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Three-Body Recombination of Identical Bosons with a Large Positive
Scattering Length at Nonzero Temperature: For identical bosons with a large scattering length, the dependence of the
3-body recombination rate on the collision energy is determined in the
zero-range limit by universal functions of a single scaling variable. There are
six scaling functions for angular momentum zero and one scaling function for
each higher partial wave. We calculate these universal functions by solving the
Skorniakov--Ter-Martirosian equation. The results for the 3-body recombination
as a function of the collision energy are in good agreement with previous
results from solving the 3-body Schroedinger equation for 4He atoms. The
universal scaling functions can be used to calculate the 3-body recombination
rate at nonzero temperature. We obtain an excellent fit to the data from the
Innsbruck group for 133Cs atoms with a large positive scattering length. | cond-mat_other |
Correlation energy of two electrons in a ball: We study the ground-state correlation energy $E_{\rm c}$ of two electrons of
opposite spin confined within a $D$-dimensional ball ($D \ge 2$) of radius $R$.
In the high-density regime, we report accurate results for the exact and
restricted Hartree-Fock energy, using a Hylleraas-type expansion for the former
and a simple polynomial basis set for the latter. By investigating the exact
limiting correlation energy $E_{\rm c}^{(0)} = \lim_{R \to 0} \Ec$ for various
values of $D$, we test our recent conjecture [J. Chem. Phys. {\bf 131} (2009)
241101] that, in the large-$D$ limit, $E_{\rm c}^{(0)} \sim -\delta^2/8$ for
any spherically-symmetric confining external potential, where $\delta=1/(D-1)$. | cond-mat_other |
The influence of the optical Stark effect on chiral tunneling in
graphene: The influences of intense coherent laser fields on the transport properties
of a single layer graphene are investigated by using the finite-difference
time-domain method. Under an intense laser field, the valence band and
conduction band states mix via the optical Stark effect. The chiral symmetry of
Dirac electrons is broken and the perfect chiral tunneling is strongly
suppressed. | cond-mat_other |
Formation of vortices in a dense Bose-Einstein condensate: A relaxation method is employed to study a rotating dense Bose-Einstein
condensate beyond Thomas-Fermi approximation. We use a slave-boson model to
describe the strongly interacting condensate and derive a generalized
non-linear Schr\"odinger equation with kinetic term for the rotating
condensate. In comparison with previous calculations, based on Thomas-Fermi
approximation, significant improvements are found in regions, where the
condensate in a trap potential is not smooth. The critical angular velocity of
the vortex formation is higher than in the Thomas-Fermi prediction. | cond-mat_other |
Electrically probing photonic bandgap phenomena in contacted defect
nanocavities: We demonstrate an electrically tunable two dimensional photonic crystal
nanocavity containing InAs self assembled quantum dots. Photoluminescence and
electroluminescence measurements are combined to probe the cavity mode
structure and demonstrate a local electrical contact to the quantum dots.
Measurements performed as a function of the electric field enable us to probe
the capture, relaxation and recombination dynamics of photogenerated carriers
inside the quantum dots emitting into a modified photonic environment.
Furthermore, the two dimensional photonic crystal is probed by spatially
dependent photocurrent spectroscopy indicating a 3.5x enhancement of the local
radiative lifetime of the QDs inside the photonic crystal environment. | cond-mat_other |
Graphite vs graphene: scientific background: Nobel Prize in Physics 2010 was given for "groundbreaking experiments
regarding the two-dimensional material graphene." In fact, before graphene has
been extracted from graphite and measured, some of its fundamental physical
properties have already been experimentally uncovered in bulk graphite. In this
Letter to the Nobel Committee we propose to include those findings in the
Scientific Background | cond-mat_other |
Collisional de-excitation in a quasi-2D degenerate Bose gas: We separate a Bose-Einstein condensate into an array of 2D sheets using a 1D
optical lattice, and then excite quantized vibrational motion in the direction
normal to the sheets. Collisions between atoms induce vibrational
de-excitation, transferring the large excitation energy into back-to-back
outgoing atoms, imaged as rings in the 2D plane. The ring diameters correspond
to vibrational energy level differences, and edge-on imaging allows
identification of the final vibrational states. Time dependence of these data
provides a nearly complete characterization of the decay process including the
energies, populations, and lifetimes of the lowest two excited vibrational
levels. The measured decay rates represent a suppression of collisional
de-excitation due to the reduced dimensionality, a matter wave analog to
inhibited spontaneous emission. | cond-mat_other |
The Fast Wandering of Slow Birds: I study a single "slow" bird moving with a flock of birds of a different, and
faster (or slower) species. I find that every "species" of flocker has a
characteristic speed $\gamma\ne v_0$, where $v_0$ is the mean speed of the
flock, such that, if the speed $v_s$ of the "slow" bird equals $\gamma$, it
will randomly wander transverse to the mean direction of flock motion far
faster than the other birds will: its mean-squared transverse displacement will
grow in $d=2$ with time $t$ like $t^{5/3}$, in contrast to $t^{4/3}$ for the
other birds. In $d=3$, the slow bird's mean squared transverse displacement
grows like $t^{5/4}$, in contrast to $t$ for the other birds. If $v_s\neq
\gamma$, the mean-squared displacement of the "slow" bird crosses over from
$t^{5/2}$ to $t^{4/3}$ scaling in $d=2$, and from $t^{5/4}$ to $t$ scaling in
$d=3$, at a time $t_c$ that scales according to $t_c \propto|v_s-\gamma|^{-2}$. | cond-mat_other |
Fermi Condensates: Ultracold atomic gases have proven to be remarkable model systems for
exploring quantum mechanical phenomena. Experimental work on gases of fermionic
atoms in particular has seen large recent progress including the attainment of
so-called Fermi condensates. In this article we will discuss this recent
development and the unique control over interparticle interactions that made it
possible. | cond-mat_other |
Matter Wave Interference Pattern in the collision of bright solitons
(Bose Einstein condensates) in a time dependent trap: We show that it is possible to observe matter wave interference patterns in
the collision of bright solitons (Bose Einstein condensates) without free
ballistic expansion for suitable choices of scattering length and time
dependent trap. | cond-mat_other |
Hyperspherical Description of the Degenerate Fermi Gas: S-wave
Interactions: We present a unique theoretical description of the physics of the spherically
trapped $N$-atom degenerate Fermi gas (DFG) at zero temperature based on an
ordinary Schr\"{o}dinger equation with a microscopic, two body interaction
potential. With a careful choice of coordinates and a variational wavefunction,
the many body Schr\"{o}dinger equation can be accurately described by a
\emph{linear}, one dimensional effective Schr\"{o}dinger equation in a single
collective coordinate, the rms radius of the gas. Comparisons of the energy,
rms radius and peak density of ground state energy are made to those predicted
by Hartree-Fock (HF). Also the lowest radial excitation frequency (the
breathing mode frequency) agrees with a sum rule calculation, but deviates from
a HF prediction. | cond-mat_other |
Boson Representation of Spin Operators: The derivation of the boson representation of spin operators is given which
reproduces the Holstein-Primakoff and Dyson-Maleev transformations in the
corresponding cases. The suggested formalism allows to address some subtle
issues which appear crucial for treating certain class of problems. Moreover,
the transformation is suggested which is naturally related to the symmetry of
the spin systems. | cond-mat_other |
On the absorption spectrum of noble gases at the arc spectrum limit: Rydberg spectral lines of an atom are sometimes superimposed on the
continuous spectrum of a different configuration. Effects of interaction among
different configurations in one of these cases are theoretically investigated,
and a formula is obtained that describes the behavior of absorption spectrum
intensity. This offers qualitative justification of some experimental results
obtained by BEUTLER in studies of absorption arc spectra of noble gases and
$I^b$ spectra of some metal vapors. | cond-mat_other |
Theory of correlations between ultra-cold bosons released from an
optical lattice: In this paper we develop a theoretical description of the correlations
between ultra-cold bosons after free expansion from confinement in an optical
lattice. We consider the system evolution during expansion and give criteria
for a far field regime. We develop expressions for first and second order
two-point correlations based on a variety of commonly used approximations to
the many-body state of the system including Bogoliubov, meanfield decoupling,
and particle-hole perturbative solution about the perfect Mott-insulator state.
Using these approaches we examine the effects of quantum depletion and pairing
on the system correlations. Comparison with the directly calculated correlation
functions is used to justify a Gaussian form of our theory from which we
develop a general three-dimensional formalism for inhomogeneous lattice systems
suitable for numerical calculations of realistic experimental regimes. | cond-mat_other |
Nonlinear Quantum Electrodynamics in Dirac materials: Classical electromagnetism is linear. However, fields can polarize the vacuum
Dirac sea, causing quantum nonlinear electromagnetic phenomena, e.g.,
scattering and splitting of photons, that occur only in very strong fields
found in neutron stars or heavy ion colliders.We show that strong nonlinearity
arises in Dirac materials at much lower fields $\sim 1\:\text{T}$, allowing us
to explore the nonperturbative, extremely high field limit of quantum
electrodynamics in solids. We explain recent experiments in a unified framework
and predict a new class of nonlinear magneto-electric effects, including a
magnetic enhancement of dielectric constant of insulators and a strong electric
modulation of magnetization. We propose experiments and discuss the
applications in novel materials. | cond-mat_other |
Classical and quantum dynamics of pulsating instability in a
Bose-Einstein condensate in an optical lattice: We study the dynamics of a Bose-Einstein condensate (BEC) in a one
dimensional optical lattice in the limit of weak atom-atom interactions.
Numerically we find that a BEC may develop a pulsating instability in which
atoms nearly periodically collect themselves into a pulse and subsequently
disperse back into the initial homogeneous state. A qualitative explanation of
the quasi-periodic behavior is given by drawing an analogy with a double-well
system. In an extension we introduce quantum effects approximately within
Truncated Wigner Approximation (TWA). In pure classical mean field theory the
condensate shows an undamped pulsating instability, whereas we have observed a
damping in the oscillation when we average over many stochastic realizations. | 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 |
Electronic Control and Readout of Qubit States in Solid State Quantum
Computing Systems: We demonstrate that an $n^+/i/n^+$ junction is the most suitable candidate
for electronic control and readout of qubit states in quantum computing systems
based on shallow impurities. The signature of this system is that the
$n^+-$regions serve as metallic electrodes separated form the $i-$region by a
self-induced barrier (internal workfunction). The $n^+/i/n^+$ system mimics the
properties of a metal-vacuum-metal junction with the qubit (impurity atom)
placed in a ``vacuum'' $i$-region between two ``metallic'' $n^+$ electrodes. We
will show that the self-induced barrier exists in a sufficiently wide range of
the concentration of dopants in the $n^+$-semiconductor (e.g. up to $10^{21}$
cm$^{-3}$ for Si) and its height can be controlled by tuning the doping level.
A shallow donor placed in a vacuum $i$-region will be populated with one
electron in equilibrium. In the case of Li donor in Si the $n^+$-electrodes
will be used for a precision placement of the Li atom during the growth
process; for voltage control and manipulation of the qubit states; and for a
qubit readout by means of the optically stimulated resonant tunnelling. Another
important feature of our system is that the qubit states (first two lowest
energy levels of Li in Si) are separated by an energy gap from a continuum of
the many-body states of the controlling electrodes. | cond-mat_other |
Numerical study of magnetic field induced ordering in BaCuSi$_2$O$_6$
and related systems: Thermodynamics of spin dimer system BaCuSi_2O_6 is studied using a quantum
Monte Carlo calculation (QMC) and a bond-operator mean field theory. We propose
that a new type of boson, which, rather than being hard-core, allows up to two
occupancy at each site, is responsible for the Bose Einstein condensation of
field induced ordering. Its superfluid density is identified as the square of
the in-plane staggered magnetization m_{xy} in the ordered phase. We also
compare our QMC result of the spin Heisenberg model to those predicted by mean
field theory as well as by the simple hard core boson model for both large and
small intra-dimer coupling J. The asymmetry of the phase diagram of m_{xy}(h)
of small coupling J in related systems such as NiCl_2-4SC(NH_2)_2 is explained
with our new boson operator. | cond-mat_other |
Magnetism and structure of magnetic multilayers based on the fully spin
polarized Heusler alloys Co2MnGe and Co2MnSn: Our Introduction starts with a short general review of the magnetic and
structural properties of the Heusler compounds which are under discussion in
this book. Then, more specifically, we come to the discussion of our
experimental results on multilayers composed of the Heusler alloys Co2MnGe and
Co2MnSn with V or Au as interlayers. The experimental methods we apply combine
magnetization and magneto-resistivity measurements, x-ray diffraction and
reflectivity, soft x-ray magnetic circular dichroism and spin polarized neutron
reflectivity. We find that below a critical thickness of the Heusler layers at
typically dcr = 1.5 nm the ferromagnetic order is lost and spin glass order
occurs instead. For very thin ferromagnetic Heusler layers there are
peculiarities in the magnetic order which are unusual when compared to
conventional ferromagnetic transition metal multilayer systems. In [Co2MnGe/Au]
multilayers there is an exchange bias shift at the ferromagnetic hysteresis
loops at low temperatures caused by spin glass ordering at the interface. In
[Co2MnGe/V] multilayers we observe an antiferromagnetic interlayer long range
ordering below a well defined Neel temperature originating from the dipolar
stray fields at the magnetically rough Heusler layer interfaces. | cond-mat_other |
Coexisting Non-Equilibrium Condensates with Long-Range Spatial Coherence
in Semiconductor Microcavities: Real and momentum space spectrally resolved images of microcavity polariton
emission in the regime of condensation are investigated under non resonant
excitation using a laser source with reduced intensity fluctuations on the
timescale of the exciton lifetime. We observe that the polariton emission
consists of many macroscopically occupied modes. Lower energy modes are
strongly localized by the photonic potential disorder on a scale of few
microns. Higher energy modes have finite k-vectors and are delocalized over
10-15 microns. All the modes exhibit long range spatial coherence comparable to
their size. We provide a theoretical model describing the behavior of the
system with the results of the simulations in good agreement with the
experimental observations. We show that the multimode emission of the polariton
condensate is a result of its nonequilibrium character, the interaction with
the local photonic potential and the reduced intensity fluctuations of the
excitation laser. | cond-mat_other |
Annealing Effect for Supersolid Fraction in $^4$He: We report on experimental confirmation of the non-classical rotational
inertia (NCRI) in solid helium samples originally reported by Kim and Chan. The
onset of NCRI was observed at temperatures below ~400 mK. The ac velocity for
initiation of the NCRI suppression is estimated to be ~10 $\mu$m/sec. After an
additional annealing of the sample at $T= 1.8$ K for 12 hours, ~ 10% relative
increase of NCRI fraction was observed. Then after repeated annealing with the
same conditions, the NCRI fraction was saturated. It differs from Reppy's
observation on a low pressure solid sample. | cond-mat_other |
Pairing fluctuations and the superfluid density through the BCS-BEC
crossover: We derive an expression for the superfluid density of a uniform two-component
Fermi gas through the BCS-BEC crossover in terms of the thermodynamic potential
in the presence of an imposed superfluid flow. Treating the pairing
fluctuations in a Gaussian approximation following the approach of Nozi\`eres
and Schmitt-Rink, we use this definition of $\rho_s$ to obtain an explicit
result which is valid at finite temperatures and over the full BCS-BEC
crossover. It is crucial that the BCS gap $\Delta$, the chemical potential
$\mu$, and $\rho_s$ all include the effect of fluctuations at the same level in
a self-consistent manner. We show that the normal fluid density $\rho_n \equiv
n - \rho_s$ naturally separates into a sum of contributions from Fermi BCS
quasiparticles ($\rho^F_{n}$) and Bose collective modes ($\rho^B_{n}$). The
expression for $\rho^F_{n}$ is just Landau's formula for a BCS Fermi superfluid
but now calculated over the BCS-BEC crossover. The expression for the Bose
contribution $\rho^B_{n}$ is more complicated and only reduces to Landau's
formula for a Bose superfluid in the extreme BEC limit, where all the fermions
have formed stable Bose pairs and the Bogoliubov excitations of the associated
molecular Bose condensate are undamped. In a companion paper, we present
numerical calculations of $\rho_s$ using an expression equivalent to the one
derived in this paper, over the BCS-BEC crossover, including unitarity, and at
finite temperatures. | cond-mat_other |
Coherent tunneling by adiabatic passage in an optical waveguide system: We report on the first experimental demonstration of light transfer in an
engineered triple-well optical waveguide structure which provides a classic
analogue of Coherent Tunnelling by Adiabatic Passage (CTAP) recently proposed
for coherent transport in space of neutral atoms or electrons among
tunneling-coupled optical traps or quantum wells [A.D. Greentree et al., Phys.
Rev. B 70, 235317 (2004); K. Eckert et al., Phys. Rev. A 70, 023606 (2004)].
The direct visualization of CTAP wavepacket dynamics enabled by our simple
optical system clearly shows that in the counterintuitive passage scheme light
waves tunnel between the two outer wells without appreciable excitation of the
middle well. | cond-mat_other |
Introduction to Spin-Polarized Ballistic Hot Electron Injection and
Detection in Silicon: Ballistic hot electron transport overcomes the well-known problems of
conductivity and spin lifetime mismatch that plagues spin injection in
semiconductors with ferromagnetic ohmic contacts. Through the spin-dependent
mean-free-path, it also provides a means for spin detection after transport.
Experimental results using these techniques (consisting of spin precession and
spin-valve measurements) with Silicon-based devices reveals the exceptionally
long spin lifetime and high spin coherence induced by drift-dominated transport
in the semiconductor. An appropriate quantitative model that accurately
simulates the device characteristics for both undoped and doped spin transport
channels is described; it can be used to determine the spin current velocity,
diffusion constant, and spin lifetime, constituting a spin "Haynes-Shockley"
experiment without time-of-flight techniques. A perspective on the future of
these methods is offered as summary. | cond-mat_other |
Semiclassical atom theory applied to solid-state physics: Using the semiclassical neutral atom theory, we extend to fourth order the
modified gradient expansion of the exchange energy of density functional
theory. This expansion can be applied both to large atoms and solid-state
problems. Moreover, we show that it can be employed to construct a simple and
non-empirical generalized gradient approximation (GGA) exchange-correlation
functional competitive with state-of-the-art GGAs for solids, but also
reasonably accurate for large atoms and ordinary chemistry. | cond-mat_other |
Quantum computing without qubit-qubit interactions: Quantum computing tries to exploit entanglement and interference to process
information more efficiently than the best known classical solutions.
Experiments demonstrating the feasibility of this approach have already been
performed. However, finding a really scalable and robust quantum computing
architecture remains a challenge for both, experimentalists and theoreticians.
In most setups decoherence becomes non-negligible when one tries to perform
entangling gate operations using the coherent control of qubit-qubit
interactions. However, in this proceedings we show that two-qubit gate
operations can be implemented even without qubit-qubit interactions and review
a recent quantum computing scheme by Lim et al. [Phys. Rev. Lett. 95, 030505
(2005)] using only single photon sources (e.g. atom-cavity systems, NV colour
centres or quantum dots) and photon pair measurements. | cond-mat_other |
Mechanism of Electric Field Induced Conductance Transition in Molecular
Organic Semiconductor Based Thin Films: We have studied the electrical field induced conductance transition in thin
film of Perylenetetracarboxylic dianhydride sandwiched between two metal
electrodes, from an insulating state to conducting state with a high ON-OFF
ratio in those devices, where one of electrodes is either Al or Cu. Temperature
dependence of resistivity shows semiconducting behavior in OFF-state, but it
shows metallic behavior in the ON-state. Devices with a thin intermediate layer
of LiF between metal electrode and organic layer, or devices fabricated in
planar configuration do not show switching behavior. All these suggest that
conducting pathways are responsible for the electric field induced conductance
transition. | cond-mat_other |
The system of mobile ions in lattice models: Screening effects,
thermodynamic and electrophysical properties: The lattice fluid model of the system with short range and long range Coulomb
interactions is suggested. In the framework of the collective variables method,
the screening of the Coulomb interactions in the bulk is considered. It is
shown that the Debye length includes additional concentration dependence
inversely proportional to the square root of the mean concentration of vacant
sites like what is known at the plane boundary. The Coulomb interaction
contribution to the free energy of the system is calculated in the approach
close to the mean spherical approximation and is given in an analytical form.
The influence of the variation of the crystal field near the system boundary on
the structure and characteristics of the electric double layer is investigated.
As compared to the system with equal crystal potentials at the lattice sites
throughout the system the pronounced difference for the electric capacitance
appears at low absolute values of the surface potential and it is more
pronounced for negative electric potentials. The capacitance diverges as the
potential values at which the electric field tends to zero and attains negative
values in regions of the surface potentials depending on their polarity and
values of the surface crystal potential. Negative values of the capacitance may
indicate the thermodynamic instability of the system that can result from
neglecting the short range interaction contribution. | cond-mat_other |
Lattice solitons in quasicondensates: We analyze finite temperature effects in the generation of bright solitons in
condensates in optical lattices. We show that even in the presence of strong
phase fluctuations solitonic structures with well defined phase profile can be
created. We propose a novel family of variational functions which describe well
the properties of these solitons and account for the non-linear effects in the
band structure. We discuss also the mobility and collisions of these localized
wave packets. | cond-mat_other |
Comment on "Exact results for survival probability in the multistate
Landau-Zener model": We correct the proof of Brundobler-Elser formula (BEF) provided in [2004
\textit{J. Phys. B: At. Mol. Opt. Phys.} \textbf{37} 4069] and continued in
Appendix of [2005 \textit{J. Phys. B: At. Mol. Opt. Phys.} \textbf{38} 907].
After showing that some changes of variables employed in these articles are
used erroneously, we propose an alternative change of variables which solves
the problem. In our proof, we reveal the connection between the BEF for a
general $N$-level Landau-Zener system and the exactly solvable bow-tie model.
The special importance of the diabatic levels with maximum/minimum slope is
emphasized throughout. | cond-mat_other |
Onset of thermally driven self-motion of a current filament in a
bistable semiconductor structure: We perform an analytical investigation of the bifurcation from static to
traveling current density filaments in a bistable semiconductor structure with
S-shaped current-voltage characteristic. Joule self-heating of a semiconductor
structure and the effect of temperature on electron transport are consistently
taken into account in the framework of a generic reaction-diffusion model with
global coupling. It is shown that the self-heating is capable to induce
translation instability which leads to spontaneous onset of lateral self-motion
of the filament along the structure. This may occur in a wide class of
semiconductor structures whose bistability is caused by impact ionization due
to the negative effect of temperature on the impact ionization rate. The
increment of the translation mode and the instability threshold are determined
analytically. | cond-mat_other |
On non-markovian nature of stock trading: Using a relationship between the moments of the probability distribution of
times between the two consecutive trades (intertrade time distribution) and the
moments of the distribution of a daily number of trades we show, that the
underlying point process generating times of the trades is an essentially
non-markovian long-range memory one. Further evidence for the long-range memory
nature of this point process is provided by the powerlike correlation between
the intertrade time intervals. The data set includes all trades in EESR stock
on the Moscow International Currency Exchange in January 2003 - September 2003
and in Siemens, Commerzbank and Karstadt stocks traded on the Xetra electronic
stock exchange of Deutsche Boerse in October 2002. | cond-mat_other |
On reality of dynamical matrix: The symmetry properties of the dynamical matrix are well described in
multiple classic textbooks. This short paper revisits the issue to demonstrate
alternative form of dynamical matrix which explicitly shows its symmetry and
reality in common cases. | cond-mat_other |
Wave equation of the scalar field and superfluids: The new formal analogy between superfluid systems and cosmology, which
emerges by taking into account the back-reaction of the vacuum to the quanta of
sound waves \cite{noi}, enables us to put forward some common features between
these two different areas of physics. We find the condition that allows us to
justify a General Relativity (GR) derivation of the hydrodynamical equation for
the superfluid in a four-dimensional space whose metric is the Unruh one
\cite{Unruh}. Furthermore we show how, in the particular case taken into
account, our hydrodynamical equation can be deduced within a four-dimensional
space from the wave equation of a massless scalar field. | cond-mat_other |
Matter sound waves in two-component Bose-Einstein condensates: The creation and propagation of sound waves in two-component Bose-Einstein
condensates (BEC) are investigated and a new method of wave generation in
binary BEC mixtures is proposed. The method is based on a fast change of the
inter-species interaction constant and is illustrated for two experimental
settings: a drop-like condensate immersed into a second large repulsive
condensate, and a binary mixture of two homogeneous repulsive BEC's. A
mathematical model based on the linearized coupled Gross-Pitaevskii equations
is developed and explicit formulae for the space and time dependence of sound
waves are provided. Comparison of the analytical and numerical results shows
excellent agreement, confirming the validity of the proposed approach. | cond-mat_other |
Spin-polarized transport in ferromagnetic multilayered semiconductor
nanostructures: The occurrence of inhomogeneous spin-density distribution in multilayered
ferromagnetic diluted magnetic semiconductor nanostructures leads to strong
dependence of the spin-polarized transport properties on these systems. The
spin-dependent mobility, conductivity and resistivity in
(Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a
function of temperature, scaled by the average magnetization of the diluted
magnetic semiconductor layers. An increase of the resistivity near the
transition temperature is obtained. We observed that the spin-polarized
transport properties changes strongly among the three materials. | cond-mat_other |
Quantum dynamics in view of Einstein's theory of Brownian motion: A quantum-mechanical version of Einstein's 1905 theory of Brownian motion is
presented. Starting from the Hamiltonian dynamics of an isolated composite of
objective and environmental systems, subdynamics for the objective system is
derived in the spirit of Einstein. The resulting master equation is found to
have the Lindblad structure. | cond-mat_other |
Transport in ultradilute solutions of $^3$He in superfluid $^4$He: We calculate the effect of a heat current on transporting $^3$He dissolved in
superfluid $^4$He at ultralow concentration, as will be utilized in a proposed
experimental search for the electric dipole moment of the neutron (nEDM). In
this experiment, a phonon wind will generated to drive (partly depolarized)
$^3$He down a long pipe. In the regime of $^3$He concentrations $\tilde <
10^{-9}$ and temperatures $\sim 0.5$ K, the phonons comprising the heat current
are kept in a flowing local equilibrium by small angle phonon-phonon
scattering, while they transfer momentum to the walls via the $^4$He first
viscosity. On the other hand, the phonon wind drives the $^3$He out of local
equilibrium via phonon-$^3$He scattering. For temperatures below $0.5$ K, both
the phonon and $^3$He mean free paths can reach the centimeter scale, and we
calculate the effects on the transport coefficients. We derive the relevant
transport coefficients, the phonon thermal conductivity and the $^3$He
diffusion constants from the Boltzmann equation. We calculate the effect of
scattering from the walls of the pipe and show that it may be characterized by
the average distance from points inside the pipe to the walls. The temporal
evolution of the spatial distribution of the $^3$He atoms is determined by the
time dependent $^3$He diffusion equation, which describes the competition
between advection by the phonon wind and $^3$He diffusion. As a consequence of
the thermal diffusivity being small compared with the $^3$He diffusivity, the
scale height of the final $^3$He distribution is much smaller than that of the
temperature gradient. We present exact solutions of the time dependent
temperature and $^3$He distributions in terms of a complete set of normal
modes. | cond-mat_other |
Comparison of strong coupling regimes in bulk GaAs, GaN and ZnO
semiconductor microcavities: Wide bandgap semiconductors are attractive candidates for polariton-based
devices operating at room temperature. We present numerical simulations of
reflectivity, transmission and absorption spectra of bulk GaAs, GaN and ZnO
microcavities, in order to compare the particularities of the strong coupling
regime in each system. Indeed the intrinsic properties of the excitons in these
materials result in a different hierarchy of energies between the valence-band
splitting, the effective Rydberg and the Rabi energy, defining the
characteristics of the exciton-polariton states independently of the quality
factor of the cavity. The knowledge of the composition of the polariton
eigenstates is central to optimize such systems. We demonstrate that, in ZnO
bulk microcavities, only the lower polaritons are good eigenstates and all
other resonances are damped, whereas upper polaritons can be properly defined
in GaAs and GaN microcavities. | cond-mat_other |
Absorbing photonic crystals for thin film photovoltaics: The absorption of thin hydrogenated amorphous silicon layers can be
efficiently enhanced through a controlled periodic patterning. Light is trapped
through coupling with photonic Bloch modes of the periodic structures, which
act as an absorbing planar photonic crystal. We theoretically demonstrate this
absorption enhancement through one or two dimensional patterning, and show the
experimental feasibility through large area holographic patterning. Numerical
simulations show over 50% absorption enhancement over the part of the solar
spectrum comprised between 380 and 750nm. It is experimentally confirmed by
optical measurements performed on planar photonic crystals fabricated by laser
holography and reactive ion etching. | cond-mat_other |
Bose Einstein Condensation of incommensurate solid 4He: It is pointed out that simulation computation of energy performed so far
cannot be used to decide if the ground state of solid 4He has the number of
lattice sites equal to the number of atoms (commensurate state) or if it is
different (incommensurate state). The best variational wave function, a shadow
wave function, gives an incommensurate state but the equilibrium concentration
of vacancies remains to be determined. In order to investigate the presence of
a supersolid phase we have computed the one--body density matrix in solid 4He
for the incommensurate state by means of the exact Shadow Path Integral Ground
State projector method. We find a vacancy induced Bose Einstein condensation of
about 0.23 atoms per vacancy at a pressure of 54 bar. This means that bulk
solid 4He is supersolid at low enough temperature if the exact ground state is
incommensurate. | cond-mat_other |
Improved modeling of Coulomb effects in nanoscale Schottky-barrier FETs: We employ a novel multi-configurational self-consistent Green's function
approach (MCSCG) for the simulation of nanoscale Schottky-barrier field-effect
transistors. This approach allows to calculate the electronic transport with a
seamless transition from the single-electron regime to room temperature
field-effect transistor operation. The particular improvement of the MCSCG
stems from a division of the channel system into a small subsystem of
resonantly trapped states for which a many-body Fock space becomes feasible and
a strongly coupled rest which can be treated adequately on a conventional
mean-field level. The Fock space description allows for the calculation of
few-electron Coulomb charging effects beyond mean-field.
We compare a conventional Hartree non-equilibrium Green's function
calculation with the results of the MCSCG approach. Using the MCSCG method
Coulomb blockade effects are demonstrated at low temperatures while under
strong nonequilibrium and room temperature conditions the Hartree approximation
is retained. | cond-mat_other |
Ohmic and non-Ohmic Andreev transport through an interface between
superconductor and hopping insulator: Dramatic role of barrier properties: At low temperatures and voltages tunneling transport through an interface
between a superconductor and hopping insulator is dominated by coherent
two-electron tunneling between the Cooper-pair condensate and pairs of
localized states, see Kozub et al., PRL 96, 107004 (2006). By detailed analysis
of such transport we show that the interface resistance is extremely sensitive
to the properties of the tunneling barriers, as well as to asymptotic behavior
of the localized states. In particular, dramatic cancellation takes place for
hydrogen-like impurities and ideal barrier. However, some disorder can lift the
cancellations restoring the interface transport. We also study non-Ohmic
behavior of the interface resistor and show that it is sensitive to the Coulomb
correlation of the occupation probabilities of the involved localized states.
It is expected that non-Ohmic contribution to I-V-curve will experience
pronounced mesoscopic (fingerprint) fluctuations. | cond-mat_other |
Chiral Symmetry and Electron Spin Relaxation of Lithium Donors in
Silicon: We report theoretical and experimental studies of the longitudinal electron
spin and orbital relaxation time of interstitial Li donors in $^{28}$Si. We
predict that despite the near-degeneracy of the ground-state manifold the spin
relaxation times are extremely long for the temperatures below 0.3 K. This
prediction is based on a new finding of the chiral symmetry of the donor
states, which presists in the presence of random strains and magnetic fields
parallel to one of the cubic axes. Experimentally observed kinetics of
magnetization reversal at 2.1 K and 4.5 K are in a very close agreement with
the theory. To explain these kinetics we introduced a new mechanism of spin
decoherence based on a combination of a small off-site displacement of the Li
atom and an umklapp phonon process. Both these factors weakly break chiral
symmetry and enable the long-term spin relaxation. | cond-mat_other |
Magnetic Phase Transition in FeRh: Density functional calculations are performed to investigate the phase
transition in FeRh alloy. The effective exchange coupling, the critical
temperature of magnetic phase transition and the adiabatic spin wave spectrum
have been obtained. Different contributions to the free energy of different
phases are estimated. It has been found that the antiferro-ferromagnetic
transition in FeRh occurs mostly due to the spin wave excitations. | cond-mat_other |
Self-Adaptive Spike-Time-Dependent Plasticity of Metal-Oxide Memristors: Metal-oxide memristors have emerged as promising candidates for hardware
implementation of artificial synapses - the key components of high-performance,
analog neuromorphic networks - due to their excellent scaling prospects. Since
some advanced cognitive tasks require spiking neuromorphic networks, which
explicitly model individual neural pulses (spikes) in biological neural
systems, it is crucial for memristive synapses to support the
spike-time-dependent plasticity (STDP), which is believed to be the primary
mechanism of Hebbian adaptation. A major challenge for the STDP implementation
is that, in contrast to some simplistic models of the plasticity, the
elementary change of a synaptic weight in an artificial hardware synapse
depends not only on the pre-synaptic and post-synaptic signals, but also on the
initial weight (memristor's conductance) value. Here we experimentally
demonstrate, for the first time, STDP protocols that ensure self-adaptation of
the average memristor conductance, making the plasticity stable, i.e.
insensitive to the initial state of the devices. The experiments have been
carried out with 200-nm Al2O3/TiO2-x memristors integrated into 12x12
crossbars. The experimentally observed self-adaptive STDP behavior has been
complemented with numerical modeling of weight dynamics in a simple system with
a leaky-integrate-and-fire neuron with a random spike-train input, using a
compact model of memristor plasticity, fitted for quantitatively correct
description of our memristors. | cond-mat_other |
Fröhlich Polarons. Lecture course including detailed theoretical
derivations -- 10th edition: In the present course, an overview is presented of the fundamentals of
continuum-polaron physics, which provide the basis of the analysis of polaron
effects in ionic crystals and polar semiconductors. These Lecture Notes deal
with "large", or "continuum", polarons, as described by the Fr\"ohlich
Hamiltonian. The emphasis is on the polaron optical absorption, with detailed
mathematical derivations. | cond-mat_other |
Self-sustained Levitation of Dust Aggregate Ensembles by Temperature
Gradient Induced Overpressures: In laboratory experiments we observe dust aggregates from 100 \mu m to 1 cm
in size composed of micrometer sized grains levitating over a hot surface.
Depending on the dust sample aggregates start to levitate at a temperature of
400 K. Levitation of dust aggregates is restricted to a pressure range between
1--40 mbar. The levitating is caused by a Knudsen compressor effect. Based on
thermal transpiration through the dust aggregates the pressure increases
between surface and aggregates. Dust aggregates are typically balanced 100 \mu
m over the surface. On a slightly concave surface individual aggregates are
trapped at the center. Ensembles of aggregates are confined in a 2D plane.
Aggregates are subject to systematic and random translational and rotational
motion. The levitated aggregates are well suited to study photophoretic or
thermophoretic forces on dust aggregates or the mutual interaction between dust
aggregates. | cond-mat_other |
Nonlinear tunneling of BEC in an optical lattice: signatures of quantum
collapse and revival: Quantum theory of the intraband resonant tunneling of a Bose-Einstein
condensate loaded in a twodimensional optical lattice is considered. It is
shown that the phenomena of quantum collapse and revival can be observed in the
fully quantum problem. The mean-field limit of the theory is analyzed using the
WKB approximation for discrete equations, establishing in this way a direct
connection between the two approaches conventionally used in very different
physical contexts. More specifically we show that there exist two different
regimes of tunneling and study dependence of quantum collapse and revival on
the number of condensed atoms. | cond-mat_other |
Dynamic Tsallis entropy for simple model systems: In this paper we consider the dynamic Tsallis entropy and employ it for four
model systems: (i) the motion of Brownian oscillator, (ii) the motion of
Brownian oscillator with noise, (iii) the fluctuation of particle density in
hydrodynamics limit as well as in (iv) ideal gas. We show that the small value
of parameter nonextensivity $0<q<1$ works as non-linear magnifier for small
values of the entropy. The frequency spectra become more sharp and it is
possible to extract useful information in the case of noise. We show that the
ideal gas remains non-Markovian for arbitrary values of $q$. | cond-mat_other |
Edge Transport in 2D Cold Atom Optical Lattices: We theoretically study the observable response of edge currents in two
dimensional cold atom optical lattices. As an example we use Gutzwiller
mean-field theory to relate persistent edge currents surrounding a Mott
insulator in a slowly rotating trapped Bose-Hubbard system to time of flight
measurements. We briefly discuss an application, the detection of Chern number
using edge currents of a topologically ordered optical lattice insulator. | cond-mat_other |
Momentum transfer to small particles by aloof electron beams: The force exerted on nanoparticles and atomic clusters by fast passing
electrons like those employed in transmission electron microscopes are
calculated and integrated over time to yield the momentum transferred from the
electrons to the particles. Numerical results are offered for metallic and
dielectric particles of different sizes (0-500 nm in diameter) as well as for
carbon nanoclusters. Results for both linear and angular momentum transfers are
presented. For the electron beam currents commonly employed in electron
microscopes, the time-averaged forces are shown to be comparable in magnitude
to laser-induced forces in optical tweezers. This opens up the possibility to
study optically-trapped particles inside transmission electron microscopes. | cond-mat_other |
A Model to Study Finite-Size and Magnetic Effects on the Phase
Transition of a Fermion Interacting System: We present a model to study effects from an external magnetic field, chemical
potential, and finite size, on the phase structure of a massive four- and
six-fermion interacting system. These effects are introduced by a method of
compactification of coordinates, a generalization of the standard Matsubara
prescription. Through the compactification of the $z$ coordinate and of
imaginary time, we describe a heated system with the shape of a film of
thickness $L$, at temperature $\beta^{-1}$ undergoing first- or second-order
phase transition. We have found a strong dependence of the temperature
transition on the constants couplings $\lambda$ and $\eta$. Besides magnetic
catalysis and symmetry breaking for both kinds of transition, we have found an
inverse symmetry breaking phenomenon with respect to first-order phase
transition. | cond-mat_other |
Coherent optical control of spin-spin interaction in doped
semiconductors: We provide a theory of laser-induced interaction between spins localized by
impurity centers in a semiconductor host. By solving exactly the problem of two
localized spins interacting with one itinerant exciton, an analytical
expression for the induced spin-spin interaction is given as a function of the
spin separation, laser energy, and intensity. We apply the theory to shallow
neutral donors (Si) and deep rare-earth magnetic impurities (Yb) in III-V
semiconductors. When the photon energy approaches a resonance related to
excitons bound to the impurities, the coupling between the localized spins
increases, and may change from ferromagnetic to anti-ferromagnetic. This
light-controlled spin interaction provides a mechanism for the quantum control
of spins in semiconductors for quantum information processing; it suggests the
realization of spin systems whose magnetic properties can be controlled by
changing the strength and the sign of the spin-spin interaction. | cond-mat_other |
Predicted mobility edges in one-dimensional incommensurate optical
lattices: An exactly solvable model of Anderson localization: Localization properties of non-interacting quantum particles in
one-dimensional incommensurate lattices are investigated with an exponential
short-range hopping that is beyond the minimal nearest-neighbor tight-binding
model. Energy dependent mobility edges are analytically predicted in this model
and verified with numerical calculations. The results are then mapped to the
continuum Schrodinger equation, and an approximate analytical expression for
the localization phase diagram and the energy dependent mobility edges in the
ground band obtained. | cond-mat_other |
Phase diagram of a cold polarized Fermi gas: We propose a phase diagram for a cold polarized atomic Fermi gas with
zero-range interaction. We identify four main phases in the plane of density
and polarization: the superfluid phase, the normal phase, the gapless
superfluid phase, and the modulated phase. We argue that there exists a
Lifshitz point at the junction of the normal, the gapless superfluid, and the
modulated phases, and a splitting point where the superfluid, the gapless
superfluid, and the modulated phases meet. We show that the physics near the
splitting point is universal and derive an effective field theory describing
it. We also show that subregions with one and two Fermi surfaces exist within
the normal and the gapless superfluid phases. | cond-mat_other |
The atomic Bose gas in Flatland: We describe a recent experiment performed with rubidium atoms ($^{87}$Rb),
aiming at studying the coherence properties of a two-dimensional gas of bosonic
particles at low temperature. We have observed in particular a
Berezinskii--Kosterlitz--Thouless (BKT) type crossover in the system, using a
matter wave heterodyning technique. At low temperatures, the gas is
quasi-coherent on the length scale set by the system size. As the temperature
is increased, the loss of long-range coherence coincides with the onset of the
proliferation of free vortices, in agreement with the microscopic BKT theory. | cond-mat_other |
Quadrupole Excitations in hysteresis loops of magnetic NanoCluster
$Fe_8$: Experiments show several steps in hysteresis loops of high spin nanocluster
$Fe_8$. It is thought that these steps are due to thermally assisted resonant
tunneling between different quanta spin states. Up to now, in calculatiing it,
only dipole excitations were considered. Because of the symmetry and the power
of spin operators in Hamiltonian, we think that other multipole excitation must
be considered too. In this paper we consider both dipole and quadrupole
excitations in Hamiltonian and then quantum resonant tunneling phenomena are
obtained numerically. As we can show in these figures, this phenomenon is
different in these two conditions and the second condition is nearly to the
fact. | cond-mat_other |
Logic Gates with Bright Dissipative Polariton Solitons in Bragg-Cavity
Systems: Optical solitons are an ideal platform for the implementation of
communication lines, since they can be packed extremely close one to another
without risking partial loss of the encoded information due to their
interaction. On the other hand, soliton-soliton interactions are needed to
implement computations and achieve all-optical information processing. Here we
study how bright dissipative polariton solitons interact and exploit their
interaction to implement AND and OR gates with state of the art technology.
Moreover, we show that soliton-soliton interaction can be used to determine the
sign of {\alpha}2, the parameter describing the interaction between polaritons
with opposite spin. | cond-mat_other |
Scaling and Universality of the Complexity of Analog Computation: We apply a probabilistic approach to study the computational complexity of
analog computers which solve linear programming problems. We analyze
numerically various ensembles of linear programming problems and obtain, for
each of these ensembles, the probability distribution functions of certain
quantities which measure the computational complexity, known as the convergence
rate, the barrier and the computation time. We find that in the limit of very
large problems these probability distributions are universal scaling functions.
In other words, the probability distribution function for each of these three
quantities becomes, in the limit of large problem size, a function of a single
scaling variable, which is a certain composition of the quantity in question
and the size of the system. Moreover, various ensembles studied seem to lead
essentially to the same scaling functions, which depend only on the variance of
the ensemble. These results extend analytical and numerical results obtained
recently for the Gaussian ensemble, and support the conjecture that these
scaling functions are universal. | cond-mat_other |
Probing Fermionic Condensates by Fast-Sweep Projection onto Feshbach
Molecules: Fast sweep projection onto Feshbach molecules has been widely used as a probe
of fermionic condensates. By determining the exact dynamics of a pair of atoms
in time varying magnetic fields, we calculate the number of condensed and
noncondensed molecules created after fast magnetic field sweeps from the BCS to
the BEC side of the resonances in $^{40}$K and $^{6}$Li, for different sweep
rates and a range of initial and final fields. We discuss the relation between
the initial fermionic condensate fraction and the molecular condensate fraction
measured after the sweep. | cond-mat_other |
Investigating interaction-induced chaos using time-dependent density
functional theory: Systems whose underlying classical dynamics are chaotic exhibit signatures of
the chaos in their quantum mechanics. We investigate the possibility of using
time-dependent density functional theory (TDDFT) to study the case when chaos
is induced by electron-interaction alone. Nearest-neighbour level-spacing
statistics are in principle exactly and directly accessible from TDDFT. We
discuss how the TDDFT linear response procedure can reveal the mechanism of
chaos induced by electron-interaction alone. A simple model of a two-electron
quantum dot highlights the necessity to go beyond the adiabatic approximation
in TDDFT. | cond-mat_other |
Acoustic attenuation rate in the Fermi-Bose model with a finite-range
fermion-fermion interaction: We study the acoustic attenuation rate in the Fermi-Bose model describing a
mixtures of bosonic and fermionic atom gases. We demonstrate the dramatic
change of the acoustic attenuation rate as the fermionic component is evolved
through the BEC-BCS crossover, in the context of a mean-field model applied to
a finite-range fermion-fermion interaction at zero temperature, such as
discussed previously by M.M. Parish et al. [Phys. Rev. B 71, 064513 (2005)] and
B. Mihaila et al. [Phys. Rev. Lett. 95, 090402 (2005)]. The shape of the
acoustic attenuation rate as a function of the boson energy represents a
signature for superfluidity in the fermionic component. | cond-mat_other |
Saltation transport on Mars: We present the first calculation of saltation transport and dune formation on
Mars and compare it to real dunes. We find that the rate at which grains are
entrained into saltation on Mars is one order of magnitude higher than on
Earth. With this fundamental novel ingredient, we reproduce the size and
different shapes of Mars dunes, and give an estimate for the wind velocity on
Mars. | cond-mat_other |
Collective Excitations of Harmonically Trapped Ideal Gases: We theoretically study the collective excitations of an ideal gas confined in
an isotropic harmonic trap. We give an exact solution to the Boltzmann-Vlasov
equation; as expected for a single-component system, the associated mode
frequencies are integer multiples of the trapping frequency. We show that the
expressions found by the scaling ansatz method are a special case of our
solution. Our findings, however, are most useful in case the trap contains more
than one phase: we demonstrate how to obtain the oscillation frequencies in
case an interface is present between the ideal gas and a different phase. | cond-mat_other |
Time resolved scattering relaxation mechanisms of microcavity polaritons: We study the polariton relaxation dynamics for different scattering
mechanisms as: Phonon and electron scattering procesess. The relaxation
polariton is obtained at very short times by solving the Boltzman equation.
Instead of the well-known relaxation process by phonons, we show that the
bottleneck effect relaxes to the ground state more efficiently at low pump
power intensity when the electron relaxation process is included. In this way,
we clearly demonstrate that different relaxation times exist, for which any of
these two mechanism is more efficient to relax the polariton population to the
ground state. | cond-mat_other |
Soliton response to transient trap variations: The response of bright and dark solitons to rapid variations in an expulsive
longitudinal trap is investigated. We concentrate on the effect of transient
changes in the trap frequency in the form of temporal delta kicks and the
hyperbolic cotangent functions. Exact expressions are obtained for the soliton
profiles. This is accomplished using the fact that a suitable linear
Schrodinger stationary state solution in time can be effectively combined with
the solutions of non-linear Schrodinger equation, for obtaining solutions of
the Gross-Pitaevskii equation with time dependent scattering length in a
harmonic trap. Interestingly, there is rapid pulse amplification in certain
scenarios. | cond-mat_other |
An extended formalism for preferential attachment in heterogeneous
complex networks: In this paper we present a framework for the extension of the preferential
attachment (PA) model to heterogeneous complex networks. We define a class of
heterogeneous PA models, where node properties are described by fixed states in
an arbitrary metric space, and introduce an affinity function that biases the
attachment probabilities of links. We perform an analytical study of the
stationary degree distributions in heterogeneous PA networks. We show that
their degree densities exhibit a richer scaling behavior than their homogeneous
counterparts, and that the power law scaling in the degree distribution is
robust in presence of heterogeneity. | cond-mat_other |
Momentum distribution and condensate fraction of a Fermi gas in the
BCS-BEC crossover: By using the diffusion Monte Carlo method we calculate the one- and two-body
density matrix of an interacting Fermi gas at T=0 in the BCS-BEC crossover.
Results for the momentum distribution of the atoms, as obtained from the
Fourier transform of the one-body density matrix, are reported as a function of
the interaction strength. Off-diagonal long-range order in the system is
investigated through the asymptotic behavior of the two-body density matrix.
The condensate fraction of fermionic pairs is calculated in the unitary limit
and on both sides of the BCS-BEC crossover. | cond-mat_other |
Interplay of Rotational, Relaxational, and Shear Dynamics in Solid 4He: Using a high-sensitivity torsional oscillator technique, we mapped the
rotational and relaxational dynamics of solid helium-4 throughout the parameter
range of the proposed supersolidity. We found evidence that the same
microscopic excitations controlling the torsional oscillator motions are
generated independently by thermal and mechanical stimulation. Moreover, a
measure for the relaxation times of these excitations diverges smoothly without
any indication for a critical temperature or critical velocity of a supersolid
transition. Finally, we demonstrated that the combined temperature-velocity
dependence of the TO response is indistinguishable from the combined
temperature-strain dependence of the solid's shear modulus. This implies that
the rotational responses of solid helium-4 attributed to supersolidity are
associated with generation of the same microscopic excitations as those
produced by direct shear strain. | cond-mat_other |
Optimization by Quantum Annealing: Lessons from Simple Cases: This paper investigates the basic behavior and performance of simulated
quantum annealing (QA) in comparison with classical annealing (CA). Three
simple one dimensional case study systems are considered, namely a parabolic
well, a double well, and a curved washboard. The time dependent Schr\"odinger
evolution in either real or imaginary time describing QA is contrasted with the
Fokker Planck evolution of CA. The asymptotic decrease of excess energy with
annealing time is studied in each case, and the reasons for differences are
examined and discussed. The Huse-Fisher classical power law of double well CA
is replaced with a different power law in QA. The multi-well washboard problem
studied in CA by Shinomoto and Kabashima and leading classically to a
logarithmic annealing even in the absence of disorder, turns to a power law
behavior when annealed with QA. The crucial role of disorder and localization
is briefly discussed. | cond-mat_other |
Casimir interaction at liquid nitrogen temperature: Comparison between
experiment and theory: We have measured the normalized gradient of the Casimir force between
Au-coated surfaces of the sphere and the plate and equivalent Casimir pressure
between two parallel Au plates at T=77K. These measurements have been performed
by means of dynamic force microscope adapted for operating at low temperatures
in the frequency shift technique. It was shown that the measurement results at
T=77K are in a very good agreement with those at T=300K and with computations
at T=77K using both theoretical approaches to the thermal Casimir force
proposed in the literature. No thermal effect in the Casimir pressure was
observed in the limit of experimental errors with the increase of temperature
from T=77K to T=300K. Taking this into account, we have discussed the possible
role of patch potentials in the comparison between measured and calculated
Casimir pressures. | cond-mat_other |
Density hardening plasticity and mechanical aging of silica glass under
pressure: A Raman spectroscopic study: In addition of a flow, plastic deformation of structural glasses (in
particular amorphous silica) is characterized by a permanent densification.
Raman spectroscopic estimators are shown to give a full account of the plastic
behavior of silica under pressure. While the permanent densification of silica
has been widely discussed in terms of amorphous-amorphous transition, from a
plasticity point of view, the evolution of the residual densification with the
maximum pressure of a pressure cycle can be discussed as a density hardening
phenomenon. In the framework of such a mechanical aging effect, we propose that
the glass structure could be labelled by the maximum pressure experienced by
the glass and that the saturation of densification could be associated with the
densest packing of tetrahedra only linked by their vertices. | cond-mat_other |
Matter-wave solitons in radially periodic potentials: We investigate two-dimensional (2D) states of Bose-Einstein condensates (BEC)
with self-attraction or self-repulsion, trapped in an axially symmetric
optical-lattice potential periodic along the radius. Unlike previously studied
2D models with Bessel lattices, no localized states exist in the linear limit
of the present model, hence all localized states are truly nonlinear ones. We
consider the states trapped in the central potential well, and in remote
circular troughs. In both cases, a new species, in the form of \textit{radial
gap solitons}, are found in the repulsive model (the gap soliton trapped in a
circular trough may additionally support stable dark-soliton pairs). In remote
troughs, stable localized states may assume a ring-like shape, or shrink into
strongly localized solitons. The existence of stable annular states, both
azimuthally uniform and weakly modulated ones, is corroborated by simulations
of the corresponding Gross-Pitaevskii equation. Dynamics of strongly localized
solitons circulating in the troughs is also studied. While the solitons with
sufficiently small velocities are stable, fast solitons gradually decay, due to
the leakage of matter into the adjacent trough under the action of the
centrifugal force. Collisions between solitons are investigated too. Head-on
collisions of in-phase solitons lead to the collapse; $\pi $-out of phase
solitons bounce many times, but eventually merge into a single soliton without
collapsing. The proposed setting may also be realized in terms of spatial
solitons in photonic-crystal fibers with a radial structure. | cond-mat_other |
Dynamical Instabilities in a two-component Bose condensate in a 1d
optical lattice: In this paper we carry out a stability analysis of the Bloch states of a
two-component Bose-Einstein condensate confined to a 1d optical lattice. We
consider two concrete systems: a mixture of two hyperfine states of Rubidium-87
and a mixture of Sodium-23 and Rubidium-87. The former is seen to exhibit
similar phenomena to a single component condensate while the latter also
suffers an instability to phase separation at small Bloch wave vectors. It is
shown that sufficiently deep optical lattices can remove this latter
instability, potentially allowing imiscible cold atoms species to be held in
intimate contact and transported within an experimental system. | cond-mat_other |
Inverse Statistics in the Foreign Exchange Market: We investigate intra-day foreign exchange (FX) time series using the inverse
statistic analysis developed in [1,2]. Specifically, we study the time-averaged
distributions of waiting times needed to obtain a certain increase (decrease)
$\rho$ in the price of an investment. The analysis is performed for the Deutsch
mark (DM) against the $US for the full year of 1998, but similar results are
obtained for the Japanese Yen against the $US. With high statistical
significance, the presence of "resonance peaks" in the waiting time
distributions is established. Such peaks are a consequence of the trading
habits of the markets participants as they are not present in the corresponding
tick (business) waiting time distributions. Furthermore, a new {\em stylized
fact}, is observed for the waiting time distribution in the form of a power law
Pdf. This result is achieved by rescaling of the physical waiting time by the
corresponding tick time thereby partially removing scale dependent features of
the market activity. | cond-mat_other |
Coherent spin mixing dynamics in a spin-1 atomic condensate: We study the coherent off-equilibrium spin mixing inside an atomic
condensate. Using mean field theory and adopting the single spatial mode
approximation (SMA), the condensate spin dynamics is found to be well described
by that of a nonrigid pendulum, and displays a variety of periodic oscillations
in an external magnetic field. Our results illuminate several recent
experimental observations and provide critical insights into the observation of
coherent interaction-driven oscillations in a spin-1 condensate. | cond-mat_other |
Green's function of a dressed particle: We present a new, highly efficient yet accurate approximation for the Green's
functions of dressed particles, using the Holstein polaron as an example.
Instead of summing a subclass of diagrams (e.g. the non-crossed ones, in the
self-consistent Born approximation (SCBA)), we sum all the diagrams, but with
each diagram averaged over its free propagators' momenta. The resulting Green's
function satisfies exactly the first six spectral weight sum rules. All higher
sum rules are satisfied with great accuracy, becoming asymptotically exact for
coupling both much larger and much smaller than the free particle bandwidth.
Possible generalizations to other models are also discussed. | cond-mat_other |
SPT phases for mixed states in one dimension: This study explores the specific structure of 1D SPT phases with $G\times H$
symmetry ($G, H$ are finite non-Abelian groups) and constructs an order
parameter. We demonstrate that this order parameter can comprehensively
describe this specific structure. Utilizing this order parameter, we prove that
when the SPT phase theory of symmetry $G$ is extended to mixed states, if the
density matrix is treated as a pure SPT state with $G\times G$ symmetry, the
Hermiticity and positive semi-definiteness ensure that the SPT phase of the
mixed state is similarly described by $H^2(G)$. Finally, we propose that under
this perspective, the SPT phase of time-reversal symmetry no longer exists in
mixed states. | cond-mat_other |
Dynamical mean-field equations for strongly interacting fermionic atoms
in a potential trap: We derive a set of dynamical mean-field equations for strongly interacting
fermionic atoms in a potential trap across a Feshbach resonance. Our derivation
is based on a variational ansatz, which generalizes the crossover wavefunction
to the inhomogeneous case, and the assumption that the order parameter is
slowly varying over the size of the Cooper pairs. The equations reduce to a
generalized time-dependent Gross-Pitaevskii equation on the BEC side of the
resonance. We discuss an iterative method to solve these mean-field equations,
and present the solution for a harmonic trap as an illustrating example to
self-consistently verify the approximations made in our derivation. | cond-mat_other |
Observation of Self-binding in Monolayer $^3$He: We report clear experimental signatures of the theoretically unexpected
gas-liquid transition in the first three monolayers of $^3$He adsorbed on
graphite. The transition is inferred from the linear density dependence of the
$\gamma$-coefficient of the heat capacity measured in the degenerate region (2
$\le T \le$80 mK) below a critical liquid density ($\rho_{c0}$). Surprisingly,
the measured $\rho_{c0}$ values (0.6$\sim$0.9 nm$^{-2}$) are nearly the same
for all these monolayers in spite of their quite different environments. We
conclude that the ground-state of $^3$He in strict two dimensions is not a
dilute quantum gas but a self-bound quantum liquid with the lowest density ever
found. | cond-mat_other |
Improved modeling of Coulomb effects in nanoscale Schottky-barrier FETs: We employ a novel multi-configurational self-consistent Green's function
approach (MCSCG) for the simulation of nanoscale Schottky-barrier field-effect
transistors. This approach allows to calculate the electronic transport with a
seamless transition from the single-electron regime to room temperature
field-effect transistor operation. The particular improvement of the MCSCG
stems from a division of the channel system into a small subsystem of
resonantly trapped states for which a many-body Fock space becomes feasible and
a strongly coupled rest which can be treated adequately on a conventional
mean-field level. The Fock space description allows for the calculation of
few-electron Coulomb charging effects beyond mean-field.
We compare a conventional Hartree non-equilibrium Green's function
calculation with the results of the MCSCG approach. Using the MCSCG method
Coulomb blockade effects are demonstrated at low temperatures while under
strong nonequilibrium and room temperature conditions the Hartree approximation
is retained. | cond-mat_other |
Production of a degenerate Fermi gas of metastable helium-3 atoms: We give an overview of the experiments at the Laser Centre of the Vrije
Universiteit in Amsterdam on ultracold gases of metastable helium-4 and
helium-3 as well as mixtures of both isotopes. We describe our experimental
setup and discuss our experiments on Bose-Einstein condensation of 4He* (more
than 10 million atoms in a BEC) and sympathetic cooling of 3He* towards Fermi
degeneracy (more than 1 million atoms below the Fermi temperature as well as
realization of a degenerate boson-fermion mixture). We also present our results
on Penning Ionization in ~1 mK clouds containing a single isotope (homonuclear
losses) or a mixture of both isotopes (heteronuclear losses). | cond-mat_other |
Long-range donor-acceptor electron transport mediated by alpha-helices: We study the long-range electron and energy transfer mediated by a polaron on
an $\alpha$-helix polypeptide chain coupled to donor and acceptor molecules at
opposite ends of the chain. We show that for specific parameters of the system,
an electron initially located on the donor can tunnel onto the $\alpha$-helix,
forming a polaron which then travels to the other extremity of the polypeptide
chain where it is captured by the acceptor. We consider three families of
couplings between the donor, acceptor and the chain, and show that one of them
can lead to a 90\% efficiency of the electron transport from donor to acceptor.
We also show that this process remains stable at physiological temperatures in
the presence of thermal fluctuations in the system. | cond-mat_other |
Average search time bound in cue based search strategy: In this work we consider the problem of searches that utilises past
information gathered during searching, to evaluate the probability distribution
of finding the source at each step. We start with a sample strategy where the
movement at each step is in the immediate neighbourhood direction, with a
probability proportional to the normalised difference in probability of finding
the source with the present position source finding probability. We evaluate a
lower bound for the average search time for this strategy . We next consider
the problem of the lowerbound on any strategy that utilities information of the
probability distribution evaluated by the searcher at any instant. We derive an
expression for the same. Finally we present an analytic expression for this
lower bound in the case of homogeneous diffusion of particles by a source. For
a general probability distribution with entropy-E, we find that the lower bound
goes as exp(E/2). | cond-mat_other |
Universality in edge-source diffusion dynamics: We show that in edge-source diffusion dynamics the integrated concentration
N(t) has a universal dependence with a characteristic time-scale tau=(A/P)^2
pi/(4D), where D is the diffusion constant while A and P are the
cross-sectional area and perimeter of the domain, respectively. For the
short-time dynamics we find a universal square-root asymptotic dependence
N(t)=N0 sqrt(t/tau) while in the long-time dynamics N(t) saturates
exponentially at N0. The exponential saturation is a general feature while the
associated coefficients are weakly geometry dependent. | cond-mat_other |
Driven optical lattices as strong-field simulators: We argue that ultracold atoms in strongly shaken optical lattices can be
subjected to conditions similar to those experienced by electrons in
laser-irradiated crystalline solids, but without introducing secondary
polarization effects. As a consequence one can induce nonperturbative
multiphoton-like resonances due to the mutual penetration of ac-Stark-shifted
Bloch bands. These phenomena can be detected with a combination of currently
available laboratory techniques. | cond-mat_other |
Infinite average lifetime of an unstable bright state in the green
fluorescent protein: The time evolution of the fluorescence intensity emitted by well-defined
ensembles of Green Fluorescent Proteins has been studied by using a standard
confocal microscope. In contrast with previous results obtained in single
molecule experiments, the photo-bleaching of the ensemble is well described by
a model based on Levy statistics. Moreover, this simple theoretical model
allows us to obtain information about the energy-scales involved in the aging
process. | cond-mat_other |
Finite size Berezinski-Kosterlitz-Thouless transition at grain
boundaries in solid $^4$He and role of $^3$He impurities: We analyze the complex phenomenology of the Non-Classical Rotational Inertia
(NCRI) observed at low temperature in solid $^4$He within the context of a two
dimensional Berezinski-Kosterlitz-Thouless transition in a premelted $^4$He
film at the grain boundaries. We show that both the temperature and $^3$He
doping dependence of the NCRI fraction (NCRIF) can be ascribed to finite size
effects induced by the finite grain size. We give an estimate of the average
size of the grains which we argue to be limited by the isotopic $^3$He
impurities and we provide a simple power-law relation between the NCRIF and the
$^3$He concentration. | cond-mat_other |
Breathing mode for systems of interacting particles: We study the breathing mode in systems of trapped interacting particles. Our
approach, based on a dynamical ansatz in the first equation of the
Bogolyubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy allows us to tackle at
once a wide range of power law interactions and interaction strengths, at
linear and non linear levels. This both puts in a common framework various
results scattered in the literature, and by widely generalizing these,
emphasizes universal characters of this breathing mode. Our findings are
supported by direct numerical simulations. | cond-mat_other |
Ion-Size Effect at the Surface of a Silica Hydrosol: The author used synchrotron x-ray reflectivity to study the ion-size effect
for alkali ions (Na$^+$, K$^+$, Rb$^+$, and Cs$^+$), with densities as high as
$4 \times 10^{18}- 7 \times 10^{18}$ m$^{-2}$, suspended above the surface of a
colloidal solution of silica nanoparticles in the field generated by the
surface electric-double layer. According to the data, large alkali ions
preferentially accumulate at the sol's surface replacing smaller ions, a
finding that qualitatively agrees with the dependence of the Kharkats-Ulstrup
single-ion electrostatic free energy on the ion's radius. | cond-mat_other |
Giant viscosity enhancement in a spin-polarized Fermi liquid: The viscosity is measured for a Fermi liquid, a dilute $^3$He-$^4$He mixture,
under extremely high magnetic field/temperature conditions ($B \leq 14.8$ T, $T
\geq 1.5$ mK). The spin splitting energy $\mu B$ is substantially greater than
the Fermi energy $k_B T_F$; as a consequence the polarization tends to unity
and s-wave quasiparticle scattering is suppressed for $T \ll T_F$. Using a
novel composite vibrating-wire viscometer an enhancement of the viscosity is
observed by a factor of more than 500 over its low-field value. Good agreement
is found between the measured viscosity and theoretical predictions based upon
a $t$-matrix formalism. | cond-mat_other |
High frequency dynamics modulated by collective magnetization reversal
in artificial spin ice: Spin-torque ferromagnetic resonance (ST-FMR) arises in heavy
metal/ferromagnet heterostructures when an alternating charge current is passed
through the bilayer stack. The methodology to detect the resonance is based on
the anisotropic magnetoresistance, which is the change in the electrical
resistance due to different orientations of the magnetization. In connected
networks of ferromagnetic nanowires, known as artificial spin ice, the
magnetoresistance is rather complex owing to the underlying collective behavior
of the geometrically frustrated magnetic domain structure. Here, we demonstrate
ST-FMR investigations in a square artificial spin-ice system and correlate our
observations to magnetotransport measurements. The experimental findings are
described using a simulation approach that highlights the importance of the
correlated dynamics response of the magnetic system. Our results open the
possibility of designing reconfigurable microwave oscillators and
magnetoresistive devices based on connected networks of nanomagnets. | cond-mat_other |
Enhanced low energy fusion rate in palladium (Pd) due to vibrational
deuteron dipole-dipole interactions and associated resonant tunneling that
over-cancels the Jastrow factor between deuteron pair wavefunctions: We show that interstitial hydrogen nucleii on a metallic lattice are strongly
coupled to their near neighbours by the unscreened electromagnetic field
mediating transitions between low-lying states. We then show that in
almost-stoichiometric PdD clusters, in which most interstitial sites are
occupied by a deuteron, certain specific superpositions of many-site product
states exist that are lower in energy than the single-site ground state,
suggesting the existence of a new low temperature phase. The modified behaviour
of the two-particle wavefunction at small separations is investigated and
prelimary results suggesting an over-canceling of the effective Coulomb barrier
are presented. | cond-mat_other |
Band structure and optical properties of opal photonic crystals: A theoretical approach for the interpretation of reflectance spectra of opal
photonic crystals with fcc structure and (111) surface orientation is
presented. It is based on the calculation of photonic bands and density of
states corresponding to a specified angle of incidence in air. The results
yield a clear distinction between diffraction in the direction of light
propagation by (111) family planes (leading to the formation of a stop band)
and diffraction in other directions by higher-order planes (corresponding to
the excitation of photonic modes in the crystal). Reflectance measurements on
artificial opals made of self-assembled polystyrene spheres are analyzed
according to the theoretical scheme and give evidence of diffraction by
higher-order crystalline planes in the photonic structure. | cond-mat_other |
Surface electron band structure and VLEED reflectivity for Al(111): The 2D layer Green function scattering method is used to calculate the energy
of surface states and resonances at Gamma-bar for Al(111) for both below and
above the vacuum level. The surface barrier potential is represented by an
empirical form. The above vacuum level surface electron band structure for this
surface has not been calculated before and it is important in understanding
many surface phenomena. The geometric structure of the Al(111) surface is known
from intensity analysis in low-energy electron diffraction at energies 60 --
450 eV. The details of the surface barrier for Al(111) were obtained from a
match with the below vacuum level experimental energy position of the first
Rydberg surface resonance and the Shockley surface state at k_|| = 0
(Gamma-bar). The calculation was then extended to the above vacuum level case
for 0 -- 27 eV with the inclusion of inelastic electron interactions. Tamm-type
resonances at 6.9 eV and possibly also at 8.3 eV, a Shockley-type resonance at
14.0\pm0.5 eV and a series of Rydberg (image) resonances near 24 eV all above
vacuum level are found at k_|| = 0. The same 2D layer Green function scattering
method using the same input data was then used to calculate the intensity of
the 00 beam for k_|| = 0 (normal incidence) in very low energy electron
diffraction (VLEED) from this surface in the energy range 0 -- 65 eV. Features
in the VLEED intensities are found due to the Shockley and Rydberg resonances.
Experimental data from over 26 years ago found surface features near the
energies found in this work. Beam intensities from low energy electron
microscope (LEEM) measurements at normal incidence and new data from other
surface spectroscopies could provide experimental confirmation of the
resonances predicted in this work. | 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 |
Spontaneous Radiation and Amplification of Kelvin Waves on Quantized
Vortices in Bose-Einstein Condensates: We propose a different type of Landau instability in trapped Bose-Einstein
condensates by a helically moving environment. In the presence of quantized
vortices, the instability can cause spontaneous radiation and amplification of
Kelvin waves. This study gives a microscopic understanding of the
Donnelly-Glaberson instability which was known as a hydrodynamic instability in
superfluid helium.
The Donnelly-Glaberson instability can be a powerful tool for observing the
dispersion relation of Kelvin waves, vortex reconnections, and quantum
turbulence in atomic Bose-Einstein condensates. | cond-mat_other |
Observation of Spin Wave Soliton Fractals in Magnetic Film Active
Feedback Rings: The manifestation of fractals in soliton dynamics has been observed for the
first time. The experiment utilized self-generated spin wave envelope solitons
in a magnetic film based active feedback ring. At high ring gain, the soliton
that circulates in the ring breathes in a fractal pattern. The corresponding
power frequency spectrum shows a comb structure, with each peak in the comb
having its own comb, and so on, to finer and finer scales. | cond-mat_other |
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