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Fluctuation electromagnetic conservative -dissipative interaction and
heating of two closely spaced parallel plates in relative motion.
Nonrelativistic approximation.1: For the first time, we calculate the heating rate, attractive conservative
and tangential dissipative fluctuation electromagnetic forces felt by a thick
plate moving parallel to a closely spaced another plate in rest using a
nonrelativistic approximation of fluctuation electrodynamics. These results can
be considered as the high lights when solving general relativistic problem of
the fluctuation electromagnetic interaction in configuration of two perfectly
smooth parallel thick plates in relative motion. | cond-mat_other |
Device modeling of long-channel nanotube electro-optical emitter: We present a simple analytic model of nanotube electro-optical emitters,
along with improved experimental measurements using PMMA-passivated devices
with reduced hysteresis. Both the ambipolar electrical characteristics and the
motion of the infrared-emission spot are well described. The model indicates
that the electric field is strongly enhanced at the emission spot, and that
device performance can be greatly improved by the use of thinner gate oxides. | cond-mat_other |
Chaos due to parametric excitation: phase space symmetry and photon
correlations: We discuss dissipative chaos showing symmetries in the phase space and
nonclassical statistics for a parametrically driven nonlinear Kerr resonator
(PDNR). In this system an oscillatory mode is created in the process of
degenerate down-conversion of photons under interaction with a train of
external Gaussian pulses. For chaotic regime we demonstrate, that the
Poincar\'e section showing a strange attractor, as well as the resonator mode
contour plots of the Wigner functions display two-fold symmetry in the phase
space. We show that quantum-to-classical correspondence is strongly violated
for some chaotic regimes of the PDNR. Considering the second-order correlation
function we show that the high-level of photons correlation leading to
squeezing in the regular regime strongly decreases if the system transits to
the chaotic regime. Thus, observation of the photon-number correlation allows
to extract information about the chaotic regime. | cond-mat_other |
Effects of spatial and temporal noise on a cubic-autocatalytic
reaction-diffusion model: We characterize the influence that external noise, with both spatial and
temporal correlations, has on the scale dependence of the reaction parameters
of a cubic autocatalytic reaction diffusion (CARD) system. Interpreting the
CARD model as a primitive reaction scheme for a living system, the results
indicate that power-law correlations in environmental fluctuations can either
decrease or increase the rates of nutrient decay and the rate of autocatalysis
(replication) on small spatial and temporal scales. | cond-mat_other |
Quantum degenerate two-species Fermi-Fermi mixture coexisting with a
Bose-Einstein condensate: We report on the generation of a quantum degenerate Fermi-Fermi mixture of
two different atomic species. The quantum degenerate mixture is realized
employing sympathetic cooling of fermionic Li-6 and K-40 gases by an
evaporatively cooled bosonic Rb-87 gas. We describe the combination of trapping
and cooling methods that proved crucial to successfully cool the mixture. In
particular, we study the last part of the cooling process and show that the
efficiency of sympathetic cooling of the Li-6 gas by Rb-87 is increased by the
presence of K-40 through catalytic cooling. Due to the differing physical
properties of the two components, the quantum degenerate Li-6 K-40 Fermi-Fermi
mixture is an excellent candidate for a stable, heteronuclear system allowing
to study several so far unexplored types of quantum matter. | cond-mat_other |
Dipolar spinor Bose-Einstein condensates: Under many circumstances, the only important two-body interaction between
atoms in ultracold dilute atomic vapors is the short-ranged isotropic s-wave
collision. Recent studies have shown, however, that situations may arise where
the dipolar interaction between atomic magnetic or electric dipole moments can
play a significant role. The long-range anisotropic nature of the dipolar
interaction greatly enriches the static and dynamic properties of ultracold
atoms. In the case of dipolar spinor condensates, the interplay between the
dipolar interaction and the spin exchange interaction may lead to nontrivial
spin textures. Here we pay particular attention to the spin vortex state that
is analogous to the magnetic vortex found in thin magnetic films. | cond-mat_other |
Emergence of oscillons in kink-impurity interactions: The (1+1)-dimensional classical $\varphi^4$ theory contains stable,
topological excitations in the form of solitary waves or kinks, as well as
stable but non-topological solutions, such as the oscillon. Both are used in
effective descriptions of excitations throughout myriad fields of physics. The
oscillon is well-known to be a coherent, particle-like solution when introduced
as an Ansatz in the $\varphi^4$ theory. Here, we show that oscillons also arise
naturally in the dynamics of the theory, in particular as the result of
kink-antikink collisions in the presence of an impurity. We show that in
addition to the scattering of kinks and the formation of a breather, both bound
oscillon pairs and propagating oscillons may emerge from the collision. We
discuss their resonances and critical velocity as a function of impurity
strength and highlight the role played by the impurity in the scattering
process. | cond-mat_other |
Novel Methods to Create Multielectron Bubbles in Superfluid Helium: An equilibrium multielectron bubble in liquid helium is a fascinating object
with a spherical two-dimensional electron gas on its surface. We describe two
ways of creating them. MEBs have been observed in the dome of a cylindrical
cell with an unexpectedly short lifetime; we show analytically why these MEBs
can discharge by tunneling. Using a novel method, MEBs have been extracted from
a vapor sheath around a hot filament in superfluid helium by applying electric
fields up to 15 kV/cm, and photographed with high-speed video. Charges as high
as 1.6x10-9 C (~1010 electrons) have been measured. The latter method provides
a means of capture in an electromagnetic trap to allow the study of the
extensive exciting properties of these elusive objects. | cond-mat_other |
Limits to the analogue Hawking temperature in a Bose-Einstein condensate: Quasi-one dimensional outflow from a dilute gas Bose-Einstein condensate
reservoir is a promising system for the creation of analogue Hawking radiation.
We use numerical modeling to show that stable sonic horizons exist in such a
system under realistic conditions, taking into account the transverse
dimensions and three-body loss. We find that loss limits the analogue Hawking
temperatures achievable in the hydrodynamic regime, with sodium condensates
allowing the highest temperatures. A condensate of 30,000 atoms, with
transverse confinement frequency omega_perp=6800*2*pi Hz, yields horizon
temperatures of about 20 nK over a period of 50 ms. This is at least four times
higher than for other atoms commonly used for Bose-Einstein condensates. | cond-mat_other |
Two approaches for describing the Casimir interaction with graphene:
density-density correlation function versus polarization tensor: The comparison studies of theoretical approaches to the description of the
Casimir interaction in layered systems including graphene is performed. It is
shown that at zero temperature the approach using the polarization tensor leads
to the same results as the approach using the longitudinal density-density
correlation function of graphene. An explicit expression for the
zero-temperature transverse density-density correlation function of graphene is
provided. We further show that the computational results for the Casimir free
energy of graphene-graphene and graphene-Au plate interactions at room
temperature, obtained using the temperature-dependent polarization tensor,
deviate significantly from those using the longitudinal density-density
correlation function defined at zero temperature. We derive both the
longitudinal and transverse density-density correlation functions of graphene
at nonzero temperature. The Casimir free energy in layered structures including
graphene, computed using the temperature-dependent correlation functions, is
exactly equal to that found using the polarization tensor. | cond-mat_other |
Simulations of Surface X-ray Diffraction from a Monolayer 4He Film
Adsorbed on Graphite: We carried out simulations of crystal truncation rod (CTR) scatterings, i.e.,
one of the surface X-ray diffraction techniques with atomic resolution, from a
monolayer He film adsorbed on graphite. Our simulations reveal that the 00L rod
scatterings from the He monolayer exhibit notable intensity modifications for
those from a graphite surface in the ranges of approximately L = 0.6 - 1.7 and
L = 2.2 - 3.5. The height of the He monolayer from the graphite surface largely
affects the CTR scattering profiles, indicating that CTR scatterings have
enough sensitivities to determine the surface structure of the various phases
in the He layer. In particular, in the incommensurate solid phase, our
preliminary experimental data show the intensity modulations that are expected
from the present simulations. | cond-mat_other |
Motion of vortices in ferromagnetic spin-1 BEC: The paper investigates dynamics of nonsingular vortices in a ferromagnetic
spin-1 BEC, where spin and mass superfluidity coexist in the presence of
uniaxial anisotropy (linear and quadratic Zeeman effect). The analysis is based
on hydrodynamics following from the Gross-Pitaevskii theory. Cores of
nonsingular vortices are skyrmions with charge, which is tuned by uniaxial
anisotropy and can have any fractal value between 0 and 1. There are
circulations of mass and spin currents around these vortices. The results are
compared with the equation of vortex motion derived earlier in the
Landau-Lifshitz-Gilbert theory for magnetic vortices in easy-plane
ferromagnetic insulators. In the both cases the transverse gyrotropic force
(analog of the Magnus force in superfluid and classical hydrodynamics) is
proportional to the charge of skyrmions in vortex cores. | cond-mat_other |
Dissipation-managed soliton in a quasi-one-dimensional Bose-Einstein
condensate: We use the time-dependent mean-field Gross-Pitaevskii equation to study the
formation of a dynamically-stabilized dissipation-managed bright soliton in a
quasi-one-dimensional Bose-Einstein condensate (BEC). Because of three-body
recombination of bosonic atoms to molecules, atoms are lost (dissipated) from a
BEC. Such dissipation leads to the decay of a BEC soliton. We demonstrate by a
perturbation procedure that an alimentation of atoms from an external source to
the BEC may compensate for the dissipation loss and lead to a
dynamically-stabilized soliton. The result of the analytical perturbation
method is in excellent agreement with mean-field numerics. It seems possible to
obtain such a dynamically-stabilized BEC soliton without dissipation in
laboratory. | cond-mat_other |
Mesoscopic Aspects of Strongly Interacting Cold Atoms: Harmonically trapped lattice bosons with strong repulsive interactions
exhibit a superfluid-Mott-insulator heterostructure in the form of a "wedding
cake". We discuss the mesoscopic aspects of such a system within a
one-dimensional scattering matrix approach and calculate the scattering
properties of quasi-particles at a superfluid-Mott-insulator interface as an
elementary building block to describe transport phenomena across such a
boundary. We apply the formalism to determine the heat conductivity through a
Mott layer, a quantity relevant to describe thermalization processes in the
optical lattice setup. We identify a critical hopping below which the heat
conductivity is strongly suppressed. | cond-mat_other |
Attractive Fermi gases with unequal spin populations in highly elongated
traps: We investigate two-component attractive Fermi gases with imbalanced spin
populations in trapped one dimensional configurations. The ground state
properties are determined within local density approximation, starting from the
exact Bethe-ansatz equations for the homogeneous case. We predict that the
atoms are distributed according to a two-shell structure: a partially polarized
phase in the center of the trap and either a fully paired or a fully polarized
phase in the wings. The partially polarized core is expected to be a superfluid
of the FFLO type. The size of the cloud as well as the critical spin
polarization needed to suppress the fully paired shell, are calculated as a
function of the coupling strength. | cond-mat_other |
Sufficient conditions for two-dimensional localization by arbitrarily
weak defects in periodic potentials with band gaps: We prove, via an elementary variational method, 1d and 2d localization within
the band gaps of a periodic Schrodinger operator for any mostly negative or
mostly positive defect potential, V, whose depth is not too great compared to
the size of the gap. In a similar way, we also prove sufficient conditions for
1d and 2d localization below the ground state of such an operator. Furthermore,
we extend our results to 1d and 2d localization in d dimensions; for example, a
linear or planar defect in a 3d crystal. For the case of D-fold degenerate band
edges, we also give sufficient conditions for localization of up to D states. | cond-mat_other |
Electrical Tuning of Terahertz Plasmonic Crystal Phases: We present an extensive study of resonant two-dimensional (2D) plasmon
excitations in grating-gated quantum well heterostructures, which enable an
electrical control of periodic charge carrier density profile. Our study
combines theoretical and experimental investigations of nanometer-scale
AlGaN/GaN grating-gate structures and reveals that all terahertz (THz)
plasmonic resonances in these structures can be explained only within the
framework of the plasmonic crystal model. We identify two different plasmonic
crystal phases. The first is the delocalized phase, where THz radiation
interacts with the entire grating-gate structure that is realized at a weakly
modulated 2D electron gas (2DEG) regime. In the second, the localized phase,
THz radiation interacts only with the ungated portions of the structure. This
phase is achieved by fully depleting the gated regions, resulting in strong
modulation. By gate-controlling of the modulation degree, we observe a
continuous transition between these phases. We also discovered that
unexpectedly the resonant plasma frequencies of ungated parts (in the localized
phase) still depend on the gate voltage. We attribute this phenomenon to the
specific depletion of the conductive profile in the ungated region of the 2DEG,
the so-called edge gating effect. Although we study a specific case of plasmons
in AlGaN/GaN grating-gate structures, our results have a general character and
are applicable to any other semiconductor-based plasmonic crystal structures.
Our work represents the first demonstration of an electrically tunable
transition between different phases of THz plasmonic crystals, which is a
crucial step towards a deeper understanding of THz plasma physics and the
development of all-electrically tunable devices for THz optoelectronics. | cond-mat_other |
Quantum complementarity of microcavity polaritons: We present an experiment that probes polariton quantum correlations by
exploiting quantum complementarity. Specifically, we find that polaritons in
two distinct idler-modes interfere if and only if they share the same
signal-mode so that "which-way" information cannot be gathered. The
experimental results prove the existence of polariton pair correlations that
store the "which-way" information. This interpretation is confirmed by a
theoretical analysis of the measured interference visibility in terms of
quantum Langevin equations. | cond-mat_other |
Quantum Phases of Ultracold Bosonic Atoms in a Two-Dimensional Optical
Superlattice: We study quantum phases of ultracold bosonic atoms in a two-dimensional
optical superlattice. The extended Bose-Hubbard model derived from the system
of ultracold bosonic atoms in an optical superlattice is solved numerically
with Gutzwiller approach. We find that the modulated superfluid(MS),
Mott-insulator (MI) and density-wave(DW) phases appear in some regimes of
parameters. The experimental detection of the first order correlations and the
second order correlations of different quantum phases with time-of-flight and
noise-correlation techniques is proposed. | cond-mat_other |
A possibility for precise Weinberg angle measurement in centrosymmetric
crystals with axis: We demonstrate that parity nonconserving interaction due to the nuclear weak
charge Q_W leads to nonlinear magnetoelectric effect in centrosymmetric
paramagnetic crystals. It is shown that the effect exists only in crystals with
special symmetry axis k. Kinematically, the correlation (correction to energy)
has the form H_PNC ~ Q_W (E,[B,k])(B,k), where B and E are the external
magnetic and electric fields. This gives rise to magnetic induction M_PNC ~ Q_W
{k(B,[k,E]) + [k,E](B,k)}. To be specific we consider rare-earth trifluorides
and, in particular, dysprosium trifluoride which looks the most suitable for
experiment. We estimate the optimal temperature for the experiment to be of a
few kelvin. For the magnetic field B = 1 T and the electric field E = 10 kV/cm,
the expected magnetic induction is 4 \pi M_PNC = 0.5 * 10^-11 G, six orders of
magnitude larger than the best sensitivity currently under discussion.
Dysprosium has several stable isotopes, and so, comparison of the effects for
different isotopes provides possibility for precise measurement of the Weinberg
angle. | cond-mat_other |
Charge density wave in graphene: magnetic-field-induced Peierls
instability: We suggest that a magnetic-field-induced Peierls instability accounts for the
recent experiment of Zhang et al. in which unexpected quantum Hall plateaus
were observed at high magnetic fields in graphene on a substrate. This Peierls
instability leads to an out-of-plane lattice distortion resulting in a charge
density wave (CDW) on sublattices A and B of the graphene honeycomb lattice. We
also discuss alternative microscopic scenarios proposed in the literature and
leading to a similar CDW ground state in graphene. | cond-mat_other |
Conformal Mapping on Rough Boundaries I: Applications to harmonic
problems: The aim of this study is to analyze the properties of harmonic fields in the
vicinity of rough boundaries where either a constant potential or a zero flux
is imposed, while a constant field is prescribed at an infinite distance from
this boundary. We introduce a conformal mapping technique that is tailored to
this problem in two dimensions. An efficient algorithm is introduced to compute
the conformal map for arbitrarily chosen boundaries. Harmonic fields can then
simply be read from the conformal map. We discuss applications to "equivalent"
smooth interfaces. We study the correlations between the topography and the
field at the surface. Finally we apply the conformal map to the computation of
inhomogeneous harmonic fields such as the derivation of Green function for
localized flux on the surface of a rough boundary. | cond-mat_other |
The Projected Gross-Pitaevskii Equation for harmonically confined Bose
gases: We extend the Projected Gross Pitaevskii equation formalism of Davis et al.
[Phys. Rev. Lett. \bf{87}, 160402 (2001)] to the experimentally relevant case
of harmonic potentials. We outline a robust and accurate numerical scheme that
can efficiently simulate this system. We apply this method to investigate the
equilibrium properties of a harmonically trapped three-dimensional Bose gas at
finite temperature, and consider the dependence of condensate fraction,
position and momentum distributions, and density fluctuations on temperature.
We apply the scheme to simulate an evaporative cooling process in which the
preferential removal of high energy particles leads to the growth of a
Bose-Einstein condensate. We show that a condensate fraction can be inferred
during the dynamics even in this non-equilibrium situation. | cond-mat_other |
Critical dislocation speed in helium-4 crystals: Our experiments show that in $^4$He crystals, the binding of $^3$He
impurities to dislocations does not necessarily imply their pinning. Indeed, in
these crystals, there are two different regimes of the motion of dislocations
when impurities bind to them. At lowdriving strain $\epsilon$ and frequency
$\omega$, where the dislocation speed is less than a critical value (45
$\mu$m/s), dislocations and impurities apparently move together. Impurities
really pin the dislocations only at higher values of $\omega$. The critical
speed separating the two regimes is two orders of magnitude smaller than the
average speed of free $^3$He impurities in the bulk crystal lattice.We obtained
this result by studying the dissipation of dislocation motion as a function of
the frequency and amplitude of a driving strain applied to a crystal at low
temperature. Our results solve an apparent contradiction between some
experiments, which showed a frequency-dependent transition temperature from a
soft to a stiff state, and other experiments or models where this temperature
was assumed to be independent of frequency. The impurity pinning mechanism for
dislocations appears to be more complicated than previously assumed. | cond-mat_other |
The interplay between magnetism, structure, and strong electron-phonon
coupling in binary FeAs under pressure: Unlike the ferropnictide superconductors, which crystallize in a tetragonal
crystal structure, binary FeAs forms in an orthorhombic crystal structure,
where the local atomic environment resembles a highly distorted variant of the
FeAs4 tetrahdedral building block of the ferropnictide superconductors.
However, like the parent compounds of the ferropnictide superconductors, FeAs
undergoes magnetic ordering at low temperatures, with no evidence favoring a
superconducting ground state at ambient pressure. We employ pressure-dependent
electrical transport and x-ray diffraction measurements using diamond anvil
cells to characterize the magnetic state and the structure as a function of
pressure. While the MnP-type structure of FeAs persists up to 25 GPa,
compressing continuously with no evidence of structural transformations under
pressure, features in the magnetotransport measurements associated with
magnetism are not observed for pressures in excess of 11 GPa. Where observable,
the features associated with magnetic order at ambient pressure show remarkably
little pressure dependence, and transport measurements suggest that a dynamical
structural instability coupled to the Fermi surface via a strong
electron-phonon interaction may play an important role in enabling magnetism in
FeAs. | cond-mat_other |
Simulation study of the two-dimensional Burridge-Knopoff model of
earthquakes: Spatiotemporal correlations of the two-dimensional spring-block
(Burridge-Knopoff) model of earthquakes are extensively studied by means of
numerical computer simulations. The model is found to exhibit either
``subcritical'' or ``supercritical'' behavior, depending on the values of the
model parameters. Transition between these regimes is either continuous or
discontinuous. Seismic events in the ``subcritical'' regime and those in the
``supercritical'' regime at larger magnitudes exhibit universal scaling
properties. In the ``supercritical'' regime, eminent spatiotemporal
correlations, {\it e.g.}, remarkable growth of seismic activity preceding the
mainshock, arise in earthquake occurrence, whereas such spatiotemporal
correlations are significantly suppressed in the ``subcritical'' regime.
Seismic activity is generically suppressed just before the mainshock in a close
vicinity of the epicenter of the upcoming event while it remains to be active
in the surroundings (the Mogi doughnut). It is also observed that, before and
after the mainshock, the apparent $B$-value of the magnitude distribution
decreases or increases in the ``supercritical'' or ``subcritical'' regimes,
respectively. Such distinct precursory phenomena may open a way to the
prediction of the upcoming large event. | cond-mat_other |
Artificial electromagnetism for neutral atoms: Escher staircase and
Laughlin liquids: We show how lasers may create fields which couple to neutral atoms in the
same way that the electromagnetic fields couple to charged particles. These
fields are needed for using neutral atoms as an analog quantum computer for
simulating the properties of many-body systems of charged particles. They allow
for seemingly paradoxical geometries, such as a ring where atoms continuously
reduce their potential energy while moving in a closed path. We propose neutral
atom experiments which probe quantum Hall effects and the interplay between
magnetic fields and periodic potentials. | cond-mat_other |
Tracking Intrinsic Non-Hermitian Skin Effect in Lossy Lattices: Non-Hermitian skin effect (NHSE), characterized by a majority of eigenstates
localized at open boundaries, is one of the most iconic phenomena in
non-Hermitian lattices. Despite notable experimental studies implemented, most
of them witness only certain signs of the NHSE rather than the intrinsic
exponential localization inherent in eigenstates, owing to the ubiquitous and
inevitable background loss. Even worse, the experimental observation of the
NHSE would be completely obscured in highly lossy cases. Here, we theoretically
propose a dual test approach to eliminate the destructive loss effect and track
the intrinsic NHSE that is essentially irrelevant to background loss.
Experimentally, the effectiveness of this approach is precisely validated by
one- and two-dimensional non-Hermitian acoustic lattices. Our study sheds new
light on the previously untapped intrinsic aspect of the NHSE, which is of
particular significance in non-Hermitian topological physics. | cond-mat_other |
On the Transition to Turbulence of Oscillatory Flow of Liquid Helium-4: Oscillating solid bodies have frequently been used for studying the
properties of normal and superfluid helium. In particular, the transition from
laminar flow to turbulence has attracted much interest recently. The purpose of
this note is to review several central features of this transition in
oscillatory flow, which have been inaccurately formulated in some recent work. | cond-mat_other |
Limits to inertial vibration power harvesting: power-spectral-density
approach and its applications: Maximum output powers of vibration-driven inertial power harvesters reported
in literature exhibit sizable variations, even when normalized by the device
weight or their maximum linear size. To help establish a common benchmark, we
present a power-spectral-density based approach for estimating the maximum
power that can be obtained using a resonant inertial power harvester from a
random (aperiodic) vibration source with a given power spectral density. In the
simplest case of unlimited harvester size, the maximum obtainable power is
simply proportional to the maximum value of the power spectral density of
vibration acceleration. We describe in detail the underlying theory and the
practical method for evaluating these limits. We also present a simple
analytical formula to estimate the minimum harvester size required for
obtaining the maximum possible power. Specific power limits are derived as
function of harvester size for three practical examples of vibration sources:
(a) pneumatic power tool, (b) the body of an idling Mazda RX7 sports car, and
(c) human walking motion. Characteristic power spectra and optimum design
parameters (quality factor and resonant frequency) are presented for both
translational and rotational harvesters. Translational harvesters generally
outperform rotational ones for realistic harvester sizes, with the power tool
vibrations yielding a practical power limit of ~300 mW per gram of inertial
mass, followed by walking at ~1mW/g, while the vibrations of a car body yield
~0.1 mW/g or less. | cond-mat_other |
Turbulent superfluid profiles in a counterflow channel: We have developed a two-dimensional model of quantised vortices in helium II
moving under the influence of applied normal fluid and superfluid in a
counterflow channel. We predict superfluid and vortex-line density profiles
which could be experimentally tested using recently developed visualization
techniques. | cond-mat_other |
Genuine phase diffusion of a Bose-Einstein condensate in the
microcanonical ensemble: A classical field study: Within the classical field model, we find that the phase of a Bose-Einstein
condensate undergoes a true diffusive motion in the microcanonical ensemble,
the variance of the condensate phase change between time zero and time $t$
growing linearly in $t$. The phase diffusion coefficient obeys a simple scaling
law in the double thermodynamic and Bogoliubov limit. We construct an
approximate calculation of the diffusion coefficient, in fair agreement with
the numerical results over the considered temperature range, and we extend this
approximate calculation to the quantum field. | cond-mat_other |
A simple position operator for periodic systems: We present a position operator that is compatible with periodic boundary
conditions (PBC). It is a one-body operator that can be applied in calculations
of correlated materials by simply replacing the traditional position vector by
the new definition. We show that it satisfies important fundamental as well as
practical constraints. To illustrate the usefulness of the PBC position
operator we apply it to the localization tensor, a key quantity that is able to
differentiate metallic from insulating states. In particular, we show that the
localization tensor given in terms of the PBC position operator yields the
correct expression in the thermodynamic limit. Moreover, we show that it
correctly distinguishes between finite precursors of metals and insulators. | cond-mat_other |
Calculations of two-color interband optical injection and control of
carrier population, spin, current, and spin current in bulk semiconductors: Quantum interference between one- and two-photon absorption pathways allows
coherent control of interband transitions in unbiased bulk semiconductors;
carrier population, carrier spin polarization, photocurrent injection, and spin
current injection can all be controlled. We calculate injection spectra for
these effects using a 14x14 k.p Hamiltonian including remote band effects for
five bulk semiconductors of zinc-blende symmetry: InSb, GaSb, InP, GaAs, and
ZnSe. Microscopic expressions for spin-current injection and spin control
accounting for spin split bands are presented. We also present analytical
expressions for the injection spectra derived in the parabolic-band
approximation and compare these with the calculation nonperturbative in k. | cond-mat_other |
The S shape of a granular pile in a rotating drum: The shape of a granular pile in a rotating drum is investigated. Using
Discrete Elements Method (DEM) simulations we show that the "S shape" obtained
for high rotation speed can be accounted for by the friction on the end plates.
A theoretical model which accounts for the effect of the end plates is
presented and the equation of the shape of the free surface is derived. The
model reveals a dimensionless number which quantifies the influence of the end
plates on the shape of the pile. Finally, the scaling laws of the system are
discussed and numerical results support our conclusions. | cond-mat_other |
Landau damping: instability mechanism of superfluid Bose gases moving in
optical lattices: We investigate Landau damping of Bogoliubov excitations in a dilute Bose gas
moving in an optical lattice at finite temperatures. Using a 1D tight-binding
model, we explicitly obtain the Landau damping rate, the sign of which
determines the stability of the condensate. We find that the sign changes at a
certain condensate velocity, which is exactly the same as the critical velocity
determined by the Landau criterion of superfluidity. This coincidence of the
critical velocities reveals the microscopic mechanism of the Landau
instability. This instability mechanism is also consistent with the recent
experiment suggesting that a thermal cloud plays a crucial role in breakdown of
superfluids, since the thermal cloud is also vital in the Landau damping
process. We also examine the possibility of simultaneous disappearance of all
damping processes. | cond-mat_other |
The Dynamic Structure Factor of the 1D Bose Gas near the Tonks-Girardeau
Limit: While the 1D Bose gas appears to exhibit superfluid response under certain
conditions, it fails the Landau criterion according to the elementary
excitation spectrum calculated by Lieb. The apparent riddle is solved by
calculating the dynamic structure factor of the Lieb-Liniger 1D Bose gas. A
pseudopotential Hamiltonian in the fermionic representation is used to derive a
Hartree-Fock operator, which turns out to be well-behaved and local. The
Random-Phase approximation for the dynamic structure factor based on this
derivation is calculated analytically and is expected to be valid at least up
to first order in $1/\gamma$, where $\gamma$ is the dimensionless interaction
strength of the model. The dynamic structure factor in this approximation
clearly indicates a crossover behavior from the non-superfluid Tonks to the
superfluid weakly-interacting regime, which should be observable by Bragg
scattering in current experiments. | cond-mat_other |
Measurements on Melting Pressure, Metastable Solid Phases, and Molar
Volume of Univariant Saturated Helium Mixture: A concentration-saturated helium mixture at the melting pressure consists of
two liquid phases and one or two solid phases. The equilibrium system is
univariant, whose properties depend uniquely on temperature. Four coexisting
phases can exist on singular points, which are called quadruple points. As a
univariant system, the melting pressure could be used as a thermometric
standard. It would provide some advantages compared to the current reference,
namely pure $^3$He, especially at the lowest temperatures below 1 mK. We have
extended the melting pressure measurements of the concentration-saturated
helium mixture from 10 mK to 460 mK. The density of the dilute liquid phase was
also recorded. The effect of the equilibrium crystal structure changing from
hcp to bcc was clearly seen at T=294 mK at the melting pressure P=2.638 MPa. We
observed the existence of metastable solid phases around this point. No
evidence was found for the presence of another, disputed, quadruple point at
around 400 mK. The experimental results agree well with our previous
calculations at low temperatures, but deviate above 200 mK. | cond-mat_other |
Theoretical analysis of beta- emission from 63-Ni nano-particles in
glassy 15-P: The energy loss of $\beta^-$ emission emitting from a 63-Ni source in a
phosphorus 15-P scintillation medium is theoretically studied. It has shown the
$\beta$ energy spectrum absorption in ${15}P$ had nearly 100% efficiency for
$\leq$ 28 keV in 800 {\mu}m scintillator thickness. This can eventually lead to
the production of light sources using these beta-emitting radiation sources as
a low energy source in the near future. | cond-mat_other |
Feshbach spectroscopy of a K-Rb atomic mixture: We perform extensive magnetic Feshbach spectroscopy of an ultracold mixture
of fermionic 40K and bosonic 87Rb atoms. The magnetic-field locations of 14
interspecies resonances is used to construct a quantum collision model able to
predict accurate collisional parameters for all K-Rb isotopic pairs. In
particular we determine the interspecies s-wave singlet and triplet scattering
lengths for the 40K-87Rb mixture as -111 +/- 5 Bohr and -215 +/- 10 Bohr
respectively. We also predict accurate scattering lengths and position of
Feshbach resonances for the other K-Rb isotopic pairs. We discuss the
consequences of our results for current and future experiments with ultracold
K-Rb mixtures. | cond-mat_other |
Limits to the analogue Hawking temperature in a Bose-Einstein condensate: Quasi-one dimensional outflow from a dilute gas Bose-Einstein condensate
reservoir is a promising system for the creation of analogue Hawking radiation.
We use numerical modeling to show that stable sonic horizons exist in such a
system under realistic conditions, taking into account the transverse
dimensions and three-body loss. We find that loss limits the analogue Hawking
temperatures achievable in the hydrodynamic regime, with sodium condensates
allowing the highest temperatures. A condensate of 30,000 atoms, with
transverse confinement frequency omega_perp=6800*2*pi Hz, yields horizon
temperatures of about 20 nK over a period of 50 ms. This is at least four times
higher than for other atoms commonly used for Bose-Einstein condensates. | cond-mat_other |
Computational spectroscopy of helium-solvated molecules: effective
inertia, from small He clusters toward the nano-droplet regime: Accurate computer simulations of the rotational dynamics of linear molecules
solvated in He clusters indicate that the large-size (nano-droplet) regime is
attained quickly for light rotors (HCN, CO) and slowly for heavy ones (OCS,
N$_2$O, CO$_2$), thus challenging previously reported results. Those results
spurred the view that the different behavior of light rotors with respect to
heavy ones - including a smaller reduction of inertia upon solvation of the
former - would result from the lack of adiabatic following of the He density
upon molecular rotation. We have performed computer experiments in which the
rotational dynamics of OCS and HCN molecules was simulated using a fictitious
inertia appropriate to the other molecule. These experiments indicate that the
approach to the nano-droplet regime, as well as the reduction of the molecular
inertia upon solvation, is determined by the anistropy of the potential, more
than by the molecular weight. Our findings are in agreement with recent
infrared and/or microwave experimental data which, however, are not yet totally
conclusive by themselves. | cond-mat_other |
Strong extinction of a far-field laser beam by a single quantum dot: Through the utilization of index-matched GaAs immersion lens techniques we
demonstrate a record extinction (12%) of a far-field focused laser by a single
InAs/GaAs quantum dot. This contrast level enables us to report for the first
time resonant laser transmission spectroscopy on a single InAs/GaAs quantum dot
without the need for phase-sensitive lock-in detection. | cond-mat_other |
Stability of the density-wave state of a dipolar condensate in a pancake
trap: We study a dipolar boson-fermion mixture in a pancake geometry at absolute
zero temperature, generalizing our previous work on the stability of polar
condensates and the formation of a density-wave state in cylindrical traps.
After examining the dependence of the polar condensate stability on the
strength of the fermion-induced interaction, we determine the transition point
from a ground-state Gaussian to a hexagonal density-wave state. We use a
variational principle to analyze the stability properties of those density-wave
state. | cond-mat_other |
Dense electron-hole plasma in silicon light emitting diodes: Efficient electroluminescence of silicon light emitting p-n diodes with
different sizes and shapes is investigated at room temperature. High quantum
efficiency of the diodes, a long linear dependence of the electroluminescence
intensity on the diode current and a low energy shift of the emission line in
electroluminescence spectra with increasing diode current are explained by the
self-compression of injected electron-hole plasma into dense electron-hole
plasma drops. Experiments on space scanning of the electroluminescence
intensity of the diodes support this conclusion. The plasma self-compression is
explained by existence of an attraction in electron-hole plasma, compensating
the plasma pressure. A decrease of the semiconductor energy gap due to a local
lattice overheating, produced by the plasma, and the exchange-correlation
interaction could contribute to this attraction. The self-focusing of the
injection current can accompany the plasma self-compression. | cond-mat_other |
Feshbach resonances with large background scattering length: interplay
with open-channel resonances: Feshbach resonances are commonly described by a single-resonance Feshbach
model, and open-channel resonances are not taken into account explicitly.
However, an open-channel resonance near threshold limits the range of validity
of this model. Such a situation exists when the background scattering length is
much larger than the range of the interatomic potential. The open-channel
resonance introduces strong threshold effects not included in the
single-resonance description. We derive an easy-to-use analytical model that
takes into account both the Feshbach resonance and the open-channel resonance.
We apply our model to $^{85}$Rb, which has a large background scattering
length, and show that the agreement with coupled-channels calculations is
excellent. The model can be readily applied to other atomic systems with a
large background scattering length, such as $^6$Li and $^{133}$Cs. Our approach
provides full insight into the underlying physics of the interplay between
open-channel (or potential) resonances and Feshbach resonances. | cond-mat_other |
Semi-analytic Faddeev solution to the $N$-boson problem with zero-range
interactions: We study two-body correlations for $N$ identical bosons by use of the
hyperspherical adiabatic expansion method. We use the zero-range interaction
and derive a transcendental equation determining the key ingredient of the
hyperradial potential. The necessary renormalization is for both repulsive and
attractive interactions achieved with an effective range expansion of the
two-body phase-shifts. Our solutions including correlations provide the
properties of Bose-Einstein condensates exemplified by stability conditions as
established by mean-field Gross-Pitaevskii calculations. The many-body Efimov
states are unavoidable for large scattering lengths. | cond-mat_other |
Vortex Lattice in a Rotating Bose-Einstein Condensate: Numerical simulations of vortex motion in a trapped Bose-Einstein condensate
were performed by solving the two-dimensional Gross-Pitaevskii equation in the
presence of a simple phenomenological model of interaction between the
condensate and the finite temperature thermal cloud. The log (base e) of total
energy, trap energy, quantum energy, kinetic energy, internal energy and
z-component of the angular momentum vs time were compared with f(x)=a+bx for
that time when the vortices come in in the condensate. The increasing/decay
rate of these energies and L_{z} were studied as a function of dissipation. | cond-mat_other |
Dipolar spinor Bose-Einstein condensates: Under many circumstances, the only important two-body interaction between
atoms in ultracold dilute atomic vapors is the short-ranged isotropic s-wave
collision. Recent studies have shown, however, that situations may arise where
the dipolar interaction between atomic magnetic or electric dipole moments can
play a significant role. The long-range anisotropic nature of the dipolar
interaction greatly enriches the static and dynamic properties of ultracold
atoms. In the case of dipolar spinor condensates, the interplay between the
dipolar interaction and the spin exchange interaction may lead to nontrivial
spin textures. Here we pay particular attention to the spin vortex state that
is analogous to the magnetic vortex found in thin magnetic films. | cond-mat_other |
The Structure of Integrable One-Dimensional Systems: We explain the relationship between the classical description of an
integrable system in terms of invariant tori and action-angle variables, and
the quantum description in terms of the asymptotic Bethe ansatz. | cond-mat_other |
Classical Region of a Trapped Bose Gas: The classical region of a Bose gas consists of all single-particle modes that
have a high average occupation and are well-described by a classical field.
Highly-occupied modes only occur in massive Bose gases at ultra-cold
temperatures, in contrast to the photon case where there are highly-occupied
modes at all temperatures. For the Bose gas the number of these modes is
dependent on the temperature, the total number of particles and their
interaction strength. In this paper we characterize the classical region of a
harmonically trapped Bose gas over a wide parameter regime. We use a
Hartree-Fock approach to account for the effects of interactions, which we
observe to significantly change the classical region as compared to the
idealized case. We compare our results to full classical field calculations and
show that the Hartree-Fock approach provides a qualitatively accurate
description of classical region for the interacting gas. | cond-mat_other |
Phase-controlled, heterodyne laser-induced transient grating
measurements of thermal transport properties in opaque material: The methodology for a heterodyned laser-induced transient thermal grating
technique for non-contact, non-destructive measurements of thermal transport in
opaque material is presented. Phase-controlled heterodyne detection allows us
to isolate pure phase or amplitude transient grating signal contributions by
varying the relative phase between reference and probe beams. The phase grating
signal includes components associated with both transient reflectivity and
surface displacement whereas the amplitude grating contribution is governed by
transient reflectivity alone. By analyzing the latter with the two-dimensional
thermal diffusion model, we extract the in-plane thermal diffusivity of the
sample. Measurements on a 5 {\mu}m thick single crystal PbTe film yielded
excellent agreement with the model over a range of grating periods from 1.6 to
2.8 {\mu}m. The measured thermal diffusivity of 1.3 \times 10-6 m2/s was found
to be slightly lower than the bulk value. | cond-mat_other |
Multiband superfluidity and superfluid to band-insulator transition of
strongly interacting fermions in an optical lattice: We study the multiband superfluid and the superfluid (SF) to band insulator
(BI) transition of strongly interacting fermionic atoms in an optical lattice
at a filling of two fermions per well. We present physical arguments to show
that a consistent mean field description of this problem is obtained by
retaining only intraband pairing between the fermions. Using this approach we
obtain a reasonable account of the experimentally observed critical lattice
depth for the SF-BI transition and the modulated components of the condensate
density, and make predictions for the lattice depth dependence of the
quasiparticle gap which can be tested in future experiments. We also highlight
some interesting features unique to cold atom superfluids within this intraband
pairing approximation; for instance, the pair field is forced to be uniform in
space and the Hartree field vanishes identically. These arise as a result of
the fact that while the pairing interaction is cut off at the scale of the
Debye frequency in conventional superconductors, or at the lattice scale in
tight binding model Hamiltonians, such a cutoff is absent for cold Fermi gases. | cond-mat_other |
On the Transition from Potential Flow to Turbulence Around a Microsphere
Oscillating in Superfluid ^4He: The flow of superfluid $^4$He around a translationally oscillating sphere,
levitating without mechanical support, can either be laminar or turbulent,
depending on the velocity amplitude. Below a critical velocity $v_c$ that
scales as $\omega ^{1/2}$, and is temperature independent below 1 K, the flow
is laminar (potential flow). Below 0.5 K the linear drag force is caused by
ballistic phonon scattering that vanishes as T$^4$ until background damping,
measured in the empty cell, becomes dominant for T $<$ 0.1 K. Increasing the
velocity amplitude above $v_c$ leads to a transition from potential flow to
turbulence, where the large turbulent drag force varies as $(v^2 - v_c^2)$. In
a small velocity interval $\Delta v / v_c \le 3 \%$ above $v_c$, the flow is
unstable below 0.5 K, switching intermittently between both patterns. From time
series recorded at constant temperature and driving force, the lifetimes of
both phases are analyzed statistically. We observe metastable states of
potential flow which, after a mean lifetime of 25 minutes, ultimately break
down due to vorticity created by natural background radioactivity. The
lifetimes of the turbulent phases have an exponential distribution, and the
mean increases exponentially with $\Delta v^2$. We investigate the frequency at
which the vortex rings are shed from the sphere. Our results are compared with
recent data of other authors on vortex shedding by moving a laser beam through
a Bose-Einstein condensate. Finally, we show that our observed transition to
turbulence belongs to the class of "supertransient chaos" where lifetimes of
the turbulent states increase faster than exponentially. Peculiar results
obtained in dilute $^3$He - $^4$He mixtures are presented in the Appendix. | cond-mat_other |
Band degeneration and evolution in nonlinear triatomic chain
superlattices: Nonlinear superlattices exhibit unique features allowing for wave
manipulations. Despite the increasing attention received, the underlying
physical mechanisms and the evolution process of the band structures and
bandgaps in strongly nonlinear superlattices remain unclear. Here we establish
and examine strongly nonlinear superlattice models (three triatomic models) to
show the evolution process of typical nonlinear band structures based on
analytical and numerical approaches. We find that the strongly nonlinear
superlattices present particular band degeneration and bifurcation, accompanied
with the vibration mode transfer in their unit cells. The evolution processes
and the physical mechanisms of the band degeneration in different models are
clarified with the consideration of the mode transfer. The observed
degeneration may occur as the shifting, bifurcating, shortening, merging or
disappearing of dispersion curves, all depending on the arrangement of the
coupled nonlinear elements. Meanwhile, the dimension of the unit cell reduces,
alongside changes in the frequency range and mechanisms (Bragg and local
resonance) of the bandgaps. These findings answer some foundamental questions
peritinent to the study of nonlinear periodic structures, nonlinear crystals
and nonlinear metamaterials, which are of interest to the broad community of
physics | cond-mat_other |
The dynamics of condensate shells: collective modes and expansion: We explore the physics of three-dimensional shell-shaped condensates,
relevant to cold atoms in "bubble traps" and to Mott insulator-superfluid
systems in optical lattices. We study the ground state of the condensate
wavefunction, spherically-symmetric collective modes, and expansion properties
of such a shell using a combination of analytical and numerical techniques. We
find two breathing-type modes with frequencies that are distinct from that of
the filled spherical condensate. Upon trap release and subsequent expansion, we
find that the system displays self-interference fringes. We estimate
characteristic time scales, degree of mass accumulation, three-body loss, and
kinetic energy release during expansion for a typical system of Rb87. | cond-mat_other |
Macroscopic quantum tunneling and quantum-classical phase transitions of
the escape rate in large spin systems: This article presents a review on the theoretical and the experimental
developments on macroscopic quantum tunneling and phase transition of the
escape rate in spin systems. We present the basic ideas with simplified
calculations so that it is readable to both specialists and nonspecialists in
this area of research. A brief derivation of the path integral formulation of
quantum mechanics in its original form using the orthonormal position and
momentum basis is reviewed. For spin systems such as single molecule magnets,
the formulation of path integral requires the use of non-orthonormal spin
coherent state in $(2s+1)$ dimensional Hilbert space, the coordinate
independent and the coordinate dependent form of the spin coherent state path
integral is derived. These two forms of spin coherent state path integral are
applied to the tunneling of single molecule magnets through its magnetic
anisotropy barrier. Most experimental and numerical results are presented. The
suppression of tunneling for half-odd integer spin (spin-parity effect) at zero
magnetic field is derived from both forms, which shows that this result
(spin-parity effect) is independent of the coordinate. At nonzero magnetic
field we present both the experimental and the theoretical results of the
oscillation of tunneling splitting as a function of the applied magnetic field
applied along the spin hard anisotropy axis direction. The experimental and the
theoretical results of the tunneling in antiferromagnetic exchange coupled
dimer model are also reviewed. As the spin coherent state path integral
formalism is a semi-classical method, an alternative exact mapping of a spin
system to a particle in a potential field (effective potential method) is
derived. This effective potential method allows for the investigation of phase
transition of the escape rate in spin systems. | cond-mat_other |
Solid-liquid phase coexistence and structural transitions in palladium
clusters: We use molecular dynamics with an embedded atom potential to study the
behavior of palladium nanoclusters near the melting point in the microcanonical
ensemble. We see transitions from both fcc and decahedral ground state
structures to icosahedral structures prior to melting over a range of cluster
sizes. In all cases this transition occurs during solid-liquid phase
coexistence and the mechanism for the transition appears to be fluctuations in
the molten fraction of the cluster and subsequent recrystallization into the
icosahedral structure. | cond-mat_other |
Superslow Self-Organized Motions in a Multimode Microwave Phonon Laser
(Phaser) under Resonant Destabilization of Stationary Acoustic Stimulated
Emission: Two qualitatively different kinds of resonant destabilization of phonon
stimulated emission (SE) are experimentally revealed for periodically forced
multimode ruby phaser (phonon laser) operating at SE frequencies about 9 GHz,
i.e. at microwave acoustic wavelengths of 1 micron. The inversion state of
Cromium(3+) spin-system in ruby was created by electromagnetic pump at 23 GHz.
Under deep modulation of pump power at low frequencies OMEGA_m = 70-200 Hz
deterministic chaotic reconfigurations of the acoustic microwave power spectra
(AMPS) were observed. This range of SE destabilization corresponds to the
relaxational resonance that is well known for optical class-B lasers. Outside
the relaxational resonance range, namely at ultra-low (infrasonic) frequencies
OMEGA_m about 10 Hz, the other type of resonant destabilization of stationary
phonon SE was observed by us for the first time. This new nonlinear resonance
(we call it lambda-resonance) manifests itself as very slow and periodically
repeated self-reconfigurations of AMPS. Near the vertex of lambda-resonance the
period of AMPS self-reconfigurations takes giant values of several hours (at
T=1.8 K). The second type of SE resonant destabilization is explained in terms
of antiphase energy exchange between acoustic SE modes in a modulated phaser.
The role of polarized nuclear spin-reservoir (formed by Aluminium-27 nuclei of
the ruby crystalline matrix) in these superslow self-organized motions is
discussed. PACS: 05.65.+b, 42.65.Sf, 43.35.+d | cond-mat_other |
Wavelet versus Detrended Fluctuation Analysis of multifractal structures: We perform a comparative study of applicability of the Multifractal Detrended
Fluctuation Analysis (MFDFA) and the Wavelet Transform Modulus Maxima (WTMM)
method in proper detecting of mono- and multifractal character of data. We
quantify the performance of both methods by using different sorts of artificial
signals generated according to a few well-known exactly soluble mathematical
models: monofractal fractional Brownian motion, bifractal Levy flights, and
different sorts of multifractal binomial cascades. Our results show that in
majority of situations in which one does not know a priori the fractal
properties of a process, choosing MFDFA should be recommended. In particular,
WTMM gives biased outcomes for the fractional Brownian motion with different
values of Hurst exponent, indicating spurious multifractality. In some cases
WTMM can also give different results if one applies different wavelets. We do
not exclude using WTMM in real data analysis, but it occurs that while one may
apply MFDFA in a more automatic fashion, WTMM has to be applied with care. In
the second part of our work, we perform an analogous analysis on empirical data
coming from the American and from the German stock market. For this data both
methods detect rich multifractality in terms of broad f(alpha), but MFDFA
suggests that this multifractality is poorer than in the case of WTMM. | cond-mat_other |
Temperature dependence of exciton recombination in semiconducting
single-wall carbon nanotubes: We study the excitonic recombination dynamics in an ensemble of (9,4)
semiconducting single-wall carbon nanotubes by high sensitivity time-resolved
photo-luminescence experiments. Measurements from cryogenic to room temperature
allow us to identify two main contributions to the recombination dynamics. The
initial fast decay is temperature independent and is attributed to the presence
of small residual bundles that create external non-radiative relaxation
channels. The slow component shows a strong temperature dependence and is
dominated by non-radiative processes down to 40 K. We propose a quantitative
phenomenological modeling of the variations of the integrated photoluminescence
intensity over the whole temperature range. We show that the luminescence
properties of carbon nanotubes at room temperature are not affected by the
dark/bright excitonic state coupling. | cond-mat_other |
Fermion Superfluids of Non-Zero Orbital Angular Momentum near Resonance: We study the pairing of Fermi gases near the scattering resonance of the
$\ell\neq 0$ partial wave. Using a model potential which reproduces the actual
two-body low energy scattering amplitude, we have obtained an analytic solution
of the gap equation. We show that the ground state of $\ell=1$ and $\ell=3$
superfluid are orbital ferromagnets with pairing wavefunctions $Y_{11}$ and
$Y_{32}$ respectively. For $\ell=2$, there is a degeneracy between $Y_{22}$ and
a "cyclic state". Dipole energy will orient the angular momentum axis. The gap
function can be determined by the angular dependence of the momentum
distribution of the fermions. | cond-mat_other |
Boson-Fermion pairing in Bose-Fermi mixtures on 1D optical lattices: Boson-fermion pairing is considered in a discrete environment of bosons and
fully spin-polarized fermions, coupled via an attractive Bose-Fermi Hubbard
Hamiltonian in one dimension. The results of the T-matrix approximation for
particles of equal mass and at double half filling are compared with the
results of exact diagonalization and with Quantum Monte Carlo results.
Satisfactory agreement for most quantities is found. The appearance of a
stable, weak-coupling pairing mode is also confirmed. | cond-mat_other |
Matrix Product Density Operators: Simulation of finite-T and dissipative
systems: We show how to simulate numerically both the evolution of 1D quantum systems
under dissipation as well as in thermal equilibrium. The method applies to both
finite and inhomogeneous systems and it is based on two ideas: (a) a
representation for density operators which extends that of matrix product
states to mixed states; (b) an algorithm to approximate the evolution (in real
or imaginary time) of such states which is variational (and thus optimal) in
nature. | cond-mat_other |
Fully Frustrated Cold Atoms: Fully frustrated Josephson Junction arrays (FF-JJA's) exhibit a subtle
compound phase transition in which an Ising transition associated with discrete
broken translational symmetry and a Berezinskii-Kosterlitz-Thouless (BKT)
transition associated with quasi-long-range phase coherence occur nearly
simultaneously. In this Letter we discuss a cold atom realization of the FF-JJA
system. We demonstrate that both orders can be studied by standard
momentum-distribution-function measurements and present numerical results,
based on a successful self-consistent spin-wave approximation, that illustrate
the expected behavior of observables. | cond-mat_other |
Generalized Mean Field Approach to a Resonant Bose-Fermi Mixture: We formulate a generalized mean-field theory of a mixture of fermionic and
bosonic atoms, in which the fermion-boson interaction can be controlled by a
Feshbach resonance. The theory correctly accounts for molecular binding
energies of the molecules in the two-body limit, in contrast to the most
straightforward mean-field theory. Using this theory, we discuss the
equilibrium properties of fermionic molecules created from atom pairs in the
gas. We also address the formation of molecules when the magnetic field is
ramped across the resonance, and present a simple Landau-Zener result for this
process. | cond-mat_other |
Controlling a magnetic Feshbach resonance with laser light: The capability to tune the strength of the elastic interparticle interaction
is crucial for many experiments with ultracold gases. Magnetic Feshbach
resonances are a tool widely used for this purpose, but future experiments
would benefit from additional flexibility such as spatial modulation of the
interaction strength on short length scales. Optical Feshbach resonances offer
this possibility in principle, but suffer from fast particle loss due to
light-induced inelastic collisions. Here we show that light near-resonant with
a molecular bound-to-bound transition can be used to shift the magnetic field
at which a magnetic Feshbach resonance occurs. This makes it possible to tune
the interaction strength with laser light and at the same time induce
considerably less loss than an optical Feshbach resonance would do. | cond-mat_other |
Search for supersolid 4He in neutron scattering experiments at ISIS: The observation of a non-classical rotation inertia (NCRI) fraction in bulk
solid 4He by M. Chan and E. Kim attracted significant interest as a possible
manifestation of supersolid state of matter. Despite numerous experimental and
theoretical studies inspired by this observation, an explicit explanation for
this phenomenon is still missing. Neutron scattering experiments on solid elium
may help to shed light on the physical grounds of NCRI and answer the question
on whether this phenomenon could be caused by Bose-Einstein Condensation. In
this paper we are going to discuss the results obtained in experiments
involving neutron scattering on solid 4He. Microscopic quantitative data such
as mean kinetic energy, mean square momentum and mean square displacement of
helium atoms as well as the lattice parameter have been obtained for the first
time for solid 4He in temperature range 70 mK - 500 mK. No change was seen in
the single atom kinetic energy within statistical error better than 1per cent
as well as change in the lattice parameter within 0.03 per cent. The mean
square displacement did not change in the region of expected supersolid
transition either. All these results suggest that the NCRI transition is quite
different from the superfluid transition in liquid 4He. | cond-mat_other |
Using Multi-Threshold Threshold Gates in RTD-based Logic Design. A Case
Study: The basic building blocks for Resonant Tunnelling Diode (RTD) logic circuits
are Threshold Gates (TGs) instead of the conventional Boolean gates (AND, OR,
NAND, NOR) due to the fact that, when designing with RTDs, threshold gates can
be implemented as efficiently as conventional ones, but realize more complex
functions. Recently, RTD structures implementing Multi-Threshold Threshold
Gates (MTTGs) have been proposed which further increase the functionality of
the original TGs while maintaining their operating principle and allowing also
the implementation of nanopipelining at the gate level. This paper describes
the design of n-bit adders using these MTTGs. A comparison with a design based
on TGs is carried out showing advantages in terms of latency, device counts and
power consumption. | cond-mat_other |
Experimental investigation on the microscopic structure of intrinsic
paramagnetic point defects in amorphous silicon dioxide: In the present Ph.D. Thesis we report an experimental investigation on the
effects of gamma- and beta-ray irradiation and of subsequent thermal treatment
on many types of a-SiO2 materials, differing in the production methods, OH- and
Al-content, and oxygen deficiencies. Our main objective is to gain further
insight on the microscopic structures of the E'_gamma, E'_delta, E'_alpha and
triplet paramagnetic centers, which are among the most important and studied
class of radiation induced intrinsic point defects in a-SiO2. To pursue this
objective, we use prevalently the EPR spectroscopy. In particular, our work is
focused on the properties of the unpaired electrons wave functions involved in
the defects, and this aspect is mainly investigated through the study of the
EPR signals originating from the interaction of the unpaired electrons with
29Si magnetic nuclei (with nuclear spin I=1/2 and natural abundance 4.7 %). In
addition, in some cases of interest, OA measurements are also performed with
the aim to further characterize the electronic properties of the defects.
Furthermore, due to its relevance for electronics application, the charge state
of the defects is investigated by looking at the processes responsible for the
generation of the defects of interest. Once these information were gained, the
possible sites that can serve as precursors for defects formation are deduced,
with the definitive purpose to obtain in the future more radiation resistant
a-SiO2 materials in which the deleterious effects connected with the point
defects are significantly reduced. | cond-mat_other |
Collision Dynamics and Rung Formation of Non-Abelian Vortices: We investigate the collision dynamics of two non-Abelian vortices and find
that, unlike Abelian vortices, they neither reconnect themselves nor pass
through each other, but create a rung between them in a topologically stable
manner. Our predictions are verified using the model of the cyclic phase of a
spin-2 spinor Bose-Einstein condensate. | cond-mat_other |
Generalized gradient approximation for solids and their surfaces: Successful modern generalized gradient approximations (GGA) are biased toward
atomic energies. Restoration of the first-principles gradient expansion for the
exchange energy over a wide range of density gradients eliminates this bias. We
introduce PBEsol, a revised Perdew-Burke-Ernzerhof GGA that improves
equilibrium properties for many densely-packed solids and their surfaces. | cond-mat_other |
Mode-locking and mode-competition in a non-equilibrium solid-state
condensate: A trapped polariton condensate with continuous pumping and decay is analyzed
using a generalized Gross-Pitaevskii model. Whereas an equilibrium condensate
is characterized by a macroscopic occupation of a ground state, here the
steady-states take more general forms. Some are characterized by a large
population in an excited state, and others by large populations in several
states. In the latter case, the highly-populated states synchronize to a common
frequency above a critical density. Estimates for the critical density of this
synchronization transition are consistent with experiments. | cond-mat_other |
Influence of domain wall interactions on nanosecond switching in
magnetic tunnel junctions: We have obtained microscopic evidence of the influence of domain wall stray
fields on the nanosecond magnetization switching in magnetic trilayer systems.
The nucleation barrier initiating the magnetic switching of the soft magnetic
Fe20Ni80 layer in magnetic tunnel junction-like FeNi/Al2O3/Co trilayers is
considerably lowered by stray fields generated by domain walls present in the
hard magnetic Co layer. This internal bias field can significantly increase the
local switching speed of the soft layer. The effect is visualized using
nanosecond time- and layer-resolved magnetic domain imaging and confirmed by
micromagnetic simulations. | cond-mat_other |
Inversion of the spin polarization of localized electrons driven by dark
excitons: The creation of free excitons by absorption of circularly polarized photons,
and their subsequent fast capture by donors, is at the origin of the spin
polarization of donor-bound electrons. The sign of the electronic spin
polarization at low density of excitation is, as expected, fixed by the
helicity of the exciting light; but at high density of excitation we show that
the spin polarization is of the opposite sign. This striking inversion is
explained, here, by the contribution of dark excitons to mechanisms of spin
polarization of localized electrons. | cond-mat_other |
Non-periodic pseudo-random numbers used in Monte Carlo calculations: The generation of pseudo-random numbers is one of the interesting problems in
Monte Carlo simulations, mostly because the common computer generators produce
periodic numbers. We used simple pseudo-random numbers generated with the
simplest chaotic system, the logistic map, with excellent results. The numbers
generated in this way are non-periodic, which we demonstrated for 10$^{13}$
numbers, and they are obtained in a deterministic way, which allows to repeat
systematically any calculation. The Monte Carlo calculations are the ideal
field to apply these numbers, and we did it for simple and more elaborated
cases. Chemistry and Information Technology use this kind of simulations, and
the application of this numbers to Quantum Monte Carlo and Cryptography is
immediate. I present here the techniques to calculate, analyze and use these
pseudo-random numbers, show that they lack periodicity up to 10$^{13}$ numbers
and that they are not correlated. | cond-mat_other |
Excitation spectrum of bosons in a finite one-dimensional circular
waveguide via the Bethe ansatz: The exactly solvable Lieb-Liniger model of interacting bosons in
one-dimension has attracted renewed interest as current experiments with
ultra-cold atoms begin to probe this regime. Here we numerically solve the
equations arising from the Bethe ansatz solution for the exact many-body wave
function in a finite-size system of up to twenty particles for attractive
interactions. We discuss the novel features of the solutions, and how they
deviate from the well-known string solutions [H. B. Thacker, Rev. Mod. Phys.\
\textbf{53}, 253 (1981)] at finite densities. We present excited state string
solutions in the limit of strong interactions and discuss their physical
interpretation, as well as the characteristics of the quantum phase transition
that occurs as a function of interaction strength in the mean-field limit.
Finally we compare our results to those of exact diagonalization of the
many-body Hamiltonian in a truncated basis. We also present excited state
solutions and the excitation spectrum for the repulsive 1D Bose gas on a ring. | cond-mat_other |
On the feasibility of studying vortex noise in 2D superconductors with
cold atoms: We investigate the feasibility of using ultracold neutral atoms trapped near
a thin superconductor to study vortex noise close to the
Kosterlitz-Thouless-Berezinskii transition temperature. Alkali atoms such as
rubidium probe the magnetic field produced by the vortices. We show that the
relaxation time $T_1$ of the Zeeman sublevel populations can be conveniently
adjusted to provide long observation times. We also show that the transverse
relaxation times $T_2$ for Zeeman coherences are ideal for studying the vortex
noise. We briefly consider the motion of atom clouds held close to the surface
as a method for monitoring the vortex motion. | cond-mat_other |
Anomalous enhancement of quasiparticle current near a potential barrier
in a Bose-Einstein condensate: We investigate tunneling properties of Bogoliubov phonons in a Bose-Einstein
condensate. We find the anomalous enhancement of the quasiparticle current
$J_{\rm q}$ carried by Bogoliubov phonons near a potential barrier, due to the
supply of the excess current from the condensate. This effect leads to the
increase of quasiparticle transmission probability in the low energy region
found by Kovrizhin {\it et al.}. We also show that the quasiparticle current
twists the phase of the condensate wavefunction across the barrier, leading to
a finite Josephson supercurrent $J_{\rm s}$ through the barrier. This induced
supercurrent flows in the opposite direction to the quasiparticle current so as
to cancel out the enhancement of $J_{\rm q}$ and conserve the total current
$J=J_{\rm q}+J_{\rm s}$. | cond-mat_other |
Effects of Community Structure on Search and Ranking in Information
Network: The World-Wide Web (WWW) is characterized by a strong community structure in
which communities of webpages (e.g. those sharing a common keyword) are densely
interconnected by hyperlinks. We study how such network architecture affects
the average Google ranking of individual webpages in the comunity. It is shown
that the Google rank of community webpages could either increase or decrease
with the density of inter-community links depending on the exact balance
between average in- and out-degrees in the community. The magnitude of this
effect is described by a simple analytical formula and subsequently verified by
numerical simulations of random scale-free networks with a desired level of the
community structure. A new algorithm allowing for generation of such networks
is proposed and studied. The number of inter-community links in such networks
is controlled by a temperature-like parameter with the strongest community
structure realized in "low-temperature" networks. | cond-mat_other |
Feshbach resonances in an ultracold $^7$Li and $^{87}$Rb mixture: We report on the observation of five Feshbach resonances in collisions
between ultracold $^7$Li and $^{87}$Rb atoms in the absolute ground state
mixture where both species are in their $|f,m_f>=|1,1>$ hyperfine states. The
resonances appear as trap losses for the $^7$Li cloud induced by inelastic
heteronuclear three-body collisions. The magnetic field values where they occur
are important quantities for an accurate determination of the interspecies
interaction potentials. Results of coupled channels calculations based on the
observed resonances are presented and refined potential parameters are given. A
very broad Feshbach resonance centered around 649 G should allow for fine
tuning of the interaction strength in future experiments. | cond-mat_other |
Accurate measurement of ^{13}C - ^{15}N distances with solid-state NMR: Solid-state NMR technique for measureing distances between hetero-nuclei in
static powder samples is described. It is based on a two-dimensional
single-echo scheme enhanced with adiabatic cross-polarization. As an example,
the results for intra-molecular distances in $\alpha$-crystalline form of
glycine are presented. The measured NMR distances ^13 C(2) - ^15 N and ^13 C(1)
- ^15 N are 1.496 $\pm$ 0.002 \AA and 2.50 $\pm$ 0.02 \AA, respectively. | cond-mat_other |
Theory of ground states for classical Heisenberg spin systems IV: We extend the theory of ground states of classical Heisenberg spin systems
previously published to the case where the interaction with an external
magnetic field is described by a Zeeman term. The ground state problem for the
Heisenberg-Zeeman Hamiltonian can be reduced first to the relative ground state
problem, and, in a second step, to the absolute ground state problem for pure
Heisenberg Hamiltonians depending on an additional Lagrange parameter. We
distinguish between continuous and discontinuous reduction. Moreover, there are
various general statements about Heisenberg-Zeeman systems that will be proven
under most general assumptions. One topic is the connection between the minimal
energy functions $E_{min}$ for the Heisenberg energy and $H_{min}$ for the
Heisenberg-Zeeman energy which turn out to be essentially mutual
Legendre-Fenchel transforms. This generalization of the traditional Legendre
transform is especially suited to cope with situations where the function
$E_{min}$ is not convex and consequently there is a magnetization jump at a
critical field. Another topic is magnetization and the occurrence of threshold
fields $B_{thr}$ and saturation fields $B_{sat}$, where we provide a general
formula for the latter. We suggest a distinction between ferromagnetic and
anti-ferromagnetic systems based on the vanishing of $B_{sat}$ for the former
ones. Parabolic systems are defined in such a way that $E_{min}$ and $H_{min}$
have a particularly simple form and studied in detail. For a large class of
parabolic systems the relative ground states can be constructed from the
absolute ground state by means of a so-called umbrella family. Finally we
provide a counter-example of a parabolic system where this construction is not
possible. | cond-mat_other |
Macroscopic quantum tunnelling of Bose-Einstein condensates in a finite
potential well: Bose-Einstein condensates are studied in a potential of finite depth which
supports both bound and quasi-bound states. This potential, which is harmonic
for small radii and decays as a Gaussian for large radii, models experimentally
relevant optical traps. The nonlinearity, which is proportional to both the
number of atoms and the interaction strength, can transform bound states into
quasi-bound ones. The latter have a finite lifetime due to tunnelling through
the barriers at the borders of the well. We predict the lifetime and stability
properties for repulsive and attractive condensates in one, two, and three
dimensions, for both the ground state and excited soliton and vortex states. We
show, via a combination of the variational and WKB approximations, that
macroscopic quantum tunnelling in such systems can be observed on time scales
of 10 milliseconds to 10 seconds. | cond-mat_other |
Thomas-Fermi versus one- and two-dimensional regimes of a trapped
dipolar Bose-Einstein condensate: We derive the criteria for the Thomas-Fermi regime of a dipolar Bose-Einstein
condensate in cigar, pancake and spherical geometries. This also naturally
gives the criteria for the mean-field one- and two-dimensional regimes. Our
predictions, including the Thomas-Fermi density profiles, are shown to be in
excellent agreement with numerical solutions. Importantly, the anisotropy of
the interactions has a profound effect on the Thomas-Fermi/low-dimensional
criteria. | cond-mat_other |
Controlled quantum stirring of Bose-Einstein condensates: By cyclic adiabatic change of two control parameters of an optical trap one
can induce a circulating current of condensed bosons. The amount of particles
that are transported per period depends on the "radius" of the cycle, and this
dependence can be utilized in order to probe the interatomic interactions. For
strong repulsive interaction the current can be regarded as arising from a
sequence of Landau-Zener crossings. For weaker interaction one observes either
gradual or coherent mega crossings, while for attractive interaction the
particles are glued together and behave like a classical ball. For the analysis
we use the Kubo approach to quantum pumping with the associated Dirac monopoles
picture of parameter space. | cond-mat_other |
An exploration of thermal counterflow in He II using particle tracking
velocimetry: Visualization of thermal counterflow in He II using PIV (particle image
velocimetry) and PTV (particle tracking velocimetry) is difficult because
tracer particle motion can be influenced by both the normal fluid and
superfluid components of He II as well as the quantized vortex tangle. For
instance, an early PTV experiment observed particles moving at the normal fluid
velocity $v_n$, while a PIV experiment observed particles moving at $v_n/2$,
though the range of heat flux applied in these experiments differed by an order
of magnitude. To resolve this apparent discrepancy and explore statistics of
particle motion in thermal counterflow, we have applied PTV to a wide range of
heat flux at several fluid temperatures. We introduce a scheme for analyzing
the velocity of particles presumably moving with the normal fluid separately
from those presumably influenced by the vortex tangle. Our results show two
distinct peaks in the streamwise particle velocity PDF (probability density
function) for lower heat flux, one centered at the normal fluid velocity $v_n$
("G2") and one near $v_n/2$ ("G1"). For higher heat flux there is a single peak
centered near $v_n/2$ ("G3"). Using our separation scheme we show there is no
size difference between particles contributing to G1 and G2. We also show that
non-classical features of the transverse velocity PDF arise entirely from G1,
while the corresponding PDF for G2 exhibits classical Gaussian form. G2
transverse velocity fluctuation, backed up by second sound attenuation in
decaying counterflow, suggests large scale turbulence in the normal fluid is
absent from the two peak region. We offer a brief discussion of physical
mechanisms that may be responsible for our observations, revealing that G1
velocity fluctuations may be linked to fluctuations of vortex line velocity,
and suggest numerical simulations that may reveal underlying physics in detail. | cond-mat_other |
Exact Coulomb cutoff technique for supercell calculations in two
dimensions: We present a reciprocal space technique for the calculation of the Coulomb
integral in two dimensions in systems with reduced periodicity, i.e., finite
systems, or systems that are periodic only in one dimension. The technique
consists in cutting off the long-range part of the interaction by modifying the
expression for the Coulomb operator in reciprocal space. The physical result
amounts in an effective screening of the spurious interactions originated by
the presence of ghost periodic replicas of the system. This work extends a
previous report [C. A. Rozzi et al., Phys. Rev. B 73, 205119 (2006)], where
three-dimensional systems were considered. We show that the use of the cutoffs
dramatically enhances the accuracy of the calculations for a given supercell
size, and it allows to describe two-dimensional systems of reduced periodicity
with substantially less computational effort. In particular, we consider
semiconductor quantum-dot arrays having potential applications in quantum
information technology. | cond-mat_other |
Photoconduction in Alq3: Photoelectronic properties of Alq3 were studied by photoconductivity
measurements in thin film, sandwich (ITO/Alq3/LiF/Al) devices. We find that the
photocurrent is dominated by bulk generation of carriers for incident photon
energies greater than 2.75 eV. The quantum efficiency of photocarrier
generation has been measured from carrier collection measurements to be about
10%. The quantum efficiency is largely independent of electric field. This
enables a direct measurement of the electric field dependence of mobility using
photoconductivity measurements, which is used for quantitative analysis of the
dark forward current in these devices. Photoconductivity measurements were also
used to obtain (\mu_{0n} \tau_n) product which can be used as a measure of
material quality. For Alq3, we find that the value of (\mu_{0n} \tau_n) product
was between 3x10^{-15} cm^2/V to 8x10^{-15} cm^2/V for different samples. In
forward bias, at high field the photocurrent shows saturation accompanied by a
phase shift. These effects are attributed to space charge effects in the
device. | cond-mat_other |
Emergent Time Scale in Entangled Quantum Dynamics of Ultracold Molecules
in Optical Lattices: We derive a novel lattice Hamiltonian, the \emph{Molecular Hubbard
Hamiltonian} (MHH), which describes the essential many body physics of
closed-shell ultracold heteronuclear molecules in their absolute ground state
in a quasi-one-dimensional optical lattice. The MHH is explicitly
time-dependent, making a dynamic generalization of the concept of quantum phase
transitions necessary. Using the Time-Evolving Block Decimation (TEBD)
algorithm to study entangled dynamics, we demonstrate that, in the case of hard
core bosonic molecules at half filling, the MHH exhibits an emergent time scale
over which spatial entanglement grows, crystalline order appears, and
oscillations between rotational states self-damp into an asymptotic
superposition. We show that this time scale is a non-monotonic function of the
physical parameters describing the lattice. We also point out that experimental
mapping of the static phase boundaries of the MHH can be used to measure the
molecular polarizability tensor. | cond-mat_other |
Disorder, defects and bandgaps in ultra thin (001) MgO tunnel barrier
layers: We report scanning tunneling spectroscopy studies of the electronic structure
of 1.5 to 3 nm (001) textured MgO layers grown on (001) Fe. Thick MgO layers
exhibit a bulk-like band gap, approximately 5-7 eV, and sparse, localized
defect states with characteristics attributable to oxygen and, in some cases,
Mg vacancies. Thin MgO layers exhibit electronic structure indicative of
interacting defect states forming band tails which in the thinnest case extend
to approximately 0.5 V of the Fermi level. These vacancy defects are ascribed
to compressive strain from the MgO/Fe lattice mismatch, accommodated as the MgO
grows. | cond-mat_other |
Electrical spin injection into p-doped quantum dots through a tunnel
barrier: We have demonstrated by electroluminescence the injection of spin polarized
electrons through Co/Al2O3/GaAs tunnel barrier into p-doped InAs/GaAs quantum
dots embedded in a PIN GaAs light emitting diode. The spin relaxation processes
in the p-doped quantum dots are characterized independently by optical
measurements (time and polarization resolved photoluminescence). The measured
electroluminescence circular polarization is about 15 % at low temperature in a
2T magnetic field, leading to an estimation of the electrical spin injection
yield of 35%. Moreover, this electroluminescence circular polarization is
stable up to 70 K. | cond-mat_other |
Viscoelastic Behavior of Solid $^4$He: Over the last five years several experimental groups have reported anomalies
in the temperature dependence of the period and amplitude of a torsional
oscillator containing solid $^4$He. We model these experiments by assuming that
$^4$He is a viscoelastic solid--a solid with frequency dependent internal
friction. We find that while our model can provide a quantitative account of
the dissipation observed in the torsional oscillator experiments, it only
accounts for about 10% of the observed period shift, leaving open the
possibility that the remaining period shift is due to the onset of
superfluidity in the sample. | cond-mat_other |
Conservative-dissipative forces and heating mediated by fluctuation
electromagnetic field: two plates in relative nonrelativistic motion: We calculate heating rate, attractive conservative and tangential dissipative
fluctuation electromagnetic forces felt by a thick plate moving with
nonrelativistic velocity parallel to a closely spaced another plate in rest
using relativistic fluctuation electrodynamics. We argue that recently
developed relativistic out of equilibrium theory of fluctuation electromagnetic
interactions (Volokitin et. al., Phys.Rev. B78, 155437 (2008))has serious
drawbacks. | cond-mat_other |
Wealth distribution in an ancient Egyptian society: Modern excavations yielded a distribution of the house areas in the ancient
Egyptian city Akhetaten, which was populated for a short period during the 14th
century BC. Assuming that the house area is a measure of the wealth of its
inhabitants allows us to make a comparison of the wealth distributions in
ancient and modern societies. | cond-mat_other |
Basic theory tools for degenerate Fermi gases: This is an introductory lecture to the theory of degenerate Fermi gases, in
the context of present experiments on atomic Fermi gases. In part one, some
properties of the ideal Fermi gas are presented, including a discussion of the
fluctuations of the number of fermions in a given spatial zone in 1D, 2D and
3D. In part two, two-body aspects of the interaction potential are discussed
and several possible models for the interaction are analyzed, including the
two-channel model for the Feshbach resonance. In part three, basic predictions
of zero temperature BCS theory are presented, including a derivation of
superfluid hydrodynamic equations from time dependent BCS theory. | cond-mat_other |
The surface-forming energy release rate based fracture criterion for
elastic-plastic crack propagation: J integral based criterion is widely used in elastic-plastic fracture
mechanics. However, it is not rigorously applicable when plastic unloading
appears during crack propagation. One difficulty is that the energy density
with plastic unloading in J integral cannot be defined unambiguously. In this
paper, we alternatively start from the analysis on the power balance, and
propose a surface-forming energy release rate (ERR), which represents the
energy directly dissipated on the surface-forming during the crack propagation
and excludes the loading-mode-dependent plastic dissipation. Therefore the
surface-forming ERR based fracture criterion has wider applicability, including
elastic-plastic crack propagation problems. Several formulae have been derived
for calculating the surface-forming ERR. From the most concise formula, it is
interesting to note that the surface-forming ERR can be computed only by the
stress and deformation of the current moment, and the definition of the energy
density or work density is avoided. When an infinitesimal contour is chosen,
the expression can be further simplified. For any fracture behaviors, the
surface-forming ERR is proven to be path-independent, and the path-independence
of its constituent term, so-called integral, is also investigated. The physical
meanings and applicability of the proposed surface-forming ERR, traditional
ERR, Js integral and J integral are compared and discussed. | cond-mat_other |
Discrete symmetry in graphene: the Dirac equation and beyond: In this pedagogical paper we review the discrete symmetries of the Dirac
equation using elementary tools, but in a comparative order: the usual 3 + 1
dimensional case and the 2 + 1 dimensional case. Motivated by new applications
of the 2d Dirac equation in condensed matter, we further analyze the discrete
symmetries of a full tight-binding model in hexagonal lattices without conical
approximations. We touch upon an effective CPT symmetry breaking that occurs
when deformations and second-neighbor corrections are considered. | cond-mat_other |
Collective Excitations of a Two-Component Bose Condensate at Finite
Temperature: We compare the collective modes for Bose-condensed systems with two
degenerate components with and without spontaneous intercomponent coherence at
finite temperature using the time-dependent Hartree-Fock approximation. We show
that the interaction between the condensate and non-condensate in these two
cases results in qualitatively different collective excitation spectra. We show
that at zero temperature the single-particle excitations of the incoherent Bose
condensate can be probed by intercomponent excitations. | cond-mat_other |
Material independent crack arrest statistics: The propagation of (planar) cracks in a heterogeneous brittle material
characterized by a random field of toughness is considered, taking into account
explicitly the effect of the crack front roughness on the local stress
intensity factor. In the so-called strong-pinning regime, the onset of crack
propagation appears to map onto a second-order phase transition characterized
by universal critical exponents which are independent of the local
characteristics of the medium. Propagation over large distances can be
described by using a simple one-dimensional description, with a correlation
length and an effective macroscopic toughness distribution that scale in a
non-trivial fashion with the crack front length. As an application of the above
concepts, the arrest of indentation cracks is addressed, and the analytical
expression for the statistical distribution of the crack radius at arrest is
derived. The analysis of indentation crack radii on alumina is shown to obey
the predicted algebraic expression for the radius distribution and its
dependence on the indentation load. | cond-mat_other |
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