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Effects of interactions on Bose-Einstein condensation of an atomic gas: The phase transition to a Bose-Einstein condensate is unusual in that it is
not necessarily driven by inter-particle interactions but can occur in an ideal
gas as a result of a purely statistical saturation of excited states. However,
interactions are necessary for any system to reach thermal equilibrium and so
are required for condensation to occur in finite time. In this Chapter we
review the role of interactions in Bose-Einstein condensation, covering both
theory and experiment. We focus on measurements performed on harmonically
trapped ultracold atomic gases, but also discuss how these results relate to
the uniform-system case, which is more theoretically studied and also more
relevant for other experimental systems.
We first consider interaction strengths for which the system can be
considered sufficiently close to equilibrium to measure thermodynamic
behaviour. In particular we discuss the effects of interactions both on the
mechanism of condensation (namely the saturation of the excited states) and on
the critical temperature at which condensation occurs. We then discuss in more
detail the conditions for the equilibrium thermodynamic measurements to be
possible, and the non-equilibrium phenomena that occur when these conditions
are controllably violated by tuning the strength of interactions in the gas. | cond-mat |
Room-temperature multiferroic hexagonal LuFeO$_3$ films: The crystal and magnetic structures of single-crystalline hexagonal LuFeO$_3$
films have been studied using x-ray, electron and neutron diffraction methods.
The polar structure of these films are found to persist up to 1050 K; and the
switchability of the polar behavior is observed at room temperature, indicating
ferroelectricity. An antiferromagnetic order was shown to occur below 440 K,
followed by a spin reorientation resulting in a weak ferromagnetic order below
130 K. This observation of coexisting multiple ferroic orders demonstrates that
hexagonal LuFeO$_3$ films are room-temperature multiferroics. | cond-mat |
Polaritons are Not Weakly Interacting: Direct Measurement of the
Polariton-Polariton Interaction Strength: Exciton-polaritons in a microcavity are composite two-dimensional bosonic
quasiparticles, arising from the strong coupling between confined light modes
in a resonant planar optical cavity and excitonic transitions, typically using
excitons in semiconductor quantum wells (QWs) placed at the antinodes of the
same cavity. Quantum phenomena such as Bose-Einstein condensation (BEC),
quantized vortices, and macroscopic quantum states have been reported at
temperatures from tens of Kelvin up to room temperatures, and polaritonic
devices such as spin switches \cite{Amo2010} and optical transistors have also
been reported. Many of these effects of exciton-polaritons depend crucially on
the polariton-polariton interaction strength. Despite the importance of this
parameter, it has been difficult to make an accurate experimental measurement,
mostly because of the difficulty of determining the absolute densities of
polaritons and bare excitons. Here we report the direct measurement of the
polariton-polariton interaction strength in a very high-Q microcavity
structure. By allowing polaritons to propagate over 40 $\mu$m to the center of
a laser-generated annular trap, we are able to separate the polariton-polariton
interactions from polariton-exciton interactions. The interaction strength is
deduced from the energy renormalization of the polariton dispersion as the
polariton density is increased, using the polariton condensation as a benchmark
for the density. We find that the interaction strength is about two orders of
magnitude larger than previous theoretical estimates, putting polaritons
squarely into the strongly-interacting regime. When there is a condensate, we
see a sharp transition to a different dependence of the renormalization on the
density, which is evidence of many-body effects. | cond-mat |
Kramers-Wannier Duality and Random Bond Ising Model: We present a new combinatorial approach to the Ising model incorporating
arbitrary bond weights on planar graphs. In contrast to existing methodologies,
the exact free energy is expressed as the determinant of a set of ordered and
disordered operators defined on vertices and dual vertices respectively,
thereby explicitly demonstrating the Kramers-Wannier duality. The implications
of our derived formula for the random bond Ising model are further elucidated. | cond-mat |
First-principles DFT+\emph{U} study of structural and electronic
properties of PbCrO$_{3}$: We have performed a systematic first-principles investigation to calculate
the structural, electronic, and magnetic properties of PbCrO$_{3}$, CrPbO$_{3}$
as well as their equiproportional combination. The local density approximation
(LDA)$+U$ and the generalized gradient approximation$+U$ theoretical formalisms
have been used to account for the strong on-site Coulomb repulsion among the
localized Cr 3d electrons. By choosing the Hubbard \emph{U} parameter around 4
eV, ferromagnetic, and/or antiferromagnetic ground states can be achieved and
our calculated lattice constants, bulk moduli, and equation of states are in
good agreement with recent experiments [W. Xiao \emph{et al.}, PNAS
\textbf{107}, 14026 (2010)]. The bonding nature of B$-$O bonds in these
ABO$_{3}$ compounds exhibit evident covalent character and our electron
transferring study indicates that the ionicity shows decreasing trend with
increasing fraction of CrO$_{6/2}$ octahedron within the
PbCrO$_{3}$-CrPbO$_{3}$ random compounds. The lengthes of B$-$O bonds determine
their lattice parameters, thus, clearly indicates that the abnormally large
volume and compressibility is due to the contain of CrPbO$_{3}$ in the
experimental sample and the transition of PbO$_{6/2}$ octahedron to CrO$_{6/2}$
upon compression. | cond-mat |
Kondo effect in "bad metals": We study the low-temperature properties of a Kondo lattice using the large-N
formalism. For a singular density of conduction states (DOS), we generalize the
single-impurity result of Withoff and Fradkin: the strong-coupling fixed point
becomes irrelevant if the DOS vanishes at the Fermi level E_F. However, for E_F
close enough to the singularity, and close to half-filling, the Kondo
temperature, $T_K$, can become much smaller than the characteristic Fermi
liquid scale. At T=0, a meta-magnetic transition occurs at the critical
magnetic field H_c ~ (k_B/mu_B) T_K. Our results provide a qualitative
explanation for the behavior of the YbInCu_4 compound below the valence-change
transition. | cond-mat |
Contrary Effect of B and N Doping into Graphene and Graphene Oxide
Heterostructures with MoS$_2$ on Interface Function and Hydrogen Evolution: Molybdenum disulfide (MoS$_2$) attracts attention as a high efficient and low
cost photocatalyst for hydrogen production, but suffers from low conductance
and high recombination rate of photo-generated charge carriers. In this work,
we investigate the MoS$_2$ heterostructures with graphene variants (GVs),
including graphene, graphene oxide, and their boron- and nitrogen-doped
variants, by using first-principles calculations. Systematic comparison between
graphene and graphene oxide composites is performed, and contrary effect of B
and N doping on interface function and hydrogen evolution is clarified. We find
that upon the formation of the interfaces some amount of electronic charge
transfers from the GV side to the MoS$_2$ layer, inducing the creation of
interface dipole and the reduction of work function, which is more pronounced
in the graphene oxide composites. Moreover, our results reveal that N doping
enhances the interface functions by forming donor-type interface states,
whereas B doping reduces those functions by forming acceptor-type interface
states. However, the B-doped systems exhibit lower Gibbs free energy difference
for hydrogen adsorption on GV side than the N-doped systems, which deserves
much consideration in the design of new functional photocatalysts. | cond-mat |
Gas sensing technologies -- status, trends, perspectives and novel
applications: The strong, continuous progresses in gas sensors and electronic noses
resulted in improved performance and enabled an increasing range of
applications with large impact on modern societies, such as environmental
monitoring, food quality control and diagnostics by breath analysis. Here we
review this field with special attention to established and emerging approaches
as well as the most recent breakthroughs, challenges and perspectives. In
particular, we focus on (1) the transduction principles employed in different
architectures of gas sensors, analysing their advantages and limitations; (2)
the sensing layers including recent trends toward nanostructured,
low-dimensional and composite materials; (3) advances in signal processing
methodologies, including the recent advent of artificial neural networks.
Finally, we conclude with a summary on the latest achievements and trends in
terms of applications. | cond-mat |
Mechanical fluctuations suppress the threshold of soft-glassy solids :
the secular drift scenario: We propose a dynamical mechanism leading to the fluidization of soft-glassy
amorphous mate-rial driven below the yield-stress by external mechanical
fluctuations. The model is based on the combination of memory effect and
non-linearity, leading to an accumulation of tiny effects over a long-term. We
test this scenario on a granular packing driven mechanically below the Coulomb
threshold. We bring evidences for an effective viscous response directly
related to small stress modulations in agreement with the theoretical
prediction of a generic secular drift. | cond-mat |
Impressive optoelectronic and thermoelectric properties of
two-dimensional XI$_2$ (X=Sn, Si): a first principle study: Two-dimensional (2D) metal halides have received more attention because of
their electronic and optoelectronic properties. Recently, researchers are
interested to investigate the thermoelectric properties of metal halide
monolayers because of their ultralow lattice conductivity, high Seebeck
coefficient and figure of merit. Here, we have investigated thermoelectric and
optoelectronic properties of XI$_2$ (X=Sn and Si) monolayers with the help of
density functional theory and Boltzmann transport equation. The structural
parameters have been optimized with relaxation of atomic positions. Excellent
thermoelectric and optical properties have been obtained for both SnI$_2$ and
SiI$_2$ monolayers. For SnI$_2$ an indirect bandgap of 2.06 eV was observed and
the absorption peak was found at 4.68 eV. For this the highest ZT value of 0.84
for p-type doping at 600K has been calculated. Similarly, for SiI$_2$ a
comparatively low indirect bandgap of 1.63 eV was observed, and the absorption
peak was obtained at 4.86 eV. The calculated ZT product for SiI$_2$ was 0.87 at
600K. Both the crystals having high absorbance and ZT value suggest that they
can be promising candidates for optoelectronic and thermoelectric devices. | cond-mat |
Stabilizing fluctuating spin-triplet superconductivity in graphene via
induced spin-orbit coupling: A recent experiment showed that proximity induced Ising spin-orbit coupling
enhances the spin-triplet superconductivity in Bernal bilayer graphene. Here,
we show that, due to the nearly perfect spin rotation symmetry of graphene, the
fluctuations of the spin orientation of the triplet order parameter suppress
the superconducting transition to nearly zero temperature. Our analysis shows
that both Ising spin-orbit coupling and in-plane magnetic field can eliminate
these low-lying fluctuations and can greatly enhance the transition
temperature, consistent with the recent experiment. Our model also suggests the
possible existence of a phase at small anisotropy and magnetic field which
exhibits quasi-long-range ordered spin-singlet charge 4e superconductivity,
even while the triplet 2e superconducting order only exhibits short-ranged
correlations. Finally, we discuss relevant experimental signatures. | cond-mat |
Many-body Multifractality throughout Bosonic Superfluid and Mott
Insulator Phases: We demonstrate many-body multifractality of the Bose-Hubbard Hamiltonian's
ground state in Fock space, for arbitrary values of the interparticle
interaction. Generalized fractal dimensions unambiguously signal, even for
small system sizes, the emergence of a Mott insulator, that cannot, however, be
naively identified with a localized phase in Fock space. We show that the
scaling of the derivative of any generalized fractal dimension with respect to
the interaction strength encodes the critical point of the superfluid to Mott
insulator transition, and provides an efficient way to accurately estimate its
position. We further establish that the transition can be quantitatively
characterized by one single wavefunction amplitude from the exponentially large
Fock space. | cond-mat |
Dynamical dimer correlations at bipartite and non-bipartite
Rokhsar-Kivelson points: We determine the dynamical dimer correlation functions of quantum dimer
models at the Rokhsar-Kivelson point on the bipartite square and cubic lattices
and the non-bipartite triangular lattice. Based on an algorithmic idea by
Henley, we simulate a stochastic process of classical dimer configurations in
continuous time and perform a stochastic analytical continuation to obtain the
dynamical correlations in momentum space and the frequency domain. This
approach allows us to observe directly the dispersion relations and the
evolution of the spectral intensity within the Brillouin zone beyond the
single-mode approximation. On the square lattice, we confirm analytical
predictions related to soft modes close to the wavevectors (pi,pi) and (pi,0)
and further reveal the existence of shadow bands close to the wavevector (0,0).
On the cubic lattice the spectrum is also gapless but here only a single soft
mode at (pi,pi,pi) is found, as predicted by the single mode approximation. The
soft mode has a quadratic dispersion at very long wavelength, but crosses over
to a linear behavior very rapidly. We believe this to be the remnant of the
linearly dispersing "photon" of the Coulomb phase. Finally the triangular
lattice is in a fully gapped liquid phase where the bottom of the dimer
spectrum exhibits a rich structure. At the M point the gap is minimal and the
spectral response is dominated by a sharp quasiparticle peak. On the other
hand, at the X point the spectral function is much broader. We sketch a
possible explanation based on the crossing of the coherent dimer excitations
into the two-vison continuum. | cond-mat |
Rotation of the dislocation grid in multilayer FeSe films and
visualization of electronic nematic domains via orbital-selective tunneling: Understanding the interplay of structural and electronic symmetry breaking in
Fe-based high temperature superconductors remains of high interest. In this
work we grow strain-patterned multilayer FeSe thin films in a range of
thicknesses using molecular beam epitaxy. We study the formation of electronic
nematic domains and spatially-varying strain using scanning tunneling
microscopy and spectroscopy. We directly visualize the formation of edge
dislocations that give rise to a two-dimensional edge dislocation network in
the films. Interestingly, we observe a 45 degree in-plane rotation of the
dislocation network as a function of film thickness, yielding antisymmetric
strain along different directions. This results in different coupling ratios
between electronic nematic domains and antisymmetric strain. Lastly, we are
able to distinguish between different orthogonal nematic domains by revealing a
small energy-dependent difference in differential conductance maps between the
two regions. This could be explained by orbital-selective tip-sample tunneling.
Our observations bring new insights into the dislocation network formation in
epitaxial thin films and provide another nanoscale tool to explore electronic
nematicity in Fe-based superconductors. | cond-mat |
Ultracold atomic Bose and Fermi spinor gases in optical lattices: We investigate magnetic properties of Mott-insulating phases of ultracold
Bose and Fermi spinor gases in optical lattices. We consider in particular the
F=2 Bose gas, and the F=3/2 and F=5/2 Fermi gases. We derive effective spin
Hamiltonians for one and two atoms per site and discuss the possibilities of
manipulating the magnetic properties of the system using optical Feshbach
resonances. We discuss low temperature quantum phases of a 87Rb gas in the F=2
hyperfine state, as well as possible realizations of high spin Fermi gases with
either 6Li or 132Cs atoms in the F=3/2 state, and with 173Yb atoms in the F=5/2
state. | cond-mat |
Clustering and fluidization in a one-dimensional granular system:
molecular dynamics and direct-simulation Monte Carlo method: We study a 1-D granular gas of point-like particles not subject to gravity
between two walls at temperatures T_left and T_right. The system exhibits two
distinct regimes, depending on the normalized temperature difference Delta =
(T_right - T_left)/(T_right + T_left): one completely fluidized and one in
which a cluster coexists with the fluidized gas. When Delta is above a certain
threshold, cluster formation is fully inhibited, obtaining a completely
fluidized state. The mechanism that produces these two phases is explained. In
the fluidized state the velocity distribution function exhibits peculiar
non-Gaussian features. For this state, comparison between integration of the
Boltzmann equation using the direct-simulation Monte Carlo method and results
stemming from microscopic Newtonian molecular dynamics gives good coincidence,
establishing that the non-Gaussian features observed do not arise from the
onset of correlations. | cond-mat |
Hysteresis behavior of the anisotropic quantum Heisenberg model driven
by periodic magnetic field: Dynamic behavior of a quantum Heisenberg ferromagnet in the presence of a
periodically oscillating magnetic field has been analyzed by means of the
effective field theory with two spin cluster. The dynamic equation of motion
has been constructed with the help of a Glauber type stochastic process and
solved for a simple cubic lattice. After the phase diagrams given, the behavior
of the hysteresis loop area, coercive field and remanent magnetization with the
anisotropy in the exchange interaction has been investigated in detail.
Especially, by comparing of the magnitudes of the hysteresis loop area in the
high anisotropy limit (i.e. Ising model) and low anisotropy limit (i.e.
isotropic Heisenberg model), detailed description of the hysteresis loop area
with the anisotropy in the exchange interaction given. Some interesting
features have been obtained about this behavior as well as in phase diagrams
such as tricritical points. | cond-mat |
Non-linear Poisson-Boltzmann Theory for Swollen Clays: The non-linear Poisson-Boltzmann equation for a circular, uniformly charged
platelet, confined together with co- and counter-ions to a cylindrical cell, is
solved semi-analytically by transforming it into an integral equation and
solving the latter iteratively. This method proves efficient, robust, and can
be readily generalized to other problems based on cell models, treated within
non-linear Poisson-like theory. The solution to the PB equation is computed
over a wide range of physical conditions, and the resulting osmotic equation of
state is shown to be in fair agreement with recent experimental data for
Laponite clay suspensions, in the concentrated gel phase. | cond-mat |
Robust entangling gate for capacitively coupled few-electron
singlet-triplet qubits: The search of a sweet spot, locus in qubit parameters where quantum control
is first-order insensitive to noises, is key to achieve high-fidelity quantum
gates. Efforts to search for such a sweet spot in conventional
double-quantum-dot singlet-triplet qubits where each dot hosts one electron
("two-electron singlet-triplet qubit"), especially for two-qubit operations,
have been unsuccessful. Here we consider singlet-triplet qubits allowing each
dot to host more than one electron, with a total of four electrons in the
double quantum dots ("four-electron singlet-triplet qubit"). We theoretically
demonstrate, using configuration-interaction calculations, that sweet spots
appear in this coupled qubit system. We further demonstrate that, under
realistic charge noise and hyperfine noise, two-qubit operation at the proposed
sweet spot could offer gate fidelities ($\sim99\%$) that are higher than
conventional two-electron singlet-triplet qubit system ($\sim90\%$). Our
results should facilitate realization of high-fidelity two-qubit gates in
singlet-triplet qubit systems. | cond-mat |
Elasticity and melting of skyrmion flux lattices in p-wave
superconductors: We analytically calculate the energy, magnetization curves (B(H)), and
elasticity of skyrmion flux lattices in p-wave superconductors near the lower
critical field H_c1, and use these results with the Lindemann criterion to
predict their melting curve. In striking contrast to vortex flux lattices,
which always melt at an external field H > H_c1, skyrmion flux lattices never
melt near H_c1. This provides a simple and unambiguous test for the presence of
skyrmions. | cond-mat |
Rational Approximations of Quasi-Periodic Problems via Projected Green's
Functions: We introduce the projected Green's function technique to study quasi-periodic
systems such as the Andre-Aubry-Harper (AAH) model and beyond. In particular,
we use projected Green's functions to construct a "rational approximate"
sequence of transfer matrix equations consistent with quasi-periodic topology,
where convergence of these sequences corresponds to the existence of extended
eigenfunctions. We motivate this framework by applying it to a few well studied
cases such as the almost-Mathieu operator (AAH model), as well as more generic
non-dual models that challenge standard routines. The technique is flexible and
can be used to extract both analytic and numerical results, e.g. we
analytically extract a modified phase diagram for Liouville irrationals. As a
numerical tool, it does not require the fixing of boundary conditions and
circumvents a primary failing of numerical techniques in quasi-periodic
systems, extrapolation from finite size. Instead, it uses finite size scaling
to define convergence bounds on the full irrational limit. | cond-mat |
Voltage-dependent first-principles simulation of insertion of chloride
ions into Al/Al$_2$O$_3$ interfaces using the Quantum Continuum Approximation: Experiments have shown that pitting corrosion can develop in aluminum
surfaces at potentials $> -0.5$ V relative to the standard hydrogen electrode
(SHE). Until recently, the onset of pitting corrosion in aluminum has not been
rigorously explored at an atomistic scale because of the difficulty of
incorporating a voltage into density functional theory (DFT) calculations. We
introduce the Quantum Continuum Approximation (QCA) which self-consistently
couples explicit DFT calculations of the metal-insulator and insulator-solution
interfaces to continuum Poisson-Boltzmann electrostatic distributions
describing the bulk of the insulating region. By decreasing the number of atoms
necessary to explicitly simulate with DFT by an order of magnitude, QCA makes
the first-principles prediction of the voltage of realistic electrochemical
interfaces feasible. After developing this technique, we apply QCA to predict
the formation energy of chloride atoms inserting into oxygen vacancies in
Al(111)/$\alpha$-Al$_2$O$_3$ (0001) interfaces as a function of applied
voltage. We predict that chloride insertion is only favorable in systems with a
grain boundary in the Al$_2$O$_3$ for voltages $> -0.2$ V (SHE). Our results
roughly agree with the experimentally demonstrated onset of corrosion,
demonstrating QCA's utility in modeling realistic electrochemical systems at
reasonable computational cost. | cond-mat |
Tsallis distributions and 1/f noise from nonlinear stochastic
differential equations: Probability distributions which emerge from the formalism of nonextensive
statistical mechanics have been applied to a variety of problems. In this paper
we unite modeling of such distributions with the model of widespread 1/f noise.
We propose a class of nonlinear stochastic differential equations giving both
the q-exponential or q-Gaussian distributions of signal intensity, revealing
long-range correlations and 1/f^beta behavior of the power spectral density.
The superstatistical framework to get 1/f^beta noise with q-exponential and
q-Gaussian distributions of the signal intensity in is proposed, as well. | cond-mat |
Topological Effect of Surface Plasmon Excitation in Gapped Isotropic
Topological Insulator Nanowires: We present a theoretical investigation of the surface plasmon (SP) at the
interface between topologically non-trivial cylindrical core and
topological-trivial surrounding material, from the axion electrodynamics and
modified constitutive relations. We find that the topological effect always
leads to a red-shift of SP energy, while the energy red-shift decreases
monotonically as core diameter decreases. A qualitative picture based on
classical perturbation theory is given to explain these phenomena, from which
we also infer that in order to enhance the shift, the difference between the
inverse of dielectric constants of two materials shall be increased. We also
find that the surrounding magnetic environment suppresses the topological
effect. All these features can be well described by a simple ansatz surface
wave, which is in good agreement with full electromagnetic eigenmodes. In
addition, bulk plasmon energy at \omega_{P}=17.5\pm0.2eV for semiconducting
Bi2Se3 nanoparticle is observed from high-resolution Electron Energy Loss
Spectrum Image measurements. | cond-mat |
Building and Destroying Symmetry in 1-D Elastic Systems: Locally periodic rods, which show approximate invariance with respect to
translations, are constructed by joining $N$ unit cells. The spectrum then
shows a band spectrum. We then break the local periodicity by including one or
more defects in the system. When the defects follow a certain definite
prescription, an analog of the Wannier-Stark ladders is gotten; when the
defects are random, an elastic rod showing Anderson localization is obtained.
In all cases experimental values match the theoretical predictions. | cond-mat |
Random sequential adsorption of trimers and hexamers: Adsorption of trimers and hexamers built of identical spheres was studied
numerically using the Random Sequential Adsorption (RSA) algorithm. Particles
were adsorbed on a two dimensional, flat and homogeneous surface. Numerical
simulations allow to establish the maximal random coverage ratio, RSA kinetics
as well as the Available Surface Function (ASF), which is crucial for
determining kinetics of the adsorption process obtained experimentally.
Additionally, the density autocorrelation function was measured. All the
results were compared with previous results obtained for spheres, dimers and
tetramers. | cond-mat |
Charge carrier localisation in disordered graphene nanoribbons: We study the electronic properties of actual-size graphene nanoribbons
subjected to substitutional disorder particularly with regard to the
experimentally observed metal-insulator transition. Calculating the local, mean
and typical density of states, as well as the time-evolution of the particle
density we comment on a possible disorder-induced localisation of charge
carriers at and close to the Dirac point within a percolation transition
scenario. | cond-mat |
Large-scale simulation of adhesion dynamics for end-graphed polymers: The adhesion between a polymer melt and substrate is studied in the presence
of chemically attached chains on the substrate surface. Extensive molecular
dynamics simulations have been carried out to study the effect of temperature,
tethered chain areal density ($\Sigma$), tethered chain length ($N_{t}$), chain
bending energy ($k_{\theta}$) and tensile pull velocity ($v$) on the adhesive
failure mechanisms of pullout and/or scission of the tethered chains. We
observe a crossover from pure chain pullout to chain scission as $N_{t}$ is
increased. Below the glass transition, the value of $N_{t}$ for which this
crossover begins approaches the bulk entanglement length $N_{e}$. For the
values of $N_{t}$ and $\Sigma$ used here, no crossover to crazing is observed. | cond-mat |
Relativity Restored: Dirac Anisotropy in QED$_3$: We show that at long lengthscales and low energies and to leading order in
1/N expansion, the anisotropic QED in 2+1 dimensions renormalizes to an
isotropic limit. Consequently, the (Euclidean) relativistic invariance of the
theory is spontaneously restored at the isotropic critical point, characterized
by the anomalous dimension exponent of the Dirac fermion propagator $\eta$. We
find $\eta=16/3\pi^2 N$. | cond-mat |
A van der Waals pn heterojunction with organic/inorganic semiconductors: van der Waals (vdW) heterojunctions formed by two-dimensional (2D) materials
have attracted tremendous attention due to their excellent electrical/optical
properties and device applications. However, current 2D heterojunctions are
largely limited to atomic crystals, and hybrid organic/inorganic structures are
rarely explored. Here, we fabricate hybrid 2D heterostructures with p-type
dioctylbenzothienobenzothiophene (C8-BTBT) and n-type MoS2. We find that
few-layer C8-BTBT molecular crystals can be grown on monolayer MoS2 by vdW
epitaxy, with pristine interface and controllable thickness down to monolayer.
The operation of the C8-BTBT/MoS2 vertical heterojunction devices is highly
tunable by bias and gate voltages between three different regimes: interfacial
recombination, tunneling and blocking. The pn junction shows diode-like
behavior with rectifying ratio up to 105 at the room temperature. Our devices
also exhibit photovoltaic responses with power conversion efficiency of 0.31%
and photoresponsivity of 22mA/W. With wide material combinations, such hybrid
2D structures will offer possibilities for opto-electronic devices that are not
possible from individual constituents. | cond-mat |
Analogue of Hamilton-Jacobi theory for the time-evolution operator: In this paper we develop an analogue of Hamilton-Jacobi theory for the
time-evolution operator of a quantum many-particle system. The theory offers a
useful approach to develop approximations to the time-evolution operator, and
also provides a unified framework and starting point for many well-known
approximations to the time-evolution operator. In the important special case of
periodically driven systems at stroboscopic times, we find relatively simple
equations for the coupling constants of the Floquet Hamiltonian, where a
straightforward truncation of the couplings leads to a powerful class of
approximations. Using our theory, we construct a flow chart that illustrates
the connection between various common approximations, which also highlights
some missing connections and associated approximation schemes. These missing
connections turn out to imply an analytically accessible approximation that is
the "inverse" of a rotating frame approximation and thus has a range of
validity complementary to it. We numerically test the various methods on the
one-dimensional Ising model to confirm the ranges of validity that one would
expect from the approximations used. The theory provides a map of the relations
between the growing number of approximations for the time-evolution operator.
We describe these relations in a table showing the limitations and advantages
of many common approximations, as well as the new approximations introduced in
this paper. | cond-mat |
Ultra cold atoms and Bose-Einstein condensation for quantum metrology: This paper is a short introduction to cold atom physics and Bose-Einstein
condensation. Light forces on atoms are presented, together with laser cooling,
and a few atom traps: the magneto-optical trap, dipole traps and magnetic
traps. A brief description of Bose-Einstein condensation is given together with
some important links with condensed matter physics. The reader is referred to
comprehensive reviews and to other lecture notes for further details on atom
cooling, trapping and Bose-Einstein condensation. | cond-mat |
Anisotropic signatures of the electronic correlations in the electrical
resistivity of UTe$_2$: Multiple unconventional superconducting phases are suspected to be driven by
magnetic fluctuations in the heavy-fermion paramagnet UTe$_2$, and a challenge
is to identify the signatures of the electronic correlations, including the
magnetic fluctuations, in the bulk physical quantities. Here, we investigate
thoroughly the anisotropy of the electrical resistivity of UTe$_2$ under
intense magnetic fields up to 70~T, for different electrical-current and
magnetic-field configurations. Two characteristic temperatures and an
anisotropic low-temperature Fermi-liquid-like coefficient $A$, controlled by
the electronic correlations, are extracted. Their critical behavior near the
metamagnetic transition induced at $\mu_0H_m\simeq35$~T for
$\mathbf{H}\parallel\mathbf{b}$ is characterized. Anisotropic scattering
processes are evidenced and magnetic fluctuations are proposed to contribute,
via a Kondo hybridization, to the electrical resistivity. Our work appeals for
a microscopic modeling of the anisotropic contributions to the electrical
resistivity as a milestone for understanding magnetically-mediated
superconductivity in UTe$_2$. | cond-mat |
Ground State Properties of Anti-Ferromagnetic Spinor Bose gases in One
Dimension: We investigate the ground state properties of anti-ferromagnetic spin-1 Bose
gases in one dimensional harmonic potential from the weak repulsion regime to
the strong repulsion regime. By diagonalizing the Hamiltonian in the Hilbert
space composed of the lowest eigenstates of single particle and spin
components, the ground state wavefunction and therefore the density
distributions, magnetization distribution, one body density matrix, and
momentum distribution for each components are obtained. It is shown that the
spinor Bose gases of different magnetization exhibit the same total density
profiles in the full interaction regime, which evolve from the single peak
structure embodying the properties of Bose gases to the fermionized shell
structure of spin-polarized fermions. But each components display different
density profiles, and magnetic domains emerge in the strong interaction limit
for $M=0.25$. In the strong interaction limit, one body density matrix and the
momentum distributions exhibit the same behaviours as those of spin-polarized
fermions. The fermionization of momentum distribution takes place, in contrast
to the $\delta$-function-like distribution of single component Bose gases in
the full interaction region. | cond-mat |
Spinodal decomposition stabilizes plastic flow in a nanocrystalline
Cu-Ti alloy: A combination of high strength and reasonable ductility has been achieved in
a copper-1.7 at.%titanium alloy deformed by high-pressure torsion. Grain
refinement and a spinodal microstructure provided a hardness of 254 +/- 2 HV ,
yield strength of 800 MPa and elongation of 10%. The spinodal structure
persisted during isothermal ageing, further increasing the yield strength to
890MPa while retaining an elongation of 7%. This work demonstrates the
potential for spinodal microstructures to overcome the difficulties in
retaining ductility in ultra-fine grained or nanocrystalline alloys, especially
upon post-deformation heating where strain softening normally results in
brittle behavior. | cond-mat |
Ultrafast Calculation of Diffuse Scattering from Atomistic Models: Diffuse scattering is a rich source of information about disorder in
crystalline materials, which can be modelled using atomistic techniques such as
Monte Carlo and molecular dynamics simulations. Modern X-ray and neutron
scattering instruments can rapidly measure large volumes of diffuse-scattering
data. Unfortunately, current algorithms for atomistic diffuse-scattering
calculations are too slow to model large data sets completely, because the fast
Fourier transform (FFT) algorithm has long been considered unsuitable for such
calculations [Butler & Welberry, J. Appl. Cryst. 25, 391 (1992)]. Here, a new
approach is presented for ultrafast calculation of atomistic diffuse-scattering
patterns. It is shown that the FFT can actually be used to perform such
calculations rapidly, and that a fast method based on sampling theory can be
used to reduce high-frequency noise in the calculations. These algorithms are
benchmarked using realistic examples of compositional, magnetic and displacive
disorder. They accelerate the calculations by a factor of at least 100, making
refinement of atomistic models to large diffuse-scattering volumes practical. | cond-mat |
Bound on Eigenstate Thermalization from Transport: We show that macroscopic thermalization and transport impose constraints on
matrix elements entering the Eigenstate Thermalization Hypothesis (ETH) ansatz
and require them to be correlated. It is often assumed that the ETH reduces to
Random Matrix Theory (RMT) below the Thouless energy scale. We show this
conventional picture is not self-consistent. We prove that energy scale at
which the RMT behavior emerges has to be parametrically smaller than the
inverse timescale of the slowest thermalization mode coupled to the operator of
interest. We argue that the timescale marking the onset of the RMT behavior is
the same timescale at which hydrodynamic description of transport breaks down. | cond-mat |
Disorder and non-Hermiticity in Kitaev spin liquids with a Majorana
Fermi surface: We study the effect of disorder on Z$_2$ quantum spin liquids with a Majorana
Fermi line (respectively surface in three dimensions) and show that depending
on the symmetries that are preserved \emph{on average} qualitatively different
scenarios will occur.
In particular, we identify the relevant non-Hermitian symmetries for which
disorder will effectively split the Fermi line into two exceptional lines, with
$\Re(E)=0$ states filling the area in between. We demonstrate the different
scenarios using both toy models as well as large-scale numerical simulations. | cond-mat |
The metastate approach to thermodynamic chaos: In realistic disordered systems, such as the Edwards-Anderson (EA) spin
glass, no order parameter, such as the Parisi overlap distribution, can be both
translation-invariant and non-self-averaging. The standard mean-field picture
of the EA spin glass phase can therefore not be valid in any dimension and at
any temperature. Further analysis shows that, in general, when systems have
many competing (pure) thermodynamic states, a single state which is a mixture
of many of them (as in the standard mean-field picture) contains insufficient
information to reveal the full thermodynamic structure. We propose a different
approach, in which an appropriate thermodynamic description of such a system is
instead based on a metastate, which is an ensemble of (possibly mixed)
thermodynamic states. This approach, modelled on chaotic dynamical systems, is
needed when chaotic size dependence (of finite volume correlations) is present.
Here replicas arise in a natural way, when a metastate is specified by its
(meta)correlations. The metastate approach explains, connects, and unifies such
concepts as replica symmetry breaking, chaotic size dependence and replica
non-independence. Furthermore, it replaces the older idea of non-self-averaging
as dependence on the bulk couplings with the concept of dependence on the state
within the metastate at fixed coupling realization. We use these ideas to
classify possible metastates for the EA model, and discuss two scenarios
introduced by us earlier --- a nonstandard mean-field picture and a picture
intermediate between that and the usual scaling/droplet picture. | cond-mat |
Equivalence of wave function matching and Green's functions methods for
quantum transport: generalized Fisher-Lee relation: We present a proof of an exact equivalence of the two approaches that are
most used in computing conductance in quantum electron and phonon transport:
the wave function matching and Green's functions methods. We can obtain all the
quantities defined in one method starting from those obtained in the other.
This completes and illuminates the work started Ando[Ando T 1991 Phys. Rev. B
44 8017] and continued later by Komyakov et al.[Khomyakov P A, Brocks G, Karpan
V, Zwierzycki M and Kelly P J 2005 Phys. Rev. B 72 035450]. The aim is to allow
for solving the transport problem with whichever approach fits most the system
at hand. One major corollary of the proven equivalence is our derivation of a
generalized Fisher-Lee formula for resolving the transmission function into
individual phonon mode contributions. As an illustration, we applied our method
to a simple model to highlight its accuracy and simplicity. | cond-mat |
Spontaneous multipole ordering by local parity mixing: Broken spatial inversion symmetry in spin-orbital coupled systems leads to a
mixing between orbitals with different parity, which results in unusual
electronic structures and transport properties. We theoretically investigate
the possibility of multipole ordering induced by a parity mixing. In
particular, we focus on the system in which the parity mixing appears in a
sublattice-dependent form. Starting from the periodic Anderson model with such
a local parity mixing, we derive an extended Kondo lattice model with
sublattice-dependent antisymmetric exchange couplings between itinerant
electrons and localized spins. By the variational calculation, simulated
annealing, and Monte Carlo simulation, we show that the model on a
quasi-one-dimensional zig-zag lattice exhibits an odd-parity multipole order
composed of magnetic toroidal and quadrupole components at and near half
filling. The multipole order causes a band deformation with the band bottom
shift and a magnetoelectric response. The results suggest that unusual
odd-parity multipole orders will be widely observed in multi-orbital systems
with local parity mixing. | cond-mat |
Cold spots in quantum systems far from equilibrium: local entropies and
temperatures near absolute zero: We consider a question motivated by the third law of thermodynamics: can
there be a local temperature arbitrarily close to absolute zero in a
nonequilibrium quantum system? We consider nanoscale quantum conductors with
the source reservoir held at finite temperature and the drain held at or near
absolute zero, a problem outside the scope of linear response theory. We obtain
local temperatures close to absolute zero when electrons originating from the
finite temperature reservoir undergo destructive quantum interference. The
local temperature is computed by numerically solving a nonlinear system of
equations describing equilibration of a scanning thermoelectric probe with the
system, and we obtain excellent agreement with analytic results derived using
the Sommerfeld expansion. A local entropy for a nonequilibrium quantum system
is introduced, and used as a metric quantifying the departure from local
equilibrium. It is shown that the local entropy of the system tends to zero
when the probe temperature tends to zero, consistent with the third law of
thermodynamics. | cond-mat |
Deconstructing Magnetization Noise: Degeneracies, Phases, and Mobile
Fractionalized Excitations in Tetris Artificial Spin Ice: Direct detection of spontaneous spin fluctuations, or "magnetization noise",
is emerging as a powerful means of revealing and studying magnetic excitations
in both natural and artificial frustrated magnets. Depending on the lattice and
nature of the frustration, these excitations can often be described as
fractionalized quasiparticles possessing an effective magnetic charge. Here, by
combining ultrasensitive optical detection of thermodynamic magnetization noise
with Monte Carlo simulations, we reveal emergent regimes of magnetic
excitations in artificial "tetris ice". A marked increase of the intrinsic
noise at certain applied magnetic fields heralds the spontaneous proliferation
of fractionalized excitations, which can diffuse independently, without cost in
energy, along specific quasi-1D spin chains in the tetris ice lattice. | cond-mat |
Winding up superfluid in a torus via Bose Einstein condensation: We simulate Bose-Einstein condensation (BEC) in a ring employing stochastic
Gross-Pitaevskii equation and show that cooling through the critical
temperature can generate spontaneous quantized circulation around the ring of
the newborn condensate. Dispersion of the resulting winding numbers follows
scaling law predicted by the Kibble-Zurek mechanism (KZM). Density growth also
exhibits scaling behavior consistent with KZM. This paves a way towards
experimental verification of KZM scalings, and experimental determination of
critical exponents for the BEC transition. | cond-mat |
Time-dependent matrix product ansatz for interacting reversible dynamics: We present an explicit time-dependent matrix product ansatz (tMPA) which
describes the time-evolution of any local observable in an interacting and
deterministic lattice gas, specifically for the rule 54 reversible cellular
automaton of [Bobenko et al., Commun. Math. Phys. 158, 127 (1993)]. Our
construction is based on an explicit solution of real-space real-time inverse
scattering problem. We consider two applications of this tMPA. Firstly, we
provide the first exact and explicit computation of the dynamic structure
factor in an interacting deterministic model, and secondly, we solve the
extremal case of the inhomogeneous quench problem, where a semi-infinite
lattice in the maximum entropy state is joined with an empty semi-infinite
lattice. Both of these exact results rigorously demonstrate a coexistence of
ballistic and diffusive transport behaviour in the model, as expected for
normal fluids. | cond-mat |
Curved non-interacting two-dimensional electron gas with anisotropic
mass: In the da Costa's thin-layer approach, a quantum particle moving in a 3D
sample is confined on a curved thin interface. At the end, the interface
effects are ignored and such quantum particle is localized on a curved surface.
A geometric potential arises and, since it manifests due to this confinement
procedure, it depends on the transverse to the surface mass component. This
inspired us to consider, in this paper, the effects due to an anisotropic
effective mass on a non-interacting two dimensional electron gas confined on a
curved surface, a fact not explored before in this context. By tailoring the
mass, many investigations carried out in the literature can be improved which
in turns can be useful to better designing electronic systems without modifying
the geometry of a given system. Some examples are examined here, as a particle
on helicoidal surface, on a cylinder, on a catenoid and on a cone, with some
possible applications briefly discussed. | cond-mat |
Elastohydrodynamics of swimming helices: effects of flexibility and
confinement: Motivated by bacterial transport through porous media, here we study the
swimming of an actuated, flexible helical filament in both three-dimensional
free space and within a cylindrical tube whose diameter is much smaller than
the length of the helix. The filament, at rest, has a native helical shape
modeled after the geometry of a typical bacterial flagellar bundle. The finite
length filament is a free swimmer, and is driven by an applied torque as well
as a counter-torque (of equal strength and opposite direction) that represents
a virtual cell body. We use a regularized Stokeslet framework to examine the
shape changes of the flexible filament in response to the actuation as well as
the swimming performance as a function of the nondimensional Sperm number that
characterizes the elastohydrodynamic system. We also show that a modified Sperm
number may be defined to characterize the swimming progression within a tube.
Finally, we demonstrate that a helical filament whose axis is not aligned with
the tube axis can exhibit centering behavior in the narrowest tubes. | cond-mat |
Novel Laves phase superconductor NbBe2: A theoretical investigation: A new Laves phase superconductor NbBe2, prototype with MgCu2, having maximum
Tc ~2.6 K has been reported very recently. Based on first-principle
calculations, we systematically study the structural, elastic, mechanical,
electronic, thermal and superconducting properties of the newly reported
superconducting intermetallic compound NbBe2. Finally, we investigate the
electron-phonon coupling constant, phonon dispersion curve and density of
states which indicates that the compound under study is a weakly coupled BCS
superconductor. | cond-mat |
Third-order optical conductivity of an electron fluid: We derive the nonlinear optical conductivity of an isotropic electron fluid
at frequencies below the interparticle collision rate. In this regime, governed
by hydrodynamics, the conductivity acquires a universal form at any
temperature, chemical potential, and spatial dimension. We show that the
nonlinear response of the fluid to a uniform field is dominated by the
third-order conductivity tensor $\sigma^{(3)}$ whose magnitude and temperature
dependence differ qualitatively from those in the conventional kinetic regime
of higher frequencies. We obtain explicit formulas for $\sigma^{(3)}$ for Dirac
materials such as graphene and Weyl semimetals. We make predictions for the
third-harmonic generation, renormalization of the collective-mode spectrum, and
the third-order circular magnetic birefringence experiments. | cond-mat |
Asymptotic Freedom and Large Spin Antiferromagnetic Chains: Building on the mapping of large-$S$ spin chains onto the O($3$) nonlinear
$\sigma$ model with coupling constant $2/S$, and on general properties of that
model (asymptotic freedom, implying that perturbation theory is valid at high
energy, and Elitzur's conjecture that rotationally invariant quantities are
infrared finite in perturbation theory), we use the Holstein-Primakoff
representation to derive analytic expressions for the equal-time and dynamical
spin-spin correlations valid at distances smaller than $S^{-1} \exp(\pi S)$ or
at energies larger than $J S^2 \exp(-\pi S)$, where $J$ is the Heisenberg
exchange coupling. This is supported by comparing the static correlations with
quantum Monte Carlo simulations for $S = 5/2$. | cond-mat |
Heat capacity of Schottky type in low-dimensional spin system: The heat capacity of low-dimensional spin systems is studied using
theoretical and numerical techniques. Keeping only two energy states, the
system is mapped onto the two -level-system (TLS) model. Using the low
temperature Lanczos method, it is confirmed that the behavior of $T_{M}$ and
the energy gap as functions of the control parameter is the same in the two
models studied; a conclusion that can probably be extrapolated to the general
case of any system that possesses an energy gap. | cond-mat |
Correlation and confinement induced itinerant ferromagnetism in chain
structures: Using a positive semidefinite operator technique one deduces exact ground
states for a zig-zag hexagon chain described by a non-integrable Hubbard model
with on-site repulsion. Flat bands are not present in the bare band structure,
and the operators $\hat B^{\dagger}_{\mu,\sigma}$ introducing the electrons
into the ground state, are all extended operators and confined in the quasi 1D
chain structure of the system. Consequently, increasing the number of carriers,
the $\hat B^{\dagger}_{\mu,\sigma}$ operators become connected i.e. touch each
other on several lattice sites. Hence the spin projection of the carriers
becomes correlated in order to minimize the ground state energy by reducing as
much as possible the double occupancy leading to a ferromagnetic ground state.
This result demonstrates in exact terms in a many-body frame that the
conjecture made at two-particle level by G. Brocks et al.
[Phys.Rev.Lett.93,146405,(2004)] that the Coulomb interaction is expected to
stabilize correlated magnetic ground states in acenes is clearly viable, and
opens new directions in the search for routes in obtaining organic
ferromagnetism. Due to the itinerant nature of the obtained ferromagnetic
ground state, the systems under discussion may have also direct application
possibilities in spintronics. | cond-mat |
Resistive state of superconducting structures with fractal clusters of a
normal phase: The effect of morphologic factors on magnetic flux dynamics and critical
currents in percolative superconducting structures is considered. The
superconductor contains the fractal clusters of a normal phase, which act as
pinning centers. The properties of these clusters are analyzed in the general
case of gamma-distribution of their areas. The statistical characteristics of
the normal phase clusters are studied, the critical current distribution is
derived, and the dependencies of the main statistical parameters on the fractal
dimension are found. The effect of fractal clusters of a normal phase on the
electric field induced by the motion of the magnetic flux after the vortices
have been broken away from pinning centers is considered. The voltage-current
characteristics of fractal superconducting structures in a resistive state for
an arbitrary fractal dimension are obtained. It is found that the fractality of
the boundaries of normal phase clusters intensifies magnetic flux trapping and
thereby increases the current-carrying capability of the superconductor. | cond-mat |
Magnetotransport near a quantum critical point in a simple metal: We use geometric considerations to study transport properties, such as the
conductivity and Hall coefficient, near the onset of a nesting-driven spin
density wave in a simple metal. In particular, motivated by recent experiments
on vanadium-doped chromium, we study the variation of transport coefficients
with the onset of magnetism within a mean-field treatment of a model that
contains nearly nested electron and hole Fermi surfaces. We show that most
transport coefficients display a leading dependence that is linear in the
energy gap. The coefficient of the linear term, though, can be small. In
particular, we find that the Hall conductivity $\sigma_{xy}$ is essentially
unchanged, due to electron-hole compensation, as the system goes through the
quantum critical point. This conclusion extends a similar observation we made
earlier for the case of completely flat Fermi surfaces to the immediate
vicinity of the quantum critical point where nesting is present but not
perfect. | cond-mat |
Flagellated bacterial motility in polymer solutions: It is widely believed that the swimming speed, $v$, of many flagellated
bacteria is a non-monotonic function of the concentration, $c$, of
high-molecular-weight linear polymers in aqueous solution, showing peaked
$v(c)$ curves. Pores in the polymer solution were suggested as the explanation.
Quantifying this picture led to a theory that predicted peaked $v(c)$ curves.
Using new, high-throughput methods for characterising motility, we have
measured $v$, and the angular frequency of cell-body rotation, $\Omega$, of
motile Escherichia coli as a function of polymer concentration in
polyvinylpyrrolidone (PVP) and Ficoll solutions of different molecular weights.
We find that non-monotonic $v(c)$ curves are typically due to low-molecular
weight impurities. After purification by dialysis, the measured $v(c)$ and
$\Omega(c)$ relations for all but the highest molecular weight PVP can be
described in detail by Newtonian hydrodynamics. There is clear evidence for
non-Newtonian effects in the highest molecular weight PVP solution.
Calculations suggest that this is due to the fast-rotating flagella `seeing' a
lower viscosity than the cell body, so that flagella can be seen as
nano-rheometers for probing the non-Newtonian behavior of high polymer
solutions on a molecular scale. | cond-mat |
Emergence of multi-body interactions in few-atom sites of a fermionic
lattice clock: Alkaline-earth (AE) atoms have metastable clock states with minute-long
optical lifetimes, high-spin nuclei, and SU($N$)-symmetric interactions that
uniquely position them for advancing atomic clocks, quantum information
processing, and quantum simulation. The interplay of precision measurement and
quantum many-body physics is beginning to foster an exciting scientific
frontier with many opportunities. Few particle systems provide a window to view
the emergence of complex many-body phenomena arising from pairwise
interactions. Here, we create arrays of isolated few-body systems in a
fermionic ${}^{87}$Sr three-dimensional (3D) optical lattice clock and use high
resolution clock spectroscopy to directly observe the onset of both elastic and
inelastic multi-body interactions. These interactions cannot be broken down
into sums over the underlying pairwise interactions. We measure
particle-number-dependent frequency shifts of the clock transition for atom
numbers $n$ ranging from 1 to 5, and observe nonlinear interaction shifts,
which are characteristic of SU($N$)-symmetric elastic multi-body effects. To
study inelastic multi-body effects, we use these frequency shifts to isolate
$n$-occupied sites and measure the corresponding lifetimes. This allows us to
access the short-range few-body physics free from systematic effects
encountered in a bulk gas. These measurements, combined with theory, elucidate
an emergence of multi-body effects in few-body systems of sites populated with
ground-state atoms and those with single electronic excitations. By connecting
these few-body systems through tunneling, the favorable energy and timescales
of the interactions will allow our system to be utilized for studies of
high-spin quantum magnetism and the Kondo effect. | cond-mat |
Phase Transitions Driven by Vortices in 2D Superfluids and
Superconductors: From Kosterlitz-Thouless to 1st Order: The Landau-Ginzburg-Wilson hamiltonian is studied for different values of the
parameter $\lambda$ which multiplies the quartic term (it turns out that this
is equivalent to consider different values of the coherence length $\xi$ in
units of the lattice spacing $a$). It is observed that amplitude fluctuations
can change dramatically the nature of the phase transition: for small values of
$\lambda$ ($\xi/a > 0.7$), instead of the smooth Kosterlitz-Thouless transition
there is a {\em first order} transition with a discontinuous jump in the vortex
density $v$ and a larger non-universal drop in the helicity modulus. In
particular, for $\lambda$ sufficiently small ($\xi/a \cong 1$), the density of
bound pairs of vortex-antivortex below $T_c$ is so low that, $v$ drops to zero
almost for all temperature $T<Tc$. | cond-mat |
Condensate Fraction and Pair Coherence Lengths of Two-Dimension Fermi
Gases with Spin-Orbit Coupling: The effects of Rashba spin-orbit coupling on BCS-BEC crossover, the
condensate fraction and pair coherence lengths for a two-component attractive
Fermi gas in two dimension are studied. The results at $T=0K$ indicate that (1)
when the strength of SOC is beyond a critical value, BCS-BEC crossover does not
happen in a conventional sense; (2) SOC enhances the condensate fraction, but
suppresses pair coherence lengths. | cond-mat |
Generation of electric current and electromotive force by an
antiferromagnetic domain wall: Dynamic magnetic textures may transfer the angular moment from the varying in
time antiferromagnetic order to spins of conduction electrons. Due to the spin
orbit coupling (SOC) these spin excitations can induce the electric current of
conduction electrons. We calculated the electric current and the electromotive
force (EMF) which are produced by a domain wall (DW) moving parallel to the
magnetically compensated interface between an antiferromagnetic insulator
(AFMI) and a two-dimensional spin orbit coupled metal. Spins of conduction
electrons interact with localized spins of a collinear AFMI through the
interface exchange interaction. The Keldysh formalism of nonequilibrium Green
functions was applied for the analysis of this system. It is shown that a Bloch
DW generates the current perpendicular to the DW motion direction. At the same
time a N\'{e}el DW creates the electric potential which builds up across the
wall. The total charge which is pumped by a Bloch DW can be expressed in terms
of a topologically invariant charge quantum. The latter does not depend on
variations of DW's velocity and shape. These effects increase dramatically when
the Fermi energy approaches the van Hove singularity of the Fermi surface. The
obtained results are important for the electrical detection and control of
dynamic magnetic textures in antiferromagnets. | cond-mat |
Hausdorff dimension and filling factor: We propose a new hierarchy scheme for the filling factor, a parameter which
characterizes the occurrence of the Fractional Quantum Hall Effect (FQHE). We
consider the Hausdorff dimension, $h$, as a parameter for classifying
fractional spin particles, such that, it is written in terms of the statistics
of the collective excitations. The number $h$ classifies these excitations with
different statistics in terms of its homotopy class. | cond-mat |
Bounding the coarse graining error in hidden Markov dynamics: Lumping a Markov process introduces a coarser level of description that is
useful in many contexts and applications. The dynamics on the coarse grained
states is often approximated by its Markovian component. In this letter we
derive finite-time bounds on the error in this approximation. These results
hold for non-reversible dynamics and for probabilistic mappings between
microscopic and coarse grained states. | cond-mat |
Drude weight in systems with open boundary conditions: For finite systems, the real part of the conductivity is usually decomposed
as the sum of a zero frequency delta peak and a finite frequency regular part.
In studies with periodic boundary conditions, the Drude weight, i.e., the
weight of the zero frequency delta peak, is found to be nonzero for integrable
systems, even at very high temperatures, whereas it vanishes for generic
(nonintegrable) systems. Paradoxically, for systems with open boundary
conditions, it can be shown that the coefficient of the zero frequency delta
peak is identically zero for any finite system, regardless of its
integrability. In order for the Drude weight to be a thermodynamically
meaningful quantity, both kinds of boundary conditions should produce the same
answer in the thermodynamic limit. We shed light on these issues by using
analytical and numerical methods. | cond-mat |
Giant current-driven domain wall mobility in (Ga,Mn)As: We study theoretically hole current-driven domain wall dynamics in (Ga,Mn)As.
We show that the spin-orbit coupling causes significant hole reflection at the
domain wall, even in the adiabatic limit when the wall is much thicker than the
Fermi wavelength, resulting in spin accumulation and mistracking between
current-carrying spins and the domain wall magnetization. This increases the
out-of-plane non-adiabatic spin transfer torque and consequently the
current-driven domain wall mobility by three to four orders of magnitude.
Trends and magnitude of the calculated domain wall current mobilities agree
with experimental findings. | cond-mat |
Effect of orbital relaxation on the band structure of cuprate
superconductors and implications for the superconductivity mechanism: Where the doped holes reside in cuprate superconductors has crucial
implications for the understanding of the mechanism responsible for their high
temperature superconductivity. It has been generally assumed that doped holes
reside in hybridized Cu $d_{x^2-y^2}$ - O $p\sigma$ orbitals in the $CuO_2$
planes, based on results of density functional band structure calculations.
Instead, we propose that doped holes in the cuprates reside in O $p\pi$
orbitals in the plane, perpendicular to the $Cu-O$ bond, that are raised to the
Fermi energy through local orbital relaxation, that is not taken into account
in band structure calculations that place the bands associated with these
orbitals well below the Fermi energy. We use a dynamic Hubbard model to
incorporate the orbital relaxation degree of freedom and find in exact
diagonalization of a small $Cu_4O_4$ cluster that holes will go to the O $p\pi$
orbitals for relaxation energies comparable to what is expected from atomic
properties of oxygen anions. The bandwidth of this band becomes significantly
smaller than predicted by band structure calculations due to the orbital
relaxation effect. Within the theory of hole superconductivity the heavy hole
carriers in this almost full band will pair and drive the system
superconducting through lowering of their quantum kinetic energy. | cond-mat |
Quantum fluctuations of the ultracold atom-molecule mixtures: We investigate evolution of the quantum coherence in the ultracold mixture of
fermionic atoms and bosonic dimer molecules. Interactions are there
experimentally controlled via tuning the external magnetic field. Consequently,
the fermionic atoms and their bosonic counterparts can be driven to a behavior
resembling the usual BCS to BEC crossover. We analyze in some detail how this
quantum coherence evolves with respect to time upon a smooth and abrupt sweep
across the Feshbach resonance inducing the atom-molecule quantum fluctuations. | cond-mat |
Phase boundary near a magnetic percolation transition: Motivated by recent experimental observations [Phys. Rev. 96, 020407 (2017)]
on hexagonal ferrites, we revisit the phase diagrams of diluted magnets close
to the lattice percolation threshold. We perform large-scale Monte Carlo
simulations of XY and Heisenberg models on both simple cubic lattices and
lattices representing the crystal structure of the hexagonal ferrites. Close to
the percolation threshold $p_c$, we find that the magnetic ordering temperature
$T_c$ depends on the dilution $p$ via the power law $T_c \sim |p-p_c|^\phi$
with exponent $\phi=1.09$, in agreement with classical percolation theory.
However, this asymptotic critical region is very narrow, $|p-p_c| \lesssim
0.04$. Outside of it, the shape of the phase boundary is well described, over a
wide range of dilutions, by a nonuniversal power law with an exponent somewhat
below unity. Nonetheless, the percolation scenario does not reproduce the
experimentally observed relation $T_c \sim (x_c -x)^{2/3}$ in
PbFe$_{12-x}$Ga$_x$O$_{19}$. We discuss the generality of our findings as well
as implications for the physics of diluted hexagonal ferrites. | cond-mat |
Charge and spin ordering in Nd{1/3}Sr{2/3}FeO{3}: We have investigated the charge and spin ordering in Nd{1/3}Sr{2/3}FeO{3}
with neutron diffraction technique. This sample undergoes a charge ordering
transition accompanying charge disproportionation of 2Fe4+ -> Fe3+ + Fe5+. We
measured the superlattice reflections due to the charge and spin ordering, and
confirmed that charges and spins order simultaneously at Tco = 185 K. The
ordering pattern of charges and spins in this sample can be viewed as three
dimensional stripe order, and is compared with two dimensional stripe order
observed in other transition metal oxides. | cond-mat |
Large deviations for the Pearson family of ergodic diffusion processes
involving a quadratic diffusion coefficient and a linear force: The Pearson family of ergodic diffusions with a quadratic diffusion
coefficient and a linear force are characterized by explicit dynamics of their
integer moments and by explicit relaxation spectral properties towards their
steady state. Besides the Ornstein-Uhlenbeck process with a Gaussian steady
state, the other representative examples of the Pearson family are the
Square-Root or the Cox-Ingersoll-Ross process converging towards the
Gamma-distribution, the Jacobi process converging towards the
Beta-distribution, the reciprocal-Gamma process (corresponding to an
exponential functional of the Brownian motion) that converges towards the
Inverse-Gamma-distribution, the Fisher-Snedecor process, and the Student
process, so that the last three steady states display heavy-tails. The goal of
the present paper is to analyze the large deviations properties of these
various diffusion processes in a unified framework. We first consider the Level
1 concerning time-averaged observables over a large time-window $T$ : we write
the first rescaled cumulants for generic observables and we identify the
specific observables whose large deviations can be explicitly computed from the
dominant eigenvalue of the appropriate deformed-generator. The explicit large
deviations at Level 2 concerning the time-averaged density are then used to
analyze the statistical inference of model parameters from data on a very long
stochastic trajectory in order to obtain the explicit rate function for the two
inferred parameters of the Pearson linear force. | cond-mat |
Three-Dimensional Percolation Modeling of Self-Healing Composites: We study the self-healing process of materials with embedded "glue"-carrying
cells, in the regime of the onset of the initial fatigue. Three-dimensional
numerical simulations within the percolation-model approach are reported. The
main numerical challenge taken up in the present work, has been to extend the
calculation of the conductance to three-dimensional lattices. Our results
confirm the general features of the process: The onset of the material fatigue
is delayed, by developing a plateau-like time-dependence of the material
quality. We demonstrate that in this low-damage regime, the changes in the
conductance and thus, in similar transport/response properties of the material
can be used as measures of the material quality degradation. A new feature
found for three dimensions, where it is much more profound than in
earlier-studied two-dimensional systems, is the competition between the healing
cells. Even for low initial densities of the healing cells, they interfere with
each other and reduce each other's effective healing efficiency. | cond-mat |
The Fermi Edge Singularity and Boundary Condition Changing Operators: The boundary conformal field theory approach to quantum impurity problems is
used to study the Fermi edge singularity, occuring in the X-ray adsorption
probablility. The deep-hole creation operator, in the effective low-energy
theory, changes the boundary condition on the conduction electrons. By a
conformal mapping, the dimension of such an operator is related to the
groundstate energy for a finite system with different boundary conditions at
the two ends. The Fermi edge singularity is solved using this method, for the
Luttinger liquid including back-scattering and for the multi-channel Kondo
problem. | cond-mat |
The second law and fluctuations of work: The case against quantum
fluctuation theorems: We study how Thomson's formulation of the second law: no work is extracted
from an equilibrium ensemble by a cyclic process, emerges in the quantum
situation through the averaging over fluctuations of work. The latter concept
is carefully defined for an ensemble of quantum systems interacting with
macroscopic sources of work. The approach is based on first splitting a mixed
quantum ensemble into pure subensembles, which according to quantum mechanics
are maximally complete and irreducible. The splitting is done by filtering the
outcomes of a measurement process. A critical review is given of two other
approaches to fluctuations of work proposed in the literature. It is shown that
in contrast to those ones, the present definition {\it i)} is consistent with
the physical meaning of the concept of work as mechanical energy lost by the
macroscopic sources, or, equivalently, as the average energy acquired by the
ensemble; {\it ii)} applies to an arbitrary non-equilibrium state. There is no
direct generalization of the classical work-fluctuation theorem to the proper
quantum domain. This implies some non-classical scenarios for the emergence of
the second law. | cond-mat |
Detecting a true quantum pump effect: Even though quantum pumping is a very promising field, it has unfortunately
not been unambiguously experimentally detected. The reason being that in the
experiments the rectification effect overshadows the pumped current. One of the
better known ways to detect it is by using the magnetic field symmetry
properties of the rectified and pumped currents. The rectified currents are
symmetric with respect to magnetic field reversal while the pumped currents do
not possess any definite symmetry with respect to field reversal. This feature
has been exploited in some recent works. In this work we look beyond this
magnetic field symmetry properties and provide examples wherein the nature or
magnitudes of the pumped and rectified currents are exactly opposite enabling
an effective distinction between the two. | cond-mat |
Quantum back-action evading measurement of collective mechanical modes: The standard quantum limit constrains the precision of an oscillator position
measurement. It arises from a balance between the imprecision and the quantum
back-action of the measurement. However, a measurement of only a single
quadrature of the oscillator can evade the back-action and be made with
arbitrary precision. Here we demonstrate quantum back-action evading
measurements of a collective quadrature of two mechanical oscillators, both
coupled to a common microwave cavity. The work allows for quantum state
tomography of two mechanical oscillators, and provides a foundation for
macroscopic mechanical entanglement and force sensing beyond conventional
quantum limits. | cond-mat |
Tailoring the flow of soft glasses by soft additives: We examine the vitrification and melting of asymmetric star polymers mixtures
by combining rheological measurements with mode coupling theory. We identify
two types of glassy states, a {\it single} glass, in which the small component
is fluid in the glassy matrix of the big one and a {\it double} glass, in which
both components are vitrified. Addition of small star polymers leads to melting
of {\it both} glasses and the melting curve has a non-monotonic dependence on
the star-star size ratio. The phenomenon opens new ways for externally steering
the rheological behavior of soft matter systems. | cond-mat |
Hidden structural transition in epitaxial
Ca$_{0.5}$Sr$_{0.5}$IrO$_{3}$/SrTiO$_{3}$ thin film: A structural transition in an ABO$_{3}$ perovskite thin film involving the
change of the BO$_{6}$ octahedral rotation pattern can be hidden under the
global lattice symmetry imposed by the substrate and often easily overlooked.
We carried out high-resolution x-ray diffraction experiments to investigate the
structures of epitaxial Ca$_{0.5}$Sr$_{0.5}$IrO$_{3}$ (CSIO) perovskite iridate
films grown on the SrTiO$_{3}$ (STO) and GdScO$_{3}$ (GSO) substrates in
detail. Although the CSIO/STO film layer displays a global tetragonal lattice
symmetry evidenced by the reciprocal space mapping, synchrotron x-ray data
indicates that its room temperature structure is monoclinic due to Glazer's
a$^{+}$a$^{-}$c$^{-}$-type rotation of the IrO$_{6}$ octahedra. In order to
accommodate the lower-symmetry structure under the global tetragonal symmetry,
the film breaks into four twinned domains, resulting in the splitting of the
(half-integer, 0, integer) superlattice reflections. Surprisingly, the
splitting of these superlattice reflections decrease with increasing
temperature, eventually disappearing at T$_{S}$ = 510(5) K, which signals a
structural transition to an orthorhombic phase with a$^{+}$a$^{-}$c$^{0}$
octahedral rotation. In contrast, the CSIO/GSO film displays a stable
monoclinic symmetry with a$^{+}$b$^{-}$c$^{-}$ octahedral rotation, showing no
structural instability caused by the substrate up to 520 K. Our study
illustrates the importance of the symmetry in addition to the lattice mismatch
of the substrate in determining the structure of epitaxial thin films. | cond-mat |
Effect of one-dimensional superlattice potentials on the band gap of
two-dimensional materials: Using the tight-binding approach, we analyze the effect of a one-dimensional
superlattice (1DSL) potential on the electronic structure of black phosphorene
and transition metal dichalcogenides. We observe that the 1DSL potential
results in a decrease of the energy band gap of the two-dimensional (2D)
materials. An analytical model is presented to relate the decrease in the
direct-band gap to the different orbital characters between the valence band
top and conduction band bottom of the 2D materials. The direct-to-indirect gap
transition, which occurs under a 1DSL potential with an unequal barrier width,
is also discussed. | cond-mat |
The Heart of Fidelity: The multi-electron wave function of an interacting electron system depends on
the size of the system, i.e. the number of electrons. Here the question
investigated is how the wave function changes for a symmetric Friedel-Anderson
impurity when the volume is doubled. It turns out that for sufficiently large
volume (when the level spacing is smaller than the resonance width) the change
in the wave function can be expressed in terms of a universal single-electron
state |q> centered at the Fermi level. This electron state is independent of
the number of electrons and independent of the parameters of the
Friedel-Anderson impurity. It is even the same universal state for a Kondo
impurity and a symmetric Friedel impurity independent of any parameter. The
only requirement is that the impurity has a resonance exactly at the Fermi
level and that the level spacing is smaller than the resonance width. This
result clarifies recent fidelity calculations. | cond-mat |
Three-body local correlation function in the Lieb-Liniger model:
bosonization approach: We develop a method for the calculation of vacuum expectation values of local
operators in the Lieb-Liniger model. This method is based on a set of new
identities obtained using integrability and effective theory (``bosonization'')
description. We use this method to get an explicit expression for the
three-body local correlation function, measured in a recent experiment [1]. | cond-mat |
On the Belitz-Kirkpatrick comment on "Specific heat of a Fermi system
near ferromagnetic quantum phase transition", by I.Grosu, D.Bodea and
M.Crisan (cond-mat/0101392): We reply to Belitz and Kirkpatrick comment cond-mat/0102064, on
cond-mat/0101392. | cond-mat |
Limit of validity of Ostwald's rule of stages in a statistical
mechanical model of crystallization: We have only rules of thumb with which to predict how a material will
crystallize, chief among which is Ostwald's rule of stages. It states that the
first phase to appear upon transformation of a parent phase is the one closest
to it in free energy. Although sometimes upheld, the rule is without
theoretical foundation and is not universally obeyed, highlighting the need for
microscopic understanding of crystallization controls. Here we study in detail
the crystallization pathways of a prototypical model of patchy particles. The
range of crystallization pathways it exhibits is richer than can be predicted
by Ostwald's rule, but a combination of simulation and analytic theory reveals
clearly how these pathways are selected by microscopic parameters. Our results
suggest strategies for controlling self-assembly pathways in simulation and
experiment. | cond-mat |
d0 Ferromagnetism in Mg-doped Rutile TiO2 Nanoparticles: In a quest of enriching the area of d0 magnetism in oxide materials, we have
undertaken to study Mg-doped TiO2 compounds. The Ti1-xMgxO2 (x=0, 0.02, 0.04
and 0.06) nanoparticles were prepared by solid-state reaction route. The X-ray
diffractions (XRD) patterns of these samples indicate single phase of
tetragonal rutile-structure of TiO2. The refinement of the XRD patterns reveals
no change in the crystallographic lattice parameters in comparison to pure TiO2
upon Mg doping and it indicates that Mg2+ ions do not enter core grains and
form core/shell structure. SEM observations reveal the uniform morphology with
nanometric grains in the range of 150-200 nm. The measurement of magnetic
properties of these compounds indicates that pure TiO2 and Ti0.98Mg0.02
compounds exhibit paramagnetic behavior and Ti0.96Mg0.04 compound exhibits
ferromagnetic (FM) phase superimposed with the dominating paramagnetic phase.
However, Ti0.94Mg0.06 compound exhibits ferromagnetic to paramagnetic
transition with FM transition temperature of 180.2 K. The measurements of zero
field and field cooled magnetization data indicate low temperature magnetic
irreversibility for x=0.06 sample and it was attributed to the competing AFM
(core) and the FM (shell) interactions. The measurement of hysteresis curves at
various temperatures indicates domain wall pinning and an exchange-bias
behavior. | cond-mat |
The Sine-Gordon Wobble: Nonperturbative, oscillatory, winding number one solutions of the Sine-Gordon
equation are presented and studied numerically. We call these nonperturbative
shape modes {\sl wobble} solitons. Perturbed Sine-Gordon kinks are found to
decay to {\sl wobble} solitons. | cond-mat |
An efficient method to calculate the anharmonicity free energy: The anharmonicity resulted from the intrinsic phonon interaction is neglected
by quasiharmonic approximation. Although the intensive researches about
anharmonicity have been done, up to now the free energy contributed by the
anharmonicity is still difficult to calculate. Here we put forward a new method
that can well include the anharmonicity. We introduce the implicit temperature
dependence of effective frequency by volume modification. The quasiharmonic
approximation becomes a special case in our method corresponding to non volume
modification. Although our method is simple and only a constant need to
determine, the anharmonicity is well included. Thermodynamic properties of MgO
predicted with our method are excellent consistent with the experiment results
at very wide temperature range. We also believe that our method will be helpful
to reveal the characteristic of anharmonicity and intrinsic phonon interaction. | cond-mat |
Spectroscopic evidence for a charge-density-wave condensate in a
charge-ordered manganite: Observation of collective excitation mode in
Pr$_{\text{0.7}}$Ca$_{\text{0.3}}$MnO$_{\text{3}}$ by using THz time-domain
spectroscopy: THz time-domain spectroscopy was used to directly probe the low-energy
(0.5--5 meV) electrodynamics of the charge-ordered manganite
Pr$_{0.7}$Ca$_{0.3}$MnO$_3$. We revealed the existence of a finite peak
structure around 2--3 meV well below the charge gap $\sim300$ meV. In analogy
to the low-energy optical properties of the well-studied low-dimensional
materials, we attributed this observed structure to the collective excitation
mode arising from the charge-density-wave condensate. This finding provides the
importance role of the quasi-one dimensional nature of the charge and orbital
ordering in Pr$_{0.7}$Ca$_{0.3}$MnO$_3$. | cond-mat |
The random model for simulation of the growth of small populations: The probability of the survival of the population of individuals of both
sexes of given mature age, procreation rate and structure stability has been
searched in the numerical experiment. The populations with long period of
reproduction and the high rate of procreation and without social mobility have
the most chance to survive. The populations with the late mature age and high
mobility dies out. The fertility rate of simple reconstruction of generations
obtained in the model (2.8) is close to the value for a human being (2.1). | cond-mat |
Modelling of field-effect transistors based on 2D materials targeting
high-frequency applications: New technologies are necessary for the unprecedented expansion of
connectivity and communications in the modern technological society. The
specific needs of wireless communication systems in 5G and beyond, as well as
devices for the future deployment of Internet of Things has caused that the
International Technology Roadmap for Semiconductors, which is the strategic
planning document of the semiconductor industry, considered since 2011,
graphene and related materials (GRMs) as promising candidates for the future of
electronics. Graphene, a one-atom-thick of carbon, is a promising material for
high-frequency applications due to its intrinsic superior carrier mobility and
very high saturation velocity. These exceptional carrier transport properties
suggest that GRM-based field-effect transistors could potentially outperform
other technologies.
This thesis presents a body of work on the modelling, performance prediction
and simulation of GRM-based field-effect transistors and circuits. The main
goal of this work is to provide models and tools to ease the following issues:
(i) gaining technological control of single layer and bilayer graphene devices
and, more generally, devices based on 2D materials, (ii) assessment of
radio-frequency (RF) performance and microwave stability, (iii) benchmarking
against other existing technologies, (iv) providing guidance for device and
circuit design, (v) simulation of circuits formed by GRM-based transistors. | cond-mat |
The Individual and Collective Effects of Exact Exchange and Dispersion
Interactions on the Ab Initio Structure of Liquid Water: In this work, we report the results of a series of density functional theory
(DFT) based ab initio molecular dynamics (AIMD) simulations of ambient liquid
water using a hierarchy of exchange-correlation (XC) functionals to investigate
the individual and collective effects of exact exchange (Exx), via the PBE0
hybrid functional, non-local vdW/dispersion interactions, via a fully
self-consistent density-dependent dispersion correction, and approximate
nuclear quantum effects (aNQE), via a 30 K increase in the simulation
temperature, on the microscopic structure of liquid water. Based on these AIMD
simulations, we found that the collective inclusion of Exx, vdW, and aNQE as
resulting from a large-scale AIMD simulation of (H$_2$O)$_{128}$ at the
PBE0+vdW level of theory, significantly softens the structure of ambient liquid
water and yields an oxygen-oxygen structure factor, $S_{\rm OO}(Q)$, and
corresponding oxygen-oxygen radial distribution function, $g_{\rm OO}(r)$, that
are now in quantitative agreement with the best available experimental data.
This level of agreement between simulation and experiment as demonstrated
herein originates from an increase in the relative population of water
molecules in the interstitial region between the first and second coordination
shells, a collective reorganization in the liquid phase which is facilitated by
a weakening of the hydrogen bond strength by the use of the PBE0 hybrid XC
functional, coupled with a relative stabilization of the resultant disordered
liquid water configurations by the inclusion of non-local vdW/dispersion
interactions. | cond-mat |
Stable multispeed lattice Boltzmann methods: We demonstrate how to produce a stable multispeed lattice Boltzmann method
(LBM) for a wide range of velocity sets, many of which were previously thought
to be intrinsically unstable. We use non-Gauss--Hermitian cubatures. The method
operates stably for almost zero viscosity, has second-order accuracy,
suppresses typical spurious oscillation (only a modest Gibbs effect is present)
and introduces no artificial viscosity. There is almost no computational cost
for this innovation.
DISCLAIMER: Additional tests and wide discussion of this preprint show that
the claimed property of coupled steps: no artificial dissipation and the
second-order accuracy of the method are valid only on sufficiently fine grids.
For coarse grids the higher-order terms destroy coupling of steps and
additional dissipation appears.
The equations are true. | cond-mat |
Gapless spin liquid ground state of spin-1/2 $J_1$-$J_2$ Heisenberg
model on square lattices: The spin-1/2 $J_1$-$J_2$ Heisenberg model on square lattices are investigated
via the finite projected entangled pair states (PEPS) method. Using the
recently developed gradient optimization method combining with Monte Carlo
sampling techniques, we are able to obtain the ground states energies that are
competitive to the best results. The calculations show that there is no N\'eel
order, dimer order and plaquette order in the region of 0.42 $\lesssim
J_2/J_1\lesssim$ 0.6, suggesting a single spin liquid phase in the intermediate
region. Furthermore, the calculated staggered spin, dimer and plaquette
correlation functions all have power law decay behaviours, which provide strong
evidences that the intermediate nonmagnetic phase is a single gapless spin
liquid state. | cond-mat |
Bose-Condensed Gases in a 1D Optical Lattice at Finite Temperatures: We study equilibrium properties of Bose-Condensed gases in a one-dimensional
(1D) optical lattice at finite temperatures. We assume that an additional
harmonic confinement is highly anisotropic, in which the confinement in the
radial directions is much tighter than in the axial direction. We derive a
quasi-1D model of the Gross-Pitaeavkill equation and the Bogoliubov equations,
and numerically solve these equations to obtain the condensate fraction as a
function of the temperature. | cond-mat |
Surface structures of the magnetostrictive D03-Fe3Ga(001): First-principles total energy calculations and experimental measurements were
performed to study the surface reconstructions of the magnetostrictive Fe3Ga
alloy. The magnetostrictive behavior was evaluated in the bulk by compressing
and stretching its lattice parameter. Results demonstrate two thermodynamically
stable surfaces, the 1x1 and 3x1 reconstructions. The 1x1 is an ideally FeGa
terminated surface whereas the 3x1 is also FeGa terminated but it has a
first-layer Fe atom substituted by a Ga atom every three unit-cells, forming
stripe-like domain patterns. Tersoff-Hamann scanning tunneling microscopy
simulations were obtained and compared with experimental results. We found good
agreement between theory and experiment, in which the distance between rows is
~1.23 nm. The substrate-induced strain increases the stability of the 3x1
reconstruction. Here we have demonstrated that Ga/Fe atomic exchanges lead to
the stripe-like domain patterns. Clarification of the atomic reconstructions
present on the magnetostrictive Fe3Ga alloys is an important step towards the
understanding of its surfaces and poses this system as a potential candidate to
be used as part of perpendicular magnetic tunnel junctions due to the existing
perpendicular magnetic anisotropy effect when grown on different substrates. | cond-mat |
Magneto-transport of large CVD-grown graphene: We present magnetoresistance measurements on large scale monolayer graphene
grown by chemical vapor deposition (CVD) on copper. The graphene layer was
transferred onto SiO2/Si via PMMA and thermal release tape for transport
measurements. The resulting centimeter-sized graphene samples were measured at
temperatures down to 30mK in a magnetic field. We observe a very sharp peak in
resistance at zero field, which is well fitted by weak localization theory. The
samples exhibit conductance fluctuations symmetric in field, which are
attributed to ensemble averaged conductance fluctuations due to large scale
inhomogeneities consistent with the grain boundaries of copper during the CVD
growth. | cond-mat |
Linear and quadratic magnetoresistance in the semimetal SiP2: Multiple mechanisms for extremely large magnetoresistance (XMR) found in many
topologically nontrivial/trivial semimetals have been theoretically proposed,
but experimentally it is unclear which mechanism is responsible in a particular
sample. In this article, by the combination of band structure calculations,
numerical simulations of magnetoresistance (MR), Hall resistivity and de
Haas-van Alphen (dHvA) oscillation measurements, we studied the MR anisotropy
of SiP$_{2}$ which is verified to be a topologically trivial, incomplete
compensation semimetal. It was found that as magnetic field, $H$, is applied
along the $a$ axis, the MR exhibits an unsaturated nearly linear $H$
dependence, which was argued to arise from incomplete carriers compensation.
For the $H$ $\parallel$ [101] orientation, an unsaturated nearly quadratic $H$
dependence of MR up to 5.88 $\times$ 10$^{4}$$\%$ (at 1.8 K, 31.2 T) and
field-induced up-turn behavior in resistivity were observed, which was
suggested due to the existence of hole open orbits extending along the $k_{x}$
direction. Good agreement of the experimental results with the simulations
based on the calculated Fermi surface (FS) indicates that the topology of FS
plays an important role in its MR. | cond-mat |
Criticality in Cell Adhesion: We illuminate the many-body effects underlying the structure, formation, and
dissolution of cellular adhesion domains in the presence and absence of forces.
We consider mixed Glauber-Kawasaki dynamics of a two-dimensional model of
nearest-neighbor interacting adhesion bonds with intrinsic binding-affinity
under the action of a shared pulling or pushing force. We consider adhesion
bonds that are immobile due to being anchored to the underlying cytoskeleton as
well as adhesion molecules that are transiently diffusing. Highly accurate
analytical results are obtained on the pair-correlation level of the
Bethe-Guggenheim approximation for the complete thermodynamics and kinetics of
adhesion clusters of any size, including the thermodynamic limit. A new kind of
dynamical phase transition is uncovered -- the mean formation and dissolution
times per adhesion bond change discontinuously with respect to the
bond-coupling parameter. At the respective critical points cluster formation
and dissolution are fastest, while the statistically dominant transition path
undergoes a qualitative change -- the entropic barrier to complete
binding/unbinding is rate-limiting below, and the phase transition between
dense and dilute phases above the dynamical critical point. In the context of
the Ising model the dynamical phase transition reflects a first-order
discontinuity in the magnetization-reversal time. Our results provide a
potential explanation for the mechanical regulation of cell adhesion, and
suggest that the quasi-static and kinetic response to changes in the membrane
stiffness or applied forces is largest near the statical and dynamical critical
point, respectively. | cond-mat |
Monte Carlo Simulation of Liquid-Crystal Alignment and of Chiral
Symmetry-Breaking: We carry out Monte Carlo simulations to investigate the effect of molecular
shape on liquid-crystal order. In our approach, each model mesogen consists of
several soft spheres bonded rigidly together. The arrangement of the spheres
may be straight (to represent uniaxial molecules), Z-shaped (for biaxial
molecules), or banana-shaped (for bent-core molecules). Using this approach, we
investigate the alignment of the nematic phase by substrates decorated with
parallel ridges. We compare results for wide and narrow ridge spacing and
examine local order near the substrates, and show that our results are
consistent with the predictions of Landau theory. We also investigate chiral
symmetry-breaking in systems of bent-core molecules. We find a chiral
crystalline phase as well as a nonchiral smectic-A phase, but not a chiral
smectic-C phase. | cond-mat |
Superfluidity and magnetism in multicomponent ultracold fermions: We study the interplay between superfluidity and magnetism in a
multicomponent gas of ultracold fermions. Ward-Takahashi identities constrain
possible mean-field states describing order parameters for both pairing and
magnetization. The structure of global phase diagrams arises from competition
among these states as functions of anisotropies in chemical potential, density,
or interactions. They exhibit first and second order phase transition as well
as multicritical points, metastability regions, and phase separation. We
comment on experimental signatures in ultracold atoms. | cond-mat |
Improved performance in quantum transport calculations: A
divide-and-conquer method based on S-matrices: We propose a divide-and-conquer algorithm to find recursively the Scattering
matrix of general tight-binding structures. The Scattering matrix allows a
direct calculation of transport properties in mesoscopic systems by using the
Landauer formula. The method is exact, and by analyzing the performance of the
algorithm in square, triangular and honeycomb lattices, we show a significant
improvement in comparison to other state-of-the-art recursive and non-recursive
methods. | cond-mat |
Near-zero-field microwave-free magnetometry with nitrogen-vacancy
centers in nanodiamonds: We study the fluorescence of nanodiamond ensembles as a function of static
external magnetic field and observe characteristic dip features close to the
zero field with potential for magnetometry applications. We analyze the
dependence of the features width and contrast of the feature on the size of the
diamond (in the range 30 nm to 3 um) and on the strength of a bias magnetic
field applied transversely to the field being scanned. We also perform
optically detected magnetic resonance (ODMR) measurements to quantify the
strain splitting of the zero-field ODMR resonance across various nanodiamond
sizes and compare it with the width and contrast measurements of the zero-field
fluorescence features for both nanodiamonds and bulk samples. The observed
properties provide compelling evidence of cross-relaxation effects in the NV
system occurring close to zero magnetic fields. Finally, the potential of this
technique for use in practical magnetometry is discussed. | cond-mat |
Evaluation of Neel temperatures from fully self-consistent
broken-symmetry GW and high-temperature expansion: application to cubic
transition-metal oxides: Using fully self-consistent thermal broken-symmetry GW we construct effective
magnetic Heisenberg Hamiltonians for a series of transition metal oxides (NiO,
CoO, FeO, MnO), capturing a rigorous but condensed description of the magnetic
states. Then applying high-temperature expansion, we find the decomposition
coefficients for spin susceptibility and specific heat. The radius of
convergence of the found series determine the Neel temperature. The NiO, CoO,
and FeO contain a small ferromagnetic interaction between the nearest neighbors
(NN) and the dominant antiferromagnetic interaction between the next-nearest
neighbors (NNN). For them the derived Neel temperatures are in a good agreement
with experiment. The case of MnO is different because both NN and NNN couplings
are antiferromagnetic and comparable in magnitude, for which the error in the
estimated Neel temperature is larger, which is a signature of additional
effects not captured by electronic structure calculations. | cond-mat |
Avoided metallicity in a hole-doped Mott insulator on a triangular
lattice: Charge carrier doping of a Mott insulator is known to give rise to a wide
variety of exotic emergent states, from high-temperature superconductivity to
various charge, spin, and orbital orders. The physics underpinning their
evolution is, however, poorly understood. A major challenge is the chemical
complexity associated with traditional routes to the addition or removal of
carriers. Here, we study the Mott insulating CrO$_2$ layer of the delafossite
oxide PdCrO$_2$, where an intrinsic polar catastrophe provides a clean route to
induce substantial doping of the surface layer. Despite this, from scanning
tunneling microscopy and angle-resolved photoemission, we find that the surface
retains an insulating character, but with a modified electronic structure and
the development of a short-range ordered state with a distinct
$(\sqrt{7}\times\sqrt{7})\mathrm{R}\pm 19.1^\circ$ periodicity. From density
functional theory, we demonstrate how this reflects the formation of an
intricate charge disproportionation that results in an insulating ground state
of the surface layer that is disparate from the hidden Mott insulator found in
the bulk. By applying voltage pulses to the surface layer, we induce
substantial local modifications to this state, which we find relax on a time
scale of tens of minutes, pointing to a glassy nature of the
charge-disproportionated insulator realised here. | cond-mat |
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