z_c=-1 to
z_c=+1. | cond-mat |
On the critical weight statistics of the Random Energy Model and of the
Directed Polymer on the Cayley Tree: We consider the critical point of two mean-field disordered models : (i) the
Random Energy Model (REM), introduced by Derrida as a mean-field spin-glass
model of $N$ spins (ii) the Directed Polymer of length $N$ on a Cayley Tree
(DPCT) with random bond energies. Both models are known to exhibit a freezing
transition between a high temperature phase where the entropy is extensive and
a low-temperature phase of finite entropy. In this paper, we study the weight
statistics at criticality via the entropy $S=-\sum w_i \ln w_i$ and the
generalized moments $Y_k=\sum w_i^k$, where the $w_i$ are the Boltzmann weights
of the $2^N$ configurations. In the REM, we find that the critical weight
statistics is governed by the finite-size exponent $\nu=2$ : the entropy scales
as $\bar{S}_N(T_c) \sim N^{1/2}$, the typical values $e^{\bar{\ln Y_k}}$ decay
as $N^{-k/2}$, and the disorder-averaged values $\bar{Y_k}$ are governed by
rare events and decay as $N^{-1/2}$ for any $k>1$. For the DPCT, we find that
the entropy scales similarly as $\bar{S}_N(T_c) \sim N^{1/2}$, whereas another
exponent $\nu'=1$ governs the $Y_k$ statistics : the typical values
$e^{\bar{\ln Y_k}}$ decay as $N^{-k}$, the disorder-averaged values $\bar{Y_k}$
decay as $N^{-1}$ for any $k>1$. As a consequence, the asymptotic probability
distribution $\bar{\pi}_{N=\infty}(q)$ of the overlap $q$, beside the delta
function $\delta(q)$ which bears the whole normalization, contains an isolated
point at $q=1$, as a memory of the delta peak $(1-T/T_c) \delta(q-1)$ of the
low-temperature phase $T<T_c$. The associated value $\bar{\pi}_{N=\infty}(q=1)$
is finite for the DPCT, and diverges as $\bar{\pi}_{N=\infty}(q=1) \sim
N^{1/2}$ for the REM. | cond-mat |
Electron flow in split-gated bilayer graphene: We present transport measurements on a bilayer graphene sheet with
homogeneous back gate and split top gate. The electronic transport data
indicates the capability to direct electron flow through graphene
nanostructures purely defined by electrostatic gating. By comparing the
transconductance data recorded for different top gate geometries - continuous
barrier and split-gate - the observed transport features for the split-gate can
be attributed to interference effects inside the narrow opening. | cond-mat |
The role of multiplicative noise in critical dynamics: We study the role of multiplicative stochastic processes in the description
of the dynamics of an order parameter near a critical point. We study
equilibrium, as well as, out-of-equilibrium properties. By means of a
functional formalism, we built the Dynamical Renormalization Group equations
for a real scalar order parameter with $Z_2$ symmetry, driven by a class o
multiplicative stochastic processes with the same symmetry. We have computed
the flux diagram, using a controlled $\epsilon$-expansion, up to order
$\epsilon^2$. We have found that, for dimensions $d=4-\epsilon$, the additive
dynamic fixed point is unstable. The flux runs to a {\em multiplicative fixed
point} driven by a diffusion function $G(\phi)=1+g^*\phi^2({\bf x})/2$, where
$\phi$ is the order parameter and $g^*=\epsilon^2/18$ is the fixed point value
of the multiplicative noise coupling constant. We show that, even though the
position of the fixed point depends on the stochastic prescription, the
critical exponents do not. Therefore, different dynamics driven by different
stochastic prescriptions (such as It\^o, Stratonovich, anti-It\^o and so on)
are in the same universality class. | cond-mat |
Anisotropic Gilbert damping in perovskite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$
thin film: The viscous Gilbert damping parameter governing magnetization dynamics is of
primary importance for various spintronics applications. Although, the damping
constant is believed to be anisotropic by theories. It is commonly treated as a
scalar due to lack of experimental evidence. Here, we present an elaborate
angle dependent broadband ferromagnetic resonance study of high quality
epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ films. Extrinsic effects are suppressed
and we show convincing evidence of anisotropic damping with twofold symmetry at
room temperature. The observed anisotropic relaxation is attributed to the
magnetization orientation dependence of the band structure. In addition, we
demonstrated that such anisotropy can be tailored by manipulating the stain.
This work provides new insights to understand the mechanism of magnetization
relaxation. | cond-mat |
Kinematically constrained vortex dynamics in charge density waves: We build a minimal model of dissipative vortex dynamics in two spatial
dimensions, subject to a kinematic constraint: dipole conservation. The
additional conservation law implies anomalously slow decay rates for vortices.
We argue that this model of vortex dynamics is relevant for a broad range of
time scales during a quench into a uniaxial charge density wave state. Our
predictions are consistent with recent experiments on uniaxial charge density
wave formation in $\mathrm{LaTe}_3$. | cond-mat |
Spin correlation functions in random-exchange s=1/2 XXZ chains: The decay of (disorder-averaged) static spin correlation functions at T=0 for
the one-dimensional spin-1/2 XXZ antiferromagnet with uniform longitudinal
coupling $J\Delta$ and random transverse coupling $J\lambda_i$ is investigated
by numerical calculations for ensembles of finite chains. At $\Delta=0$ (XX
model) the calculation is based on the Jordan-Wigner mapping to free lattice
fermions for chains with up to N=100 sites. At $\Delta \neq 0$ Lanczos
diagonalizations are carried out for chains with up to N=22 sites. The
longitudinal correlation function $<S_0^z S_r^z>$ is found to exhibit a
power-law decay with an exponent that varies with $\Delta$ and, for nonzero
$\Delta$, also with the width of the $\lambda_i$-distribution. The results for
the transverse correlation function $<S_0^x S_r^x>$ show a crossover from
power-law decay to exponential decay as the exchange disorder is turned on. | cond-mat |
Electronic and magnetic properties of the 2H-NbS$_2$ intercalated by 3d
transition metal atoms: The electronic structure and magnetic properties of the 2H-NbS$_2$ compound
intercalated by Cr, Mn and Fe, have been investigated by means of the
Korringa-Kohn-Rostoker (KKR) method. The calculations demonstrate easy plane
magneto-crystalline anisotropy (MCA) of Cr$_{1/3}$NbS$_2$ monotonously
decreasing towards the Curie temperature in line with the experimental results.
The modification of the electronic structure results in a change of the easy
axis from in-plane to out-of-plane. It is shown, that for Cr$_{1/3}$NbS$_2$ and
Mn$_{1/3}$NbS$_2$ the in-plane MCA and Dzyaloshinskii-Moriya interactions
results in a helimagnetic structure along the hexagonal $c$ axis, following the
experimental observations. The negative exchange interactions in the
Fe$_{1/3}$NbS$_2$ compound results in a non-collinear frustrated magnetic
structure if the MCA is not taken into account. It is shown, however, that a
strong MCA along the hexagonal $c$ axis leads to a magnetic ordering referred
to as an ordering of the third kind, which was observed experimentally. | cond-mat |
Rotation of the Trajectories of Bright soliton and Realignment of
Intensity Distribution in the Coupled Nonlinear Schrodinger Equation: We revisit the collisional dynamics of bright solitons in the coupled
Nonlinear Schrodinger equation. We observe that apart from the intensity
redistribution in the interaction of bright solitons, one also witnesses a
rotation of the trajectories of bright solitons . The angle of rotation can be
varied by suitably manipulating the Self-Phase Modulation (SPM) or Cross Phase
Modulation (XPM) parameters.The rotation of the trajectories of the bright
solitons arises due to the excess energy that is injected into the dynamical
system through SPM or XPM. This extra energy not only contributes to the
rotation of the trajectories, but also to the realignment of intensity
distribution between the two modes. We also notice that the angular separation
between the bright solitons can also manouvred suitably. The above results
which exclude quantum superposition for the field vectors may have wider
ramifications in nonlinear optics, Bose-Einstein condensates, Left Handed (LH)
and Right Handed (RH) meta materials. | cond-mat |
Loss of control in pattern-directed nucleation: a theoretical study: The properties of template-directed nucleation are studied close to the
transition where full nucleation control is lost and additional nucleation
occurs beyond the pre-patterned regions. First, kinetic Monte Carlo simulations
are performed to obtain information on a microscopic level. Here the
experimentally relevant cases of 1D stripe patterns and 2D square lattice
symmetry are considered. The nucleation properties in the transition region
depend in a complex way on the parameters of the system, i.e. the flux, the
surface diffusion constant, the geometric properties of the pattern and the
desorption rate. Second, the properties of the stationary concentration field
in the fully controlled case are studied to derive the remaining nucleation
probability and thus to characterize the loss of nucleation control. Using the
analytically accessible solution of a model system with purely radial symmetry,
some of the observed properties can be rationalized. A detailed comparison to
the Monte Carlo data is included. | cond-mat |
Attosecond streaking of core lines of copper dihalides: In the attosecond (as) streaking of Cu 3s core-level photoemission of copper
dihalides, we predict theoretically that the satellite ($3d^9$) is emitted
later than the main line ($3d^{10}L^{-1}$; $L$: ligand). The emission time
delay is originated from the electron correlation between the core level and 3d
shell, which leads to the difference in core-hole screening between satellite
and main lines. Further, we find that the time delay corresponds to a
quantification of the extrinsic loss of photoemission. | cond-mat |
Properties and Origins of Protein Secondary Structure: Proteins contain a large fraction of regular, repeating conformations, called
secondary structure. A simple, generic definition of secondary structure is
presented which consists of measuring local correlations along the protein
chain. Using this definition and a simple model for proteins, the forces
driving the formation of secondary structure are explored. The relative role of
energy and entropy are examined. Recent work has indicated that compaction is
sufficient to create secondary structure. We test this hypothesis, using simple
non-lattice protein models. | cond-mat |
ab-plane tilt angles in REBCO conductors: Critical current (Ic) of REBCO tapes is strongly aniso-tropic with respect to
the orientation of the magnetic field. Usually, Ic is at maximum when the
ab-plane of the REBCO crystal is parallel to the magnetic field. In commercial
REBCO tapes, it is commonly assumed that the ab-plane is coincide with the tape
plane. While in fact, the ab-plane is near but slightly tilted from the tape
plane in the transverse direction. To accurately measure Ic as a function of
the field angle {\theta} , which is defined as the angle between ab-plane and
the magnetic field direction, and to design and fabricate REBCO mag-net coils
based on the measured Ic(angle), it is important to measure the tilt angle. In
this work, we used x-ray diffraction (XRD) to measure the tilt angles at room
temperature for a large number of REBCO conductors made by SuperPower Inc.
Transmission electron mi-croscopy (TEM) was also used to investigate the origin
of this tilt. The measured data are presented, and the measurement uncer-tainty
is discussed. | cond-mat |
Quantitative analysis of Sr2RuO4 ARPES spectra: Many-body interactions
in a model Fermi liquid: ARPES spectra hold a wealth of information about the many-body interactions
in a correlated material. However, the quantitative analysis of ARPES spectra
to extract the various coupling parameters in a consistent manner is extremely
challenging, even for a model Fermi liquid system. We propose a fitting
procedure which allows quantitative access to the intrinsic lineshape,
deconvolved of energy and momentum resolution effects, of the correlated
2-dimensional material Sr2RuO4. For the first time in correlated 2-dimensional
materials, we find an ARPES linewidth that is narrower than its binding energy,
a key property of quasiparticles within Fermi liquid theory. We also find that
when the electron-electron scattering component is separated from the
electron-phonon and impurity scattering terms it decreases with a functional
form compatible with Fermi liquid theory as the Fermi energy is approached. In
combination with the previously determined Fermi surface, these results give
the first complete picture of a Fermi liquid system via ARPES. Furthermore, we
show that the magnitude of the extracted imaginary part of the self-energy is
in remarkable agreement with DC transport measurements. | cond-mat |
Different ways of dealing with Compton scattering and positron
annihilation experimental data: Different ways of dealing with one-dimensional (1D) spectra, measured e.g. in
the Compton scattering or angular correlation of positron annihilation
radiation (ACAR) experiments are presented. On the example of divalent
hexagonal close packed metals it is shown what kind of information on the
electronic structure one can get from 1D profiles, interpreted in terms of
either 2D or 3D momentum densities. 2D and 3D densities are reconstructed from
merely two and seven 1D profiles, respectively. Applied reconstruction
techniques are particular solutions of the Radon transform in terms of
orthogonal Gegenabauer polynomials. We propose their modification connected
with so-called two-step reconstruction. The analysis is performed both in the
extended p and reduced k zone schemes. It is demonstrated that if positron wave
function or many-body effects are strongly momentum dependent, analysis of 2D
densities folded into k space may lead to wrong conclusions concerning the
Fermi surface. In the case of 2D ACAR data in Mg we found very strong many-body
effects. PACS numbers: 71.18.+y, 13.60.Fz, 87.59.Fm | cond-mat |
Thermally driven ballistic rectifer: The response of electric devices to an applied thermal gradient has, so far,
been studied almost exclusively in two-terminal devices. Here we present
measurements of the response to a thermal bias of a four-terminal,
quasi-ballistic junction with a central scattering site. We find a novel
transverse thermovoltage measured across isothermal contacts. Using a
multi-terminal scattering model extended to the weakly non-linear voltage
regime, we show that the device's response to a thermal bias can be predicted
from its nonlinear response to an electric bias. Our approach forms a
foundation for the discovery and understanding of advanced, nonlocal,
thermoelectric phenomena that in the future may lead to novel thermoelectric
device concepts. | cond-mat |
Prediction of anomalous LA-TA splitting in electrides: Electrides are an emerging class of materials with excess electrons localized
in interstices and acting as anionic interstitial quasi-atoms (ISQs). The
spatial ion-electron separation means that electrides can be treated physically
as ionic crystals, and this unusual behavior leads to extraordinary physical
and chemical phenomena. Here, a completely different effect in electrides is
predicted. By recognizing the long-range Coulomb interactions between matrix
atoms and ISQs that are unique in electrides, a nonanalytic correction to the
forces exerted on matrix atoms is proposed. This correction gives rise to an
LA-TA splitting in the acoustic branch of lattice phonons near the zone center,
similar to the well-known LO-TO splitting in the phonon spectra of ionic
compounds. The factors that govern this splitting are investigated, with
isotropic fcc-Li and anisotropic hP4-Na as the typical examples. It is found
that not all electrides can induce a detectable splitting, and criteria are
given for this type of splitting. The present prediction unveils the rich
phenomena in electrides and could lead to unprecedented applications. | cond-mat |
van der Waals-like phase separation instability of a driven granular gas
in three dimensions: We show that the van der Waals-like phase separation instability of a driven
granular gas at zero gravity, previously investigated in two-dimensional
settings, persists in three dimensions. We consider a monodisperse granular gas
driven by a thermal wall of a three-dimensional rectangular container at zero
gravity. The basic steady state of this system, as described by granular
hydrodynamic equations, involves a denser and colder layer of granulate located
at the wall opposite to the driving wall. When the inelastic energy loss is
sufficiently high, the driven granular gas exhibits, in some range of average
densities, negative compressibility in the directions parallel to the driving
wall. When the lateral dimensions of the container are sufficiently large, the
negative compressibility causes spontaneous symmetry breaking of the basic
steady state and a phase separation instability. Event-driven molecular
dynamics simulations confirm and complement our theoretical predictions. | cond-mat |
Transport in Fermi Liquids Confined by Rough Walls: I present theoretical calculations of the thermal conductivity of Fermi
liquid 3He confined to a slab of thickness of order 100nm. The effect of the
roughness of the confining surfaces is included directly in terms of the
surface roughness power spectrum which may be determined experimentally.
Transport at low temperatures is limited by scattering off rough surfaces and
evolves into the known high-temperature limit in bulk through an anomalous
regime in which both inelastic quasiparticle scattering and elastic scattering
off the rough surface coexist. I show preliminary calculations for the
coefficients of thermal conductivity. These studies are applicable in the
context of electrical transport in metal nanowires as well as experiments that
probe the superfluid phase diagram of liquid 3He in a slab geometry. | cond-mat |
Anisotropy, Itineracy, and Magnetic Frustration in High-Tc Iron
Pnictides: Using first-principle density functional theory calculations combined with
insight from a tight-binding representation, dynamical mean field theory, and
linear response theory, we have extensively investigated the electronic
structures and magnetic interactions of nine ferropnictides representing three
different structural classes. The calculated magnetic interactions are found to
be short-range, and the nearest ($J_{1a}$) and next-nearest ($J_{2}$) exchange
constants follow the universal trend of J_{1a}/2J_{2}\sim 1, despite their
itinerant origin and extreme sensitivity to the z-position of As. These results
bear on the discussion of itineracy versus magnetic frustration as the key
factor in stabilizing the superconducting ground state. The calculated spin
wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast
to cuprates. | cond-mat |
Intrinsic origin of electron scattering at 4H-SiC(0001)/SiO$_2$: We introduce a first-principles study to clarify the carrier-scattering
property at the SiC/SiO$_2$. Interestingly, the electron transport at the
conduction-band edge is significantly affected by the introduction of oxygen,
even though there are no electrically active defects. The origin of the large
scattering is explained by the behavior of the internal-space states (ISSs).
Moreover, the effect of the ISSs is larger than that of the electrically active
carbon-related defects. This result indicates that an additional scattering not
considered in a conventional Si/SiO$_2$ occurs at the SiC/SiO$_2$. | cond-mat |
Ion specificity and the theory of stability of colloidal suspensions: A theory is presented which allow us to accurately calculate the critical
coagulation concentration (CCC) of hydrophobic colloidal suspensions. For
positively charged particles the CCC's follow the Hofmeister (lyotropic)
series. For negatively charged particles the series is reversed. We find that
strongly polarizable chaotropic anions are driven towards the colloidal surface
by electrostatic and hydrophobic forces. Within approximately one ionic radius
from the surface, the chaotropic anions loose part of their hydration sheath
and become strongly adsorbed. The kosmotropic anions, on the other hand, are
repelled from the hydrophobic surface. The theory is quantitatively accurate
without any adjustable parameters. We speculate that the same mechanism is
responsible for the Hofmeister series that governs stability of protein
solutions. | cond-mat |
Low-temperature structural transition in FeCr_2S_4: Transmission electron microscopy studies of [110] and [111] oriented
FeCr_2S_4 single crystals at different temperatures reveal a structural
transition at low temperatures indicating a cubic-to-triclinic symmetry
reduction within crystallographic domains. The overall crystal symmetry was
found to be reduced from Fd3m to F-43m. The triclinic distortions were
suggested to result from the combined actions of tetragonal distortions due to
the Jahn-Teller active Fe^2+ ions and trigonal distortions due to a
displacement of the Cr^3+ ions in the <111> direction. | cond-mat |
Binding a Hopfion in Chiral Magnet Nanodisk: Hopfions are three-dimensional (3D) topological textures characterized by the
integer Hopf invariant $Q_H$. Here, we present the realization of a
zero--field, stable hopfion spin texture in a magnetic system consisting of a
chiral magnet nanodisk sandwiched by two films with perpendicular magnetic
anisotropy. The preimages of the spin texture and numerical calculations of
$Q_H$ show that the hopfion has $Q_H=1$. Furthermore, another non-trivial state
that includes a monopole--antimonopole pair (MAP) is also stabilized in this
system. By applying an external magnetic field, hopfion and MAP states with the
same polarization can be switched between each other. The topological
transition between the hopfion and the MAP state involves a creation
(annihilation) of the MAP and twist of the preimages. Our work paves the way to
study non-trivial 3D topological spin textures and stimulates more
investigations in the field of 3D spintronics. | cond-mat |
Large Scale (~25 m^2) metal diffraction grating of submicron period as
possible optoelectronic detector for short scalar gravitational waves: A method of detecting of short scalar gravitational waves with a wavelength
of ~ 0.5 micrometers is proposed, in contrast to LIGO Project, aimed at
detecting of long quadrupole gravitational waves with a wavelength in interval
from 43 till 10000 km. The conduction electrons in a metal are proposed to use
as gravitational receiving antennas (pendulums) instead of massive mirrors in
LIGO. It is shown that using a Large Scale metal diffraction grating with area
of 25 m^2 you can convert the mechanical vibrations of the conduction electrons
of metal into a plane electromagnetic wave propagating along the normal to the
grating. It is shown that when the amplitude of the scalar gravitational wave
in a source (in quasar at the centre of our galaxy) is greater than Ag0 = 10^20
cm/(s^2), you can register it with the help of a large optical telescope
equipped with the proposed diffraction grating. It is shown that the special
theory of relativity allows the amplitude of the scalar gravitational waves in
this source by 5 orders of magnitude greater than the above-mentioned minimum
value. | cond-mat |
Memory of jamming - multiscale models for soft and granular matter: Soft, disordered, micro-structured materials are ubiquitous in nature and
industry, and are different from ordinary fluids or solids, with unusual,
interesting static and flow properties. The transition from fluid to solid -at
the so-called jamming density- features a multitude of complex mechanisms, but
there is no unified theoretical framework that explains them all. In this
study, a simple yet quantitative and predictive model is presented, which
allows for a variable, changing jamming density, encompassing the memory of the
deformation history and explaining a multitude of phenomena at and around
jamming. The jamming density, now introduced as a new state-variable, changes
due to the deformation history and relates the system's macroscopic response to
its microstructure. The packing efficiency can increase logarithmically slow
under gentle repeated (isotropic) compression, leading to an increase of the
jamming density. In contrast, shear deformations cause anisotropy, changing the
packing efficiency exponentially fast with either dilatancy or compactancy. The
memory of the system near jamming can be explained by a microstatistical model
that involves a multiscale, fractal energy landscape and links the microscopic
particle picture to the macroscopic continuum description, providing a unified
explanation for the qualitatively different flow-behavior for different
deformation modes. To complement our work, a recipe to extract the
history-dependent jamming density from experimentally accessible data is
proposed, and alternative state-variables are compared. The proposed simple
macroscopic constitutive model is calibrated with the memory of microstructure.
Such approach can help understanding predicting and mitigating failure of
structures or geophysical hazards, and will bring forward industrial process
design/optimization, and help solving scientific challenges in fundamental
research. | cond-mat |
Giant energy oscillations mediated by a quasiperiodically driven qubit: A qubit driven by two incommensurate frequencies can mediate a quantised
average energy current in the adiabatic limit. We show that non-adiabatic
processes result in reversals of the energy current and corresponding
oscillations in the net energy transferred between the drives. The oscillations
are bounded but giant -- much larger than the qubit energy splitting. A
Landau-Zener analysis predicts that the timescale of the oscillations is
exponentially large in the period of the drives. However, numerical analysis
reveals that this timescale is not a monotonic function of the period, and has
increasing sub-structure as the adiabatic limit is approached. We show that
this non-monotonicity arises from interference effects between subsequent
Landau-Zener transitions. Giant energy oscillations should be observable in
near-term experiments with nitrogen-vacancy centers. | cond-mat |
Entropy production rate of nonequilibrium systems from the Fokker-Planck
equation: The entropy production rate of nonequilibrium systems is studied via the
Fokker-Planck equation. This approach, based on the entropy production rate
equation given by Schnakenberg from a master equation, requires information of
the transition rate of the system under study. We obtain the transition rate
from the conditional probability extracted from the Fokker-Planck equation and
then derive a new and more operable expression for the entropy production rate.
Examples are presented as applications of our approach. | cond-mat |
Competing magnetic orders in a bilayer Hubbard model with ultracold
atoms: Fermionic atoms in optical lattices have served as a compelling model system
to study and emulate the physics of strongly-correlated matter. Driven by the
advances of high-resolution microscopy, the recent focus of research has been
on two-dimensional systems in which several quantum phases, such as
anti-ferromagnetic Mott insulators for repulsive interactions and
charge-density waves for attractive interactions have been observed. However,
the aspired emulations of real materials, such as bilayer graphene, have to
take into account that their lattice structure composes of coupled layers and
therefore is not strictly two-dimensional. In this work, we realize a bilayer
Fermi-Hubbard model using ultracold atoms in an optical lattice and demonstrate
that the interlayer coupling controls a crossover between a planar
anti-ferromagnetically ordered Mott insulator and a band insulator of
spin-singlets along the bonds between the layers. Our work will enable the
exploration of further fascinating properties of coupled-layer Hubbard models,
such as theoretically predicted superconducting pairing mechanisms. | cond-mat |
Equilibrium and Kinetics: Water Confined in Carbon Nanotube as 1D
Lattice Gas: A simple 1D lattice gas model is presented, which very well describes the
equilibrium and kinetic behaviors of water confined in a thin carbon nanotube
found in an atomistic molecular dynamics(MD) simulation {[} Nature {\bf 414},
188 (2001) {]}. The model parameters are corresponding to various physical
interactions and can be calculated or estimated in statistic mechanics. The
roles of every interaction in the water filling, emptying and transporting
processes are clearly understood. Our results indicate that the physical
picture of the single-file kinetics is very simple. | cond-mat |
Phase diagram of the quarter-filled extended Hubbard model on a two-leg
ladder: We investigate the ground-state phase diagram of the quarter-filled Hubbard
ladder with nearest-neighbor Coulomb repulsion V using the Density Matrix
Renormalization Group technique. The ground-state is homogeneous at small V, a
``checkerboard'' charge--ordered insulator at large V and not too small on-site
Coulomb repulsion U, and is phase-separated for moderate or large V and small
U. The zero-temperature transition between the homogeneous and the
charge-ordered phase is found to be second order. In both the homogeneous and
the charge-ordered phases the existence of a spin gap mainly depends on the
ratio of interchain to intrachain hopping. In the second part of the paper, we
construct an effective Hamiltonian for the spin degrees of freedom in the
strong-coupling charge-ordered regime which maps the system onto a frustrated
spin chain. The opening of a spin gap is thus connected with spontaneous
dimerization. | cond-mat |
Characterizing fractional topological phases of lattice bosons near the
first Mott lobe: The Bose-Hubbard model subjected to an effective magnetic field hosts a
plethora of phases with different topological orders when tuning the chemical
potential. Using the density matrix renormalization group method, we identify
several gapped phases near the first Mott lobe at strong interactions. They are
connected by a particle-hole symmetry to a variety of quantum Hall states
stabilized at low fillings. We characterize phases of both particle and hole
type and identify signatures compatible with Laughlin, Moore-Read, and Bosonic
Integer Quantum Hall states by calculating the quantized Hall conductance and
by extracting the topological entanglement entropy. Furthermore, we analyze the
entanglement spectrum of a Laughlin state of bosonic particles and holes for a
range of interaction strengths, as well as the entanglement spectrum of a
Moore-Read state. These results further corroborate the existence of
topological states at high fillings, close to the first Mott lobe, as hole
analogues of the respective low-filling states. | cond-mat |
Ferroelectricity from iron valence ordering in rare earth ferrites?: The possibility of multiferroicity arising from charge ordering in LuFe2O4
and structurally related rare earth ferrites is reviewed. Recent experimental
work on macroscopic indications of ferroelectricity and microscopic
determination of coupled spin and charge order indicates that this scenario
does not hold. Understanding the origin of the experimentally observed charge
and spin order will require further theoretical work. Other aspects of recent
research in these materials, such as geometrical frustration effects, possible
electric-field-induced transitions, or orbital order are also briefly treated. | cond-mat |
The Nature of Electron Transport and visible light absorption in
Strontium Niobate -- A Plasmonic Water Splitter: Semiconductor compounds are widely used for water splitting applications,
where photo-generated electron-hole pairs are exploited to induce catalysis.
Recently, powders of a metallic oxide (Sr$_{1-x}$NbO$_3$, 0.03 < x < 0.20) have
shown competitive photocatalytic efficiency, opening up the material space
available for finding optimizing performance in water-splitting applications.
The origin of the visible light absorption in these powders was reported to be
due to an interband transition and the charge carrier separation was proposed
to be due to the high carrier mobility of this material. In the current work we
have prepared epitaxial thin films of Sr$_{0.94}$NbO$_{3+{\delta}}$ and found
that the bandgap of this material is ~4.1 eV, which is very large. Surprisingly
the carrier density of the conducting phase reaches 10$^{22}$ cm$^{-3}$, which
is only one order smaller than that of elemental metals and the carrier
mobility is only 2.47 cm$^2$/(V$\cdot$s). Contrary to earlier reports, the
visible light absorption at 1.8 eV (~688 nm) is due to the bulk plasmon
resonance, arising from the large carrier density, instead of an interband
transition. Excitation of the plasmonic resonance results in a multifold
enhancement of the lifetime of charge carriers. Thus we propose that the hot
charge carriers generated from decay of plasmons produced by optical absorption
is responsible for the water splitting efficiency of this material under
visible light irradiation. | cond-mat |
Influence of heat flow directions on Nernst effects in Py/Pt bilayers: We investigated the voltages obtained in a thin Pt strip on a Permalloy film
which was subject to in-plane temperature gradients and magnetic fields. The
voltages detected by thin W-tips or bond wires showed a purely symmetric effect
with respect to the external magnetic field which can be fully explained by the
planar Nernst effect (PNE). To verify the influence of the contacts
measurements in vacuum and atmosphere were compared and gave similar results.
We explain that a slightly in-plane tilted temperature gradient only shifts the
field direction dependence but does not cancel out the observed effects.
Additionally, the anomalous Nernst effect (ANE) could be induced by using thick
Au-tips which generated a heat current perpendicular to the sample plane. The
effect can be manipulated by varying the temperature of the Au-tips. These
measurements are discussed concerning their relevance in transverse spin
Seebeck effect measurements. | cond-mat |
Fractional Brownian motion and the critical dynamics of zipping polymers: We consider two complementary polymer strands of length $L$ attached by a
common end monomer. The two strands bind through complementary monomers and at
low temperatures form a double stranded conformation (zipping), while at high
temperature they dissociate (unzipping). This is a simple model of DNA (or RNA)
hairpin formation. Here we investigate the dynamics of the strands at the
equilibrium critical temperature $T=T_c$ using Monte Carlo Rouse dynamics. We
find that the dynamics is anomalous, with a characteristic time scaling as
$\tau \sim L^{2.26(2)}$, exceeding the Rouse time $\sim L^{2.18}$. We
investigate the probability distribution function, the velocity autocorrelation
function, the survival probability and boundary behaviour of the underlying
stochastic process. These quantities scale as expected from a fractional
Brownian motion with a Hurst exponent $H=0.44(1)$. We discuss similarities and
differences with unbiased polymer translocation. | cond-mat |
Role of disorder in determining the vibrational properties of
mass-spring networks: By introducing four fundamental types of disorders into a two-dimensional
triangular lattice separately, we determine the role of each type of disorder
in the vibration of the resulting mass-spring networks. We are concerned mainly
with the origin of the boson peak and the connection between the boson peak and
the transverse Ioffe-Regel limit. For all types of disorders, we observe the
emergence of the boson peak and Ioffe-Regel limits. With increasing disorder,
the boson peak frequency $\omega_{BP}$, transverse Ioffe-Regel frequency
$\omega_{IR}^T$, and longitudinal Ioffe-Regel frequency $\omega_{IR}^L$ all
decrease. We find that there are two ways for the boson peak to form:
developing from and coexisting with (but remaining independent of) the
transverse van Hove singularity without and with local coordination number
fluctuation. In the presence of a single type of disorder, $\omega_{IR}^T\ge
\omega_{BP}$, and $\omega_{IR}^T\approx \omega_{BP}$ only when the disorder is
sufficiently strong and causes spatial fluctuation of the local coordination
number. Moreover, if there is no positional disorder, $\omega_{IR}^T\approx
\omega_{IR}^L$. Therefore, the argument that the boson peak is equivalent to
the transverse Ioffe-Regel limit is not general. Our results suggest that both
local coordination number and positional disorder are necessary for the
argument to hold, which is actually the case for most disordered solids such as
marginally jammed solids and structural glasses. We further combine two types
of disorders to cause disorder in both the local coordination number and
lattice site position. The density of vibrational states of the resulting
networks resembles that of marginally jammed solids well. However, the relation
between the boson peak and the transverse Ioffe-Regel limit is still indefinite
and condition-dependent. | cond-mat |
Thermal radiation as a probe of one-dimensional electron liquids: Motivated by recent developments in the field of plasmonics, we develop the
theory of radiation from one-dimensional electron liquids, showing that the
spectrum of thermal radiation emitted from the system exhibits signatures of
non-Fermi liquid behavior. We derive a multipole expansion for the radiation
based on the Tomonaga-Luttinger liquid model. While the dipole radiation
pattern is determined by the conductivity of the system, we demonstrate that
the quadrupole radiation can reveal important features of the quantum liquid,
such as the Luttinger parameter. Radiation offers a probe of the interactions
of the system, including Mott physics as well as non-linear Luttinger liquid
behavior. We show that these effects can be probed in current experiments on
effectively one-dimensional electron liquids, such as carbon nanotubes. | cond-mat |
Phase Diagram of a Loop on the Square Lattice: The phase diagram of the O(n) model, in particular the special case $n=0$, is
studied by means of transfer-matrix calculations on the loop representation of
the O(n) model. The model is defined on the square lattice; the loops are
allowed to collide at the lattice vertices, but not to intersect. The loop
model contains three variable parameters that determine the loop density or
temperature, the energy of a bend in a loop, and the interaction energy of
colliding loop segments. A finite-size analysis of the transfer-matrix results
yields the phase diagram in a special plane of the parameter space. These
results confirm the existence of a multicritical point and an Ising-like
critical line in the low-temperature O(n) phase. | cond-mat |
Electronic structures of ferromagnetic superconductors $\mathrm{UGe}_2$
and $\mathrm{UCoGe}$ studied by angle-resolved photoelectron spectroscopy: The electronic structures of the ferromagnetic superconductors
$\mathrm{UGe}_2$ and $\mathrm{UCoGe}$ in the paramagnetic phase were studied by
angle-resolved photoelectron spectroscopy using soft X-rays ($h\nu =400-500$).
The quasi-particle bands with large contributions from $\mathrm{U}~5f$ states
were observed in the vicinity of $E_\mathrm{F}$, suggesting that the
$\mathrm{U}~5f$ electrons of these compounds have an itinerant character. Their
overall band structures were explained by the band-structure calculations
treating all the $\mathrm{U}~5f$ electrons as being itinerant. Meanwhile, the
states in the vicinity of $E_\mathrm{F}$ show considerable deviations from the
results of band-structure calculations, suggesting that the shapes of Fermi
surface of these compounds are qualitatively different from the calculations,
possibly caused by electron correlation effect in the complicated band
structures of the low-symmetry crystals. Strong hybridization between
$\mathrm{U}~5f$ and $\mathrm{Co}~3d$ states in $\mathrm{UCoGe}$ were found by
the $\mathrm{Co}~2p-3d$ resonant photoemission experiment, suggesting that
$\mathrm{Co}~3d$ states have finite contributions to the magnetic, transport,
and superconducting properties. | cond-mat |
Random walks on weighted networks: Exploring local and non-local
navigation strategies: In this paper, we present an overview of different types of random walk
strategies with local and non-local transitions on undirected connected
networks. We present a general approach to analyzing these strategies by
defining the dynamics as a discrete time Markovian process with probabilities
of transition expressed in terms of a symmetric matrix of weights. In the first
part, we describe the matrices of weights that define local random walk
strategies like the normal random walk, biased random walks, random walks in
the context of digital image processing and maximum entropy random walks. In
addition, we explore non-local random walks like L\'evy flights on networks,
fractional transport and applications in the context of human mobility.
Explicit relations for the stationary probability distribution, the mean first
passage time and global times to characterize the random walk strategies are
obtained in terms of the elements of the matrix of weights and its respective
eigenvalues and eigenvectors. Finally, we apply the results to the analysis of
particular local and non-local random walk strategies; we discuss their
efficiency and capacity to explore different types of structures. Our results
allow to study and compare on the same basis the global dynamics of different
types of random walk strategies. | cond-mat |
Interband coherence induced correction to Thouless pumping: possible
observation in cold-atom systems: In Thouless pump, the charge transport in a one-dimensional insulator over an
adiabatic cycle is topologically quantized. For nonequilibrium initial states,
however, interband coherence will induce a previously unknown contribution to
Thouless pumping. Though not geometric in nature, this contribution is
independent of the time scale of the pumping protocol. In this work, we perform
a detailed analysis of our previous finding [Phys. Rev. B 91, 085420 (2015)] in
an already available cold-atom setup. We show that initial states with
interband coherence can be obtained via a quench of the system's Hamiltonian.
Adiabatic pumping in the post-quench system are then examined both
theoretically and numerically, in which the interband coherence is shown to
play an important role and can hence be observed experimentally. By choosing
adiabatic protocols with different switching-on speeds, we also show that the
contribution of interband coherence to adiabatic pumping can be tuned. It is
further proposed that the interband coherence induced correction to Thouless
pumping may be useful in capturing a topological phase transition point. All
our results have direct experimental interests. | cond-mat |
Evolution of ferromagnetic order in URhGe alloyed with Ru, Co and Si: We have investigated the evolution of ferromagnetic order in the correlated
metal URhGe (Curie temperature $T_{\rm C} = $9.5 K) by chemical substitution of
Ru, Co and Si. Polycrystalline samples URh$_{1-x}$Ru$_x$Ge ($x \leq $0.6),
URh$_{1-x}$Co$_x$Ge ($x \leq $0.9) and URhGe$_{1-x}$Si$_x$ ($x \leq $0.2) have
been prepared and the magnetic properties have been investigated by
magnetization and transport experiments. In the case of Ru doping, $T_{\rm C}$
initially increases, but then decreases linearly as a function of $x$ and is
completely suppressed for $x_{\rm cr} \approx 0.38$. The Curie temperature in
the URh$_{1-x}$Co$_x$Ge series has a broad maximum $T_{\rm C} = 20$ K near
$x=0.6$ and then drops to 8 K for $x=0.9$. In the case of Si doping $T_{\rm C}$
stays roughly constant. We conclude that the alloy systems URh$_{1-x}$Ru$_x$Ge
and URh$_{1-x}$Co$_x$Ge are interesting candidates to study the ferromagnetic
instability. | cond-mat |
Imaging Grains and Grain Boundaries in Single-Layer Graphene: An Atomic
Patchwork Quilt: The properties of polycrystalline materials are often dominated by the size
of their grains and by the atomic structure of their grain boundaries. These
effects should be especially pronounced in 2D materials, where even a line
defect can divide and disrupt a crystal. These issues take on practical
significance in graphene, a hexagonal two-dimensional crystal of carbon atoms;
Single-atom-thick graphene sheets can now be produced by chemical vapor
deposition on up to meter scales, making their polycrystallinity almost
unavoidable. Theoretically, graphene grain boundaries are predicted to have
distinct electronic, magnetic, chemical, and mechanical properties which
strongly depend on their atomic arrangement. Yet, because of the
five-order-of-magnitude size difference between grains and the atoms at grain
boundaries, few experiments have fully explored the graphene grain structure.
Here, we use a combination of old and new transmission electron microscope
techniques to bridge these length scales. Using atomic-resolution imaging, we
determine the location and identity of every atom at a grain boundary and find
that different grains stitch together predominantly via pentagon-heptagon
pairs. We then use diffraction-filtered imaging to rapidly map the location,
orientation, and shape of several hundred grains and boundaries, where only a
handful have been previously reported. The resulting images reveal an
unexpectedly small and intricate patchwork of grains connected by tilt
boundaries. By correlating grain imaging with scanned probe measurements, we
show that these grain boundaries dramatically weaken the mechanical strength of
graphene membranes, but do not measurably alter their electrical properties.
These techniques open a new window for studies on the structure, properties,
and control of grains and grain boundaries in graphene and other 2D materials. | cond-mat |
Controlling grain boundaries by magnetic fields: The ability to use external magnetic fields to influence the microstructure
in polycrystalline materials has potential applications in microstructural
engineering. To explore this potential and to understand the complex
interactions between electromagnetic fields and solid-state matter transport we
consider a phase-field-crystal (PFC) model. Together with efficient and
scalable numerical algorithms this allows the examination of the role that
external magnetic fields play on the evolution of defect structures and grain
boundaries, on diffusive time scales. Examples for planar and circular grain
boundaries explain the essential atomistic processes and large scale
simulations in 2D are used to obtain statistical data on grain growth under the
influence of external fields. | cond-mat |
Criterion for weak spin-orbit coupling in heavy-fermion
superconductivity: A numerical renormalization-group study: A criterion for effective irrelevancy of the spin-orbit coupling in the
heavy-fermion superconductivity is discussed on the basis of the impurity
Anderson model with two sets of Kramers doublets. Using Wilson's numerical
renormalization-group method, we demonstrate a formation of the quasiparticle
as well as the renormalization of the rotational symmetry-breaking interaction
in the lower Kramers doublet (quasispin) space. A comparison with the quasispin
conserving interaction exhibits the effective irrelevancy of the
symmetry-breaking interaction for the splitting of two doublets Delta larger
than the characteristic energy of the local spin fluctuation T_K. The formula
for the ratio of two interactions is also determined. | cond-mat |
Understanding one-dimensional topological Kondo insulator: Poor man's
non-uniform antiferromagnetic mean-field theory versus quantum Monte Carlo
simulation: Topological Kondo insulator (TKI) is an essential example of interacting
topological insulator, where electron's correlation effect plays a key role.
However, most of our understanding on this timely issue comes from numerical
simulations, (particularly in one-spatial dimension) which exactly includes
correlation effect but is black box for extracting underlying physics. In this
work, we use a non-uniform antiferromagnetic mean-field (nAFM) theory to
understand the underlying physics in a TKI model, the $1D$ $p-$wave periodic
Anderson model ($p$-PAM). Comparing with numerically exact quantum Monte Carlo
simulation, we find that nAFM theory is an excellent approximation for
ground-state properties when onsite Hubbard interaction is weak. This
emphasizes the dominating antiferromagnetic correlation in this system and
local antiferromagnetic picture captures the qualitative nature of interacting
many-body ground state. Adding extra conduction electron band to $p$-PAM leads
to a quantum phase transition from Haldane phase into topological trivial
phase. We believe these results may be helpful for understanding novel physics
in interacting TKI materials such as SmB$_{6}$ and other related compounds. | cond-mat |
Superconducting junction with tri-component pairing gap functions: We study a superconducting hetro-junction with one side characterized by the
unconventional chiral $p$-wave gap function $p_x\pm ip_y$ and the other side
the conventional $s$-wave one. Though a relative phase of $\pm \frac{\pi}{2}$
between any two components of gap functions is favored in the junction region,
mutual phase differences cannot achieve $\pm \frac{\pi}{2}$ simultaneously,
which results in frustration. Based on a Ginzburg-Landau free energy analysis,
the frustrated pattern is determined to be $s+ i\eta_1 (e^{ i\eta_2
\varphi/2}p_x +\eta_3 e^{- i\eta_2 \varphi/2}p_y)$ with $\eta_j=\pm 1$
($j=1,2,3$), where $\varphi$ is the phase difference between the $p_x$- and
$p_y$-wave gap functions. Furthermore, we find that the junction exhibits an
anisotropic magnetoelectric effect, manifesting itself as an anisotropic spin
magnetization along the edge of the junction. | cond-mat |
Charge-Kondo Effect in Mesoscopic Superconductors Coupled to Normal
Metals: We develop a theoretical proposal for the charge Kondo effect in mesoscopic
normal-superconductor-normal heterostructures, where the superconducting gap
exceeds the electrostatic charging energy. Charge-Kondo correlations in these
devices alter the conventional temperature-dependence of Andreev reflection and
electron cotunneling. We predict typical Kondo temperatures of $\gtrsim 10 {\rm
mK}$, and suggest experimental architectures that combine superconducting
charge-qubits with semiconducting nanowires at cryogenic temperatures. | cond-mat |
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