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Electromagnetically induced transparency in an atom-molecule
Bose-Einstein condensate: We propose a new measurement scheme for the atom-molecule dark state by using
electromagnetically induced transparency (EIT) technique. Based on a
density-matrix formalism, we calculate the absorption coefficient numerically.
The appearance of the EIT dip in the spectra profile gives clear evidence for
the creation of the dark state in the atom-molecule Bose-Einstein condensate. | cond-mat_other |
A Complex Chemical Potential: Signature of Decay in a Bose-Einstein
Condensate: We explore the zero-temperature statics of an atomic Bose-Einstein condensate
in which a Feshbach resonance creates a coupling to a second condensate
component of quasi-bound molecules. Using a variational procedure to find the
equation of state, the appearance of this binding is manifest in a collapsing
ground state, where only the molecular condensate is present up to some
critical density. Further, an excited state is seen to reproduce the usual
low-density atomic condensate behavior in this system, but the molecular
component is found to produce an underlying decay, quantified by the imaginary
part of the chemical potential. Most importantly, the unique decay rate
dependencies on density ($\sim \rho ^{3/2}$) and on scattering length ($\sim
a^{5/2}$) can be measured in experimental tests of this theory. | cond-mat_other |
Analysis of patterns formed by two-component diffusion limited
aggregation: We consider diffusion limited aggregation of particles of two different
kinds. It is assumed that a particle of one kind may adhere only to another
particle of the same kind. The particles aggregate on a linear substrate which
consists of periodically or randomly placed particles of different kinds. We
analyze the influence of initial patterns on the structure of growing clusters.
It is shown that at small distances from the substrate, the cluster structures
repeat initial patterns. However, starting from a critical distance the initial
periodicity is abruptly lost, and the particle distribution tends to a random
one. An approach describing the evolution of the number of branches is
proposed. Our calculations show that the initial patter can be detected only at
the distance which is not larger than approximately one and a half of the
characteristic pattern size. | cond-mat_other |
Quantization scheme of surface plasma polariton in helical liquid and
the exchanging interaction between quasi particles and emitters: The collective modes of helical electron gases interacting with light have
been studied in an extended random phase approximation. By separating two kinds
of electron density oscillations, the complicate operator dynamics coupling
electrons and photons can be simplified and solved. The inverse operator
transformation that interprets electron oscillations and photons with quasi
particles has been developed to study the interaction between surface plasma
polaritons (SPPs) and emitters. Besides the ordinary interaction induced by
electric field, we find an additional term which plays important roles at small
distance arising from electron exchanging effect. | cond-mat_other |
Evidence that rotons in helium II are interstitial atoms: Superfluid helium II contains excitations known as rotons. Their properties
have been studied experimentally for more than 70 years but their structure is
not fully understood. Feynman's 1954 description, involving rotating flow
patterns, does not fully explain later experimental data. Here we identify
volumetric, thermodynamic, colloidal, excitation, x-ray and neutron scattering
evidence that rotons are composed of interstitial helium atoms. We show in
particular that they have the same mass, effective mass and activation energy
within experimental accuracy. They readily move through the substrate, and
couple through lattice vibrations to produce quantized, loss-free flow which
corresponds to the observed superflow. Our observations revive London's 1936
conclusion that helium II has a relatively open crystal-like lattice with
enough free volume for atoms to move relative to one another, and reconcile it
with London's 1938 description of a quantum fluid. | cond-mat_other |
Combining high pressure and coherent diffraction: a first feasibility
test: We present a first experiment combining high pression and coherent X-ray
diffraction. By using a dedicated diamond anvil cell, we show that the degree
of coherence of the X-ray beam is preserved when the X-ray beam passes through
the diamond cell. This observation opens the possibility of studying the
dynamics of slow fluctuations under high pressure. | cond-mat_other |
Crossover in Broad Feshbach Resonance with Energy-Dependent Coupling: This paper has been withdrawn by the authors as the recent measurement of the
closed channel population for Li6 [Partridge et al., cond-mat/0505353]
indicates that the cutoff energy is still much larger than any other relevant
energy scales for this broad Feshbach resonance. | cond-mat_other |
Bipartite Yule Processes in Collections of Journal Papers: Collections of journal papers, often referred to as 'citation networks', can
be modeled as a collection of coupled bipartite networks which tend to exhibit
linear growth and preferential attachment as papers are added to the
collection. Assuming primary nodes in the first partition and secondary nodes
in the second partition, the basic bipartite Yule process assumes that as each
primary node is added to the network, it links to multiple secondary nodes, and
with probability, $\alpha$, each new link may connect to a newly appearing
secondary node. The number of links from a new primary node follows some
distribution that is a characteristic of the specific network. Links to
existing secondary nodes follow a preferential attachment rule. With
modifications to adapt to specific networks, bipartite Yule processes simulate
networks that can be validated against actual networks using a wide variety of
network metrics. The application of bipartite Yule processes to the simulation
of paper-reference networks and paper-author networks is demonstrated and
simulation results are shown to mimic networks from actual collections of
papers across several network metrics. | cond-mat_other |
Structural Study of Adsorbed Helium Films: New Approach with Synchrotron
Radiation X-rays: A few atomic layers of helium adsorbed on graphite have been attracting much
attention as one of the ideal quantum systems in two dimension. Although
previous reports on neutron diffraction have shown fundamental structural
information in these systems, there still remain many open questions. Here, we
propose surface crystal truncation rod (CTR) scatterings using synchrotron
radiation X-rays as a promising method to reveal surface and interface
structures of helium films on graphite at temperatures below 2 K, based on the
preliminary experimental results on a monolayer of He-4 on a thin graphite. Our
estimation on heat generation by X-ray irradiations also suggests that CTR
scatterings are applicable to even at system temperatures near 100 mK. | cond-mat_other |
Kolmogorov spectrum of superfluid turbulence: numerical analysis of the
Gross-Pitaevskii equation with the small scale dissipation: The energy spectrum of superfluid turbulence is studied numerically by
solving the Gross-Pitaevskii equation. We introduce the dissipation term which
works only in the scale smaller than the healing length, to remove short
wavelength excitations which may hinder the cascade process of quantized
vortices in the inertial range. The obtained energy spectrum is consistent with
the Kolmogorov law. | cond-mat_other |
Spectroscopy, upconversion dynamics, and applications of Er3+-doped
low-phonon materials: In this work I summarize some of the recent work carried out by our group on
the upconversion dynamics of Er3+-doped potassium lead halide crystals, which
possess very small phonons and present very efficient blue and green
upconversion. Furthermore, a non-conventional application of these RE-doped
low-phonon materials in optical refrigeration of luminescent solids is also
discussed, paying especial attention to new pathways for optical cooling that
include infrared-to-visible upconversion. Finally, I conclude with some hints
of what I think it is the next step into improving the luminescence efficiency
of solids: the use of RE-doped nanoscale photonic heterostructures for
controlling the density of photonic states. | cond-mat_other |
Digital Processing in Tunneling Spectroscopy: An alternative approach to detect very weak singularities on the
characteristics of a tunnel diode is proposed in which the numerical
differential filtering is applied directly to measured current versus voltage
dependence instead of the modulation technique commonly used with this purpose.
The gains and looses of the both approaches in the particular case of tunneling
investigations of semiconductors under pressure are discussed. The
corresponding circuitry and mathematical routines are presented. | cond-mat_other |
Phonon-Induced Quantum Magnetic Deflagration in Mn12: A comprehensive set of experiments on the effect of high-frequency surface
acoustic waves, SAWs, in the spin relaxation in Mn12-acetate is presented. We
have studied the quantum magnetic deflagration induced by SAWs under various
experimental conditions extending the data shown in a very recent paper [A.
Hernandez-Minguez et. al., Phys. Rev. Lett. 95, 217205 (2005)]. We have focused
our study on the dependence of both the ignition time and the propagation speed
of the magnetic avalanches on the frequency, amplitude, and duration of the SAW
pulses in experiments performed under different temperatures and external
magnetic fields. | cond-mat_other |
A strongly interacting Bose gas: Nozières and Schmitt-Rink theory and
beyond: We calculate the critical temperature for Bose-Einstein condensation in a gas
of bosonic atoms across a Feshbach resonance, and show how medium effects at
negative scattering lengths give rise to pairs reminiscent of the ones
responsible for fermionic superfluidity. We find that the formation of pairs
leads to a large suppression of the critical temperature. Within the formalism
developed by Nozieres and Schmitt-Rink the gas appears mechanically stable
throughout the entire crossover region, but when interactions between pairs are
taken into account we show that the gas becomes unstable close to the critical
temperature. We discuss prospects of observing these effects in a gas of
ultracold Cs133 atoms where recent measurements indicate that the gas may be
sufficiently long-lived to explore the many-body physics around a Feshbach
resonance. | cond-mat_other |
Distortion of the Stoner-Wohlfarth astroid by a spin-polarized current: The Stoner-Wohlfarth astroid is a fundamental object in magnetism. It
separates regions of the magnetic field space with two stable magnetization
equilibria from those with only one stable equilibrium and it characterizes the
magnetization reversal of nano-magnets induced by applied magnetic fields. On
the other hand, it was recently demonstrated that transfer of spin angular
momentum from a spin-polarized current provides an alternative way of switching
the magnetization. Here, we examine the astroid of a nano-magnet with uniaxial
magnetic anisotropy under the combined influence of applied fields and
spin-transfer torques. We find that spin-transfer is most efficient at
modifying the astroid when the external field is applied along the easy-axis of
magnetization. On departing from this situation, a threshold current appears
below which spin-transfer becomes ineffective yielding a current-induced dip in
the astroid along the easy-axis direction. An extension of the Stoner-Wohlfarth
model is outlined which accounts for this phenomenon. | cond-mat_other |
Phase Space Wannier Functions in Electronic Structure Calculations: We consider the applicability of phase space Wannier functions" to electronic
structure calculations. These generalized Wannier functions are analogous to
localized plane waves and constitute a complete, orthonormal set which is
exponentially localized both in position and momentum space. Their properties
are described and an illustrative application to bound states in one dimension
is presented. Criteria for choosing basis set size and lattice constant are
discussed based on semi-classical, phase space considerations. Convergence of
the ground state energy with respect to basis size is evaluated. Comparison
with plane-waves basis sets indicates that the number of phase space Wannier
functions needed for convergence can be signicantly smaller in three
dimensions. PACS: 71.10.+x, 71.50.+t | cond-mat_other |
Adiabatic quenches through an extended quantum critical region: By gradually changing the degree of the anisotropy in a XXZ chain we study
the defect formation in a quantum system that crosses an extended critical
region. We discuss two qualitatively different cases of quenches, from the
antiferromagnetic to the ferromagnetic phase and from the critical to the
antiferromegnetic phase. By means of time-dependent DMRG simulations, we
calculate the residual energy at the end of the quench as a characteristic
quantity gauging the loss of adiabaticity. We find the dynamical scalings of
the residual energy for both types of quenches, and compare them with the
predictions of the Kibble-Zurek and Landau-Zener theories. | cond-mat_other |
Barrier crossing to the small Holstein polaron regime: We investigate the dimensionality effects of the Holstein polaron from the
fully quantum regime, where the crossover between large and small polaron
solutions is known to be continuous in all dimensions, into the limit described
by the semiclassical Discrete Nonlinear Schr\"odinger (DNLS) Equation, where
the crossover is continuous in 1D but discontinuous in higher dimensions. We
use exact numerics on one hand and a two variable parametrization of the
Toyozawa ansatz on the other in order to probe the crossover region in all
parameter regimes. We find that a barrier appears also in 1D separating the two
types of solutions, seemingly in contradiction to the common paradigm for the
DNLS according to which the crossover is barrier-free. We quantify the polaron
behavior in the crossover region as a function of the exciton overlap and find
that the barrier remains small in 1D and tunnelling through it is not
rate-limiting. | cond-mat_other |
Simulating hyperbolic space on a circuit board: The Laplace operator encodes the behavior of physical systems at vastly
different scales, describing heat flow, fluids, as well as electric,
gravitational, and quantum fields. A key input for the Laplace equation is the
curvature of space. Here we discuss and experimentally demonstrate that the
spectral ordering of Laplacian eigenstates for hyperbolic (negatively curved)
and flat two-dimensional spaces has a universally different structure. We use a
lattice regularization of hyperbolic space in an electric-circuit network to
measure the eigenstates of a "hyperbolic drum", and in a time-resolved
experiment we verify signal propagation along the curved geodesics. Our
experiments showcase both a versatile platform to emulate hyperbolic lattices
in tabletop experiments, and a set of methods to verify the effective
hyperbolic metric in this and other platforms. The presented techniques can be
utilized to explore novel aspects of both classical and quantum dynamics in
negatively curved spaces, and to realise the emerging models of topological
hyperbolic matter. | cond-mat_other |
Quantum Monte-Carlo study of a two-species boson Hubbard model: We consider a two-species hard-core boson Hubbard model for a supersolid,
where the two types of bosons represent vacancies and interstitials doped into
a commensurate crystal. The on-site inter-species interaction may create bound
states of vacancies and interstitials facilitating vacancy condensation at
lower energies than in a single-species model, as suggested in an earlier mean
field study. Here we carry out quantum Monte Carlo simulation to study possible
supersolid phases of the model, corresponding to superfluid phases of the
vacancies or interstitials. At low temperatures, we find three distinct
superfluid phases. The extent of the phases and the nature of the phase
transitions are discussed in comparison to mean-field theory. | cond-mat_other |
From a nonlinear string to a weakly interacting Bose gas: We investigate a real scalar field whose dynamics is governed by a nonlinear
wave equation. We show that classical description can be applied to a quantum
system of many interacting bosons provided that some quantum ingredients are
included. An universal action has to be introduced in order to define particle
number. The value of this action should be equal to the Planck constant. This
constrain can be imposed by removing high frequency modes from the dynamics by
introducing a cut-off. We show that the position of the cut-off has to be
carefully adjusted. Finally, we show the proper choice of the cut-off ensures
that all low frequency eigenenmodes which are taken into account are
macroscopically occupied. | cond-mat_other |
Magnetic levitation induced by negative permeability: In this paper we study the interaction between a point magnetic dipole and a
semi-infinite metamaterial using the method of images. We obtain analytical
expressions for the levitation force for an arbitrarily oriented dipole.
Surprisingly the maximal levitation force for negative permeability is found to
be stronger compared to the case when the dipole is above a superconductor. | cond-mat_other |
Ground state of two electrons on concentric spheres: We extend our analysis of two electrons on a sphere [Phys. Rev. A {\bf 79},
062517 (2009); Phys. Rev. Lett. {\bf 103}, 123008 (2009)] to electrons on
concentric spheres with different radii. The strengths and weaknesses of
several electronic structure models are analyzed, ranging from the mean-field
approximation (restricted and unrestricted Hartree-Fock solutions) to
configuration interaction expansion, leading to near-exact wave functions and
energies. The M{\o}ller-Plesset energy corrections (up to third-order) and the
asymptotic expansion for the large-spheres regime are also considered. We also
study the position intracules derived from approximate and exact wave
functions. We find evidence for the existence of a long-range Coulomb hole in
the large-spheres regime, and infer that unrestricted Hartree-Fock theory
over-localizes the electrons. | cond-mat_other |
Magnetization and specific heat of TbFe3(BO3)4: Experiment and crystal
field calculations: We have studied the thermodynamic properties of single-crystalline
TbFe3(BO3)4. Magnetization measurements have been carried out as a function of
magnetic field (up to 50 T) and temperature up to 350K with the magnetic field
both parallel and perpendicular to the trigonal c-axis of the crystal. The
specific heat has been measured in the temperature range 2-300K with a magnetic
field up to 9 T applied parallel to the c-axis. The data indicate a structural
phase transition at 192 K and antiferromagnetic spin ordering at 40 K. A
Schottky anomaly is present in the specific heat data around 20 K, arising due
to two low-lying energy levels of the Tb3+ ions being split by f-d coupling.
Below TN magnetic fields parallel to the c-axis drive a spin-flop phase
transition, which is associated with a large magnetization jump. The highly
anisotropic character of the magnetic susceptibility is ascribed mainly to the
Ising-like behavior of the Tb3+ ions in the trigonal crystal field. We describe
our results in the framework of an unified approach which is based on
mean-field approximation and crystal-field calculations. | cond-mat_other |
Correlation effects on the static structure factor of a Bose gas: A theoretical treatment of the static structure factor $S(k)$ of a Bose gas
is attempted. The low order expansion theory is implemented for the
construction of the two body density distribution, while various trial
functions for the radial distribution function $g(r)$ are used. $g(r)$
introduces the atomic correlations and describes the departure from the
noninteracting gas. The Bose gas is studied as inhomogeneous one, trapped in
harmonic oscillator well, as well as homogeneous. A suitable parametrization of
the various trial functions $g(r)$ exists which leads to satisfactory
reproduction of the experimental values of $S(k)$, both in inhomogeneous case
as well as in homogeneous one. The phonon range behavior of the calculated
$S(k)$ is also addressed and discussed both in finite and infinite Bose gas. | cond-mat_other |
Study of the Magnetic Film Materials by Horizontal Scanning Mode for the
Magnetic Force Microscopy in Magnetostatic and ac Regimes: The magnetic force microscopy inverse problem for the case of horizontal
scanning of a tip on a linear magnetic film is introduced. We show the
possibility to recover the magnetic permeability of the material from the
experimental data by using the Hankel (Fourier-Bessel) transform inverse method
(HIM). This method is applied to the case of a layered slab film as well. The
inverse problem related to the ac MFM is introduced. | cond-mat_other |
Exciton BCS or BEC state in a semiconductor bilayer system?: We calculate the off-diagonal long range order (ODLRO) terms of the
exciton--exciton correlation function of a semiconductor bilayer system with
Coulomb interaction and a transverse magnetic field. We show that the formation
of a BEC state is very sensitive to the width of the interaction in momentum
space. This dependence is analytically derived and represents the key physical
ingredient for the formation (or not) of an exciton condensate state. | cond-mat_other |
Photoluminescence Spectroscopy of the Molecular Biexciton in Vertically
Stacked Quantum Dot Pairs: We present photoluminescence studies of the molecular neutral
biexciton-exciton spectra of individual vertically stacked InAs/GaAs quantum
dot pairs. We tune either the hole or the electron levels of the two dots into
tunneling resonances. The spectra are described well within a few-level,
few-particle molecular model. Their properties can be modified broadly by an
electric field and by structural design, which makes them highly attractive for
controlling nonlinear optical properties. | cond-mat_other |
Conversion Efficiencies of Heteronuclear Feshbach Molecules: We study the conversion efficiency of heteronuclear Feshbach molecules in
population imbalanced atomic gases formed by ramping the magnetic field
adiabatically. We extend the recent work [J. E. Williams et al., New J. Phys.,
8, 150 (2006)] on the theory of Feshbach molecule formations to various
combinations of quantum statistics of each atomic component. A simple
calculation for a harmonically trapped ideal gas is in good agreement with the
recent experiment [S. B. Papp and C. E. Wieman, Phys. Rev. Lett., 97, 180404
(2006)] without any fitting parameters. We also give the conversion efficiency
as an explicit function of initial peak phase space density of the majority
species for population imbalanced gases. In the low-density region where
Bose-Einstein condensation does not appear, the conversion efficiency is a
monotonic function of the initial peak phase space density, but independent of
statistics of a minority component. The quantum statistics of majority atoms
has a significant effect on the conversion efficiency. In addition,
Bose-Einstein condensation of an atomic component is the key element
determining the maximum conversion efficiency. | cond-mat_other |
Frustration of Decoherence in Open Quantum Systems: We study a model of frustration of decoherence in an open quantum system.
Contrary to other dissipative ohmic impurity models, such as the Kondo model or
the dissipative two-level system, the impurity model discussed here never
presents overdamped dynamics even for strong coupling to the environment. We
show that this unusual effect has its origins in the quantum mechanical nature
of the coupling between the quantum impurity and the environment. We study the
problem using analytic and numerical renormalization group methods and obtain
expressions for the frequency and temperature dependence of the impurity
susceptibility in different regimes. | cond-mat_other |
Floquet system, Bloch oscillation, and Stark ladder: We prove the multi-band Bloch oscillation and Stark ladder in the $nk$ and
site representation from the Floquet theorem. The proof is also possible from
the equivalence between the Floquet system, Bloch oscillation, and the rotator
with spin. We also exactly solve the periodically driven two level atom and two
band Bloch oscillation in terms of Heun function. | cond-mat_other |
Precise determination of $^6$Li cold collision parameters by
radio-frequency spectroscopy on weakly bound molecules: We employ radio-frequency spectroscopy on weakly bound $^6$Li$_2$ molecules
to precisely determine the molecular binding energies and the energy splittings
between molecular states for different magnetic fields. These measurements
allow us to extract the interaction parameters of ultracold $^6$Li atoms based
on a multi-channel quantum scattering model. We determine the singlet and
triplet scattering lengths to be $a_s=45.167(8)a_0$ and $a_t=-2140(18)a_0$ (1
$a_0$ = 0.0529177 nm), and the positions of the broad Feshbach resonances in
the energetically lowest three $s-$wave scattering channels to be 83.41(15) mT,
69.04(5) mT, and 81.12(10) mT. | cond-mat_other |
Spin dynamics triggered by sub-terahertz magnetic field pulses: Current pulses of up to 20 A and as short as 3 ps are generated by a low
temperature grown GaAs (lt-GaAs) photoconductive switch and guided through a
coplanar waveguide, resulting in a 0.6 Tesla terahertz (THz) magnetic field
pulse. The pulse length is directly calibrated using photocurrent
autocorrelation. Magnetic excitations in Fe microstructures are studied by
time-resolved Kerr spectroscopy and compared with micromagnetic simulations. A
response within less than 10 ps to the THz electromagnetic field pulse is
found. | 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 |
The Ginzburg-Landau model of Bose-Einstein condensation of magnons: We introduce a system of phenomenological equations for Bose-Einstein
condensates of magnons in the one-dimensional setting. The nonlinearly coupled
equations, written for amplitudes of the right-and left-traveling waves,
combine basic features of the Gross-Pitaevskii and complex Ginzburg-Landau
models. They include localized source terms, to represent the microwave
magnon-pumping field. With the source represented by the $\delta $-functions,
we find analytical solutions for symmetric localized states of the magnon
condensates. We also predict the existence of asymmetric states with unequal
amplitudes of the two components. Numerical simulations demonstrate that all
analytically found solutions are stable. With the $\delta $-function terms
replaced by broader sources, the simulations reveal a transition from the
single-peak stationary symmetric states to multi-peak ones, generated by the
modulational instability of extended nonlinear-wave patterns. In the
simulations, symmetric initial conditions always converge to symmetric
stationary patterns. On the other hand, asymmetric inputs may generate
nonstationary asymmetric localized solutions, in the form of traveling or
standing waves. Comparison with experimental results demonstrates that the
phenomenological equations provide for a reasonably good model for the
description of the spatiotemporal dynamics of magnon condensates. | cond-mat_other |
Fermions at unitarity and Haldane Exclusion Statistics: We consider a gas of neutral fermionic atoms at ultra-low temperatures, with
the attractive interaction tuned to Feshbach resonance. We calculate, the
variation of the chemical potential and the energy per particle as a function
of temperature by assuming the system to be an ideal gas obeying the Haldane-Wu
fractional exclusion statistics. Our results for the untrapped gas compare
favourably with the recently published Monte Carlo calculations of two groups.
For a harmonically trapped gas, the results agree with experiment, and also
with other published work. | cond-mat_other |
A Simple Experimental Setup for Simultaneous Superfluid-response and
Heat-capacity Measurements for Helium in Confined Geometries: Torsional oscillator (TO) is an experimental technique which is widely used
to investigate superfluid responses in helium systems confined in porous
materials or adsorbed on substrates. In these systems, heat capacity (HC) is
also an important quantity to study the local thermodynamic properties. We have
developed a simple method to incorporate the capability of HC measurement into
an existing TO without modifying the TO itself. By inserting a rigid thermal
isolation support made of alumina and a weak thermal link made of fine copper
wires between a standard TO and the mixing chamber of a dilution refrigerator
in parallel, we were able to carry out simultaneous TO and HC measurements on
exactly the same helium sample, i.e., four atomic layers of $^4$He adsorbed on
graphite, with good accuracies down to 30 mK. The data reproduced very well the
previous workers' results obtained independently using setups optimized for
individual measurements. This method is conveniently applicable to a variety of
experiments where careful comparisons between results of TO and HC measurements
are crucial. We describe how to design the thermal isolation support and the
weak thermal link to manage conflicting requirements in the two techniques. | cond-mat_other |
Examining electron-boson coupling using time-resolved spectroscopy: Nonequilibrium pump-probe time domain spectroscopies can become an important
tool to disentangle degrees of freedom whose coupling leads to broad structures
in the frequency domain. Here, using the time-resolved solution of a model
photoexcited electron-phonon system we show that the relaxational dynamics are
directly governed by the equilibrium self-energy so that the phonon frequency
sets a window for "slow" versus "fast" recovery. The overall temporal structure
of this relaxation spectroscopy allows for a reliable and quantitative
extraction of the electron-phonon coupling strength without requiring an
effective temperature model or making strong assumptions about the underlying
bare electronic band dispersion. | cond-mat_other |
Scaling law for seismic hazard after a main shock: After a large earthquake, the likelihood of successive strong aftershocks
needs to be estimated. Exploiting similarities with critical phenomena, we
introduce a scaling law for the decay in time following a main shock of the
expected number of aftershocks greater than a certain magnitude. Empirical
results that support our scaling hypothesis are obtained from analyzing the
record of earthquakes in California. The proposed form unifies the well-known
Omori and Gutenberg-Richter laws of seismicity, together with other
phenomenological observations. Our results substantially modify presently
employed estimates and may lead to an improved assessment of seismic hazard
after a large earthquake.} | 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 |
Potential and charge-carrier concentration distributions in solid
electrolyte between flat electrodes: Statistically studied are the equilibrium characteristics of a subsystem of
mobile charges of one sort, taking into account the subsystem of fixed charges
of the opposite sign creating a compensating electric background. The
distribution of these charges under the influence of the external field is
invariable. To represent free energy of the subsystem of mobile charges in the
form of a functional of their density and to calculate cell potentials of the
mean forces by the method of conditional distributions, a cumulant expansion
with respect to the renormalized Mayer functions is used. To take into account
the screening effects, the results of the collective variables method are used.
A system of integral equations for the potentials of mean forces is obtained
that accounts for the effects of near- and long-range interactions. The
calculations are made in the lattice approximation. The correlation component
distinguished in the expression for the binary distribution function makes it
possible to calculate the correlated and uncorrelated parts of the electric
potential using the Poisson equation. In the case of sufficiently small
electric fields, a linear expansion of the chemical potential in terms of
deviation of the charge concentration from the homogeneous distribution is
considered. In final calculations the correlation between particles is taken
into account in the approximation of the first neighbors. In this approximation
the potential and charge concentration distribution is described by a linear
differential equation of the fourth order. The results of its analytical
solution and subsequent numerical calculations for the characteristics of solid
electrolyte are analyzed. | cond-mat_other |
Reptation quantum Monte Carlo for lattice Hamiltonians with a
directed-update scheme: We provide an extension to lattice systems of the reptation quantum Monte
Carlo algorithm, originally devised for continuous Hamiltonians. For systems
affected by the sign problem, a method to systematically improve upon the
so-called fixed-node approximation is also proposed. The generality of the
method, which also takes advantage of a canonical worm algorithm scheme to
measure off-diagonal observables, makes it applicable to a vast variety of
quantum systems and eases the study of their ground-state and excited-states
properties. As a case study, we investigate the quantum dynamics of the
one-dimensional Heisenberg model and we provide accurate estimates of the
ground-state energy of the two-dimensional fermionic Hubbard model. | cond-mat_other |
Proximity Effects in Radiative Transfer: Though the dependence of near-field radiative transfer on the gap between two
planar objects is well understood, that between curved objects is still
unclear. We show, based on the analysis of the surface polariton mediated
radiative transfer between two spheres of equal radii $R$ and minimum gap $d$,
that the near--field radiative transfer scales as $R/d$ as $d/R \rightarrow 0$
and as $\ln(R/d)$ for larger values of $d/R$ up to the far--field limit. We
propose a modified form of the proximity approximation to predict near--field
radiative transfer between curved objects from simulations of radiative
transfer between planar surfaces. | cond-mat_other |
Manifestations of the Efimov Effect for Three Identical Bosons: In this paper we present results from numerical calculations for three
identical boson systems for both very large and infinite two-body s-wave
scattering length $a$. We have considered scattering lengths up to $2\times
10^5$ a.u. and solved the hyperangular part of the Schr\"odinger equation for
distances up to $10^6$ a.u.. Form these, we obtained the three-body effective
potentials, hyperspherical channel functions and the asymptotic behavior of the
nonadiabatic couplings in order to to characterize the main aspects of the
Efimov states. These results allow us to test and quantify the assumptions
related to the Efimov effect. | cond-mat_other |
Breeding and Solitary Wave Behavior of Dunes: Beautiful dune patterns can be found in deserts and along coasts due to the
instability of a plain sheet of sand under the action of the wind. Barchan
dunes are highly mobile aeolian dunes found in areas of low sand availability
and unidirectional wind fields. Up to now modelization mainly focussed on
single dunes or dune patterns without regarding the mechanisms of dune
interactions. We study the case when a small dune bumps into a bigger one.
Recently Schwammle et al. and Katsuki et al. have shown that under certain
circumstances dunes can behave like solitary waves. This means that they can
``cross'' each other which has been questioned by many researchers before. In
other cases we observe coalescence, i.e. both dune merge into one, breeding,
i.e. the creation of three baby dunes at the center and horns of a Barchan, or
budding, i.e. the small dune, after ``crossing'' the big one, is unstable and
splits into two new dunes. | cond-mat_other |
Slit-array transmission loss feasibility in airborne sound: Recent experiments conducted in water at ultrasonic frequencies showed the
possibility of overcoming the transmission loss provided by homogeneous plates
at certain frequencies by drilling periodically distributed holes on it. In
this letter, the feasibility of using slit arrays to increase the transmission
loss at certain frequencies for airborne sound is studied. Numerical results
predict a) very low transmission loss for a slit array in comparison with a
homogeneous plate in air and b) the transmission loss of a slit array can
overcome that of a homogeneous plate if the impedance mismatch is low enough. | cond-mat_other |
Spin-correlation functions in ultracold paired atomic-fermion systems:
sum rules, self-consistent approximations, and mean fields: The spin response functions measured in multi-component fermion gases by
means of rf transitions between hyperfine states are strongly constrained by
the symmetry of the interatomic interactions. Such constraints are reflected in
the spin f-sum rule that the response functions must obey. In particular, only
if the effective interactions are not fully invariant in SU(2) spin space, are
the response functions sensitive to mean field and pairing effects. We
demonstrate, via a self-consistent calculation of the spin-spin correlation
function within the framework of Hartree-Fock-BCS theory, how one can derive a
correlation function explicitly obeying the f-sum rule. By contrast, simple
one-loop approximations to the spin response functions do not satisfy the sum
rule. As we show, the emergence of a second peak at higher frequency in the rf
spectrum, as observed in a recent experiment in trapped $^6\text{Li}$, can be
understood as the contribution from the paired fermions, with a shift of the
peak from the normal particle response proportional to the square of the BCS
pairing gap. | cond-mat_other |
Quasimodes of a chaotic elastic cavity with increasing local losses: We report non-invasive measurements of the complex field of elastic
quasimodes of a silicon wafer with chaotic shape. The amplitude and phase
spatial distribution of the flexural modes are directly obtained by Fourier
transform of time measurements. We investigate the crossover from real mode to
complex-valued quasimode, when absorption is progressively increased on one
edge of the wafer. The complexness parameter, which characterizes the degree to
which a resonance state is complex-valued, is measured for non-overlapping
resonances and is found to be proportional to the non-homogeneous contribution
to the line broadening of the resonance. A simple two-level model based on the
effective Hamiltonian formalism supports our experimental results. | cond-mat_other |
Ultrafast optical Faraday effect in transparent solids: We predict a strong-field ultrafast optical Faraday effect, where a
circularly polarized ultrashort optical pulse induces transient chirality in an
achiral transparent dielectric. This effect is attractive for time-resolved
measurements because it gives access to the non-instantaneity of the nonlinear
medium response, and also because it represents relaxation of time-reversal
symmetry by all-optical means. We propose probing the induced transient
chirality with a weak linearly polarized ultraviolet pulse that is shorter than
the near-infrared pump pulse. The predicted effects are ultrafast: the induced
chirality vanishes for probe delays exceeding the duration of the near-infrared
pulse. This opens up possibilities for applications in ultrafast
circular-polarization modulators and analyzers. | cond-mat_other |
Finite-Temperature Behavior of an Inter-species Fermionic Superfluid
with Population Imbalance: We determine the superfluid transition temperature $T_c$ and related finite
temperature phase diagrams for the entire BCS-Bose Einstein condensation
crossover in a homogeneous mixture of $^{6}$Li and $^{40}$K atoms with
population imbalance. Our work is motivated by the recent observation of an
inter-species Feshbach resonance. Pairing fluctuation effects, which
significantly reduce $T_c$ from the onset temperature for pairing ($T^*$),
provide reasonable estimates of $T_c$ and indicate that the inter-species
superfluid phase should be accessible in future experiments. Although a
generalized-Sarma phase is not stable in the ground state near unitarity, our
phase diagrams show that it appears as an intermediate-temperature superfluid. | cond-mat_other |
Effective single-particle order-N scheme for the dynamics of open
non-interacting many-body systems: Quantum master equations are common tools to describe the dynamics of
many-body systems open to an environment. Due to the interaction with the
latter, even for the case of non-interacting electrons, the computational cost
to solve these equations increases exponentially with the particle number. We
propose a simple scheme, that allows to study the dynamics of $N$
non-interacting electrons taking into account both dissipation effects and
Fermi statistics, with a computational cost that scales linearly with $N$. Our
method is based on a mapping of the many-body system to a specific set of
effective single-particle systems. We provide detailed numerical results
showing excellent agreement between the effective single-particle scheme and
the exact many-body one, as obtained from studying the dynamics of two
different systems. In the first, we study optically-induced currents in quantum
rings at zero temperature, and in the second we study a linear chain coupled at
its ends to two thermal baths with different (finite) temperatures. In
addition, we give an analytical justification for our method, based on an exact
averaging over the many-body states of the original master equations. | cond-mat_other |
Theory of vortex-lattice melting in a one-dimensional optical lattice: We investigate quantum and temperature fluctuations of a vortex lattice in a
one-dimensional optical lattice. We discuss in particular the Bloch bands of
the Tkachenko modes and calculate the correlation function of the vortex
positions along the direction of the optical lattice. Because of the small
number of particles in the pancake Bose-Einstein condensates at every site of
the optical lattice, finite-size effects become very important. Moreover, the
fluctuations in the vortex positions are inhomogeneous due to the inhomogeneous
density. As a result, the melting of the lattice occurs from the outside
inwards. However, tunneling between neighboring pancakes substantially reduces
the inhomogeneity as well as the size of the fluctuations. On the other hand,
nonzero temperatures increase the size of the fluctuations dramatically. We
calculate the crossover temperature from quantum melting to classical melting.
We also investigate melting in the presence of a quartic radial potential,
where a liquid can form in the center instead of at the outer edge of the
pancake Bose-Einstein condensates. | cond-mat_other |
A generalised Landau-Lifshitz equation for isotropic SU(3) magnet: In the paper we obtain equations for large-scale fluctuations of the mean
field (the field of magnetization and quadrupole moments) in a magnetic system
realized by a square (cubic) lattice of atoms with spin s >= 1 at each site. We
use the generalized Heisenberg Hamiltonian with biquadratic exchange as a
quantum model. A quantum thermodynamical averaging gives classical effective
models, which are interpreted as Hamiltonian systems on coadjoint orbits of Lie
group SU(3). | cond-mat_other |
Magneto and ferroelectric phase transitions in HoMn2O5 monocrystals: From the physical point of view multiferroics present an extremely
interesting class of systems and problems. These are essentially of two kinds.
One is what are the microscopic conditions, and sometimes constrains, which
determine the possibility to combine in one system both magnetic and
ferroelectric properties. This turned out to be a quite nontrivial question,
and usually, in conventional systems, these two phenomena tend to exclude one
another. Why it is the case is an important and still not completely resolved
issue. In the present article we report our results from magnetic properties
measurements on HoMn2O5 with short discussion about it possible origin. | cond-mat_other |
Density-density functionals and effective potentials in many-body
electronic structure calculations: We demonstrate the existence of different density-density functionals
designed to retain selected properties of the many-body ground state in a
non-interacting solution starting from the standard density functional theory
ground state. We focus on diffusion quantum Monte Carlo applications that
require trial wave functions with optimal Fermion nodes. The theory is
extensible and can be used to understand current practices in several
electronic structure methods within a generalized density functional framework.
The theory justifies and stimulates the search of optimal empirical density
functionals and effective potentials for accurate calculations of the
properties of real materials, but also cautions on the limits of their
applicability. The concepts are tested and validated with a near-analytic
model. | cond-mat_other |
Coherent Destruction of Photon Emission from a Single Molecule Source: A
Renormalization Group Approach: The photon emission from a single molecule driven simultaneously by a laser
and a slow electric radio frequency (rf) field is studied. We use a
non-Hermitian Hamiltonian approach which accounts for the radiative decay of a
two level system modeling a single molecule source. We apply the
renormalization group method for differential equations to obtain long time
solution of the corresponding Schrdinger equation, which allows us to calculate
the average waiting time for the first photon emission. Then, we analyze the
conditions for suppression and enhancement of photon emission in this
dissipative two-level system. In particular we derive a transcendental
equation, which yields the non-trivial rf field control parameters, for which
enhancement and suppression of photon emission occurs. For finite values of
radiative decay rate an abrupt transition from the molecule's localization in
its ground state to delocalization is found for certain critical values of the
rf field parameters. Our results are shown to be in agreement with the
available experiments [Ch. Brunel et al, Phys. Rev. Lett. 81, 2679 (1998)]. | cond-mat_other |
Coherent Control of Rydberg States in Silicon: We demonstrate coherent control of donor wavefunctions in phosphorous-doped
silicon. Our experiments take advantage of a free electron laser to stimulate
and observe photon echoes from, and Rabi oscillations between the ground and
first excited state of P donors in Si. | cond-mat_other |
Comparison study of DFA and DMA methods in analysis of autocorrelations
in time series: Statistics of the Hurst scaling exponents calculated with the use of two
methods: recently introduced Detrended Moving Average Analysis(DMA) and
Detrended Fluctuation Analysis (DFA)are compared. Analysis is done for
artificial stochastic Brownian time series of various length and reveals
interesting statistical relationships between two methods. Good agreement
between DFA and DMA techniques is found for long time series $L\sim 10^{5}$,
however for shorter series we observe that two methods give different results
with no systematic relation between them. It is shown that, on the average, DMA
method overestimates the Hurst exponent comparing it with DFA technique. | cond-mat_other |
Photoacoustic ultrasound sources from diffusion-limited aggregates: Metallic diffusion-limited aggregate (DLA) films are well-known to exhibit
near-perfect broadband optical absorption. We demonstrate that such films also
manifest a substantial and relatively material-independent photoacoustic
response, as a consequence of their random nanostructure. We theoretically and
experimentally analyze photoacoustic phenomena in DLA films, and show that they
can be used to create broadband air- coupled acoustic sources. These sources
are inexpensive and simple to fabricate, and work into the ultrasonic regime.
We illustrate the device possibilities by building and testing an
optically-addressed acoustic phased array capable of producing virtually
arbitrary acoustic intensity patterns in air. | cond-mat_other |
Anomalous quantum mass flow of atoms in p-wave resonance: I analyze an atomic Fermi gas with a planar p-wave interaction, motivated by
the experimentally observed anisotropy in p-wave Feshbach resonances. An axial
superfluid state is verified. A domain wall object is discovered to be a new
topological defect of this superfluid and an explicit solution has been found.
Gapless quasiparticles appear as bound states on the wall, dispersing in the
space of reduced dimensions. Surprisingly, they are chiral, deeply related to
fermion zero modes and anomalies in quantum chromodynamics. The chirality of
the superfluid is manifested by a persistent anomalous mass current of atoms in
the groundstate. This spectacular quantum phenomenon is a prediction for future
experiments. | cond-mat_other |
Yukawa bosons in two-dimensional Harmonic confinement: The ground state property of Yukawa Bose fluid confined in a radial harmonic
trap is studied. The calculation was carried out using the density functional
theory formalism within the Kohn-Sham scheme. The excess-correlation energy for
this inhomogeneous fluid is approximated via the local density approximation. A
comparison is also made with the Gross-Piteavskii model. We found that the
system of bosons interacting in terms of Yukawa potential in a harmonic trap is
energetically favorable compared to the ones interacting via contact delta
potential. | cond-mat_other |
Inverse Borrmann effect in photonic crystals: The Borrmann effect, which is related to the microscopic distribution of the
electromagnetic field inside the primitive cell, is studied in photonic and
magnetophotonic crystals. This effect, well-known in x-ray spectroscopy, is
responsible for the enhancement or suppression of various linear and nonlinear
optical effects when the incidence angle and/or the frequency change. It is
shown that by design of the primitive cell this effect can be suppressed and
even inverted. | cond-mat_other |
Theory of Magnetodynamics Induced by Spin Torque in Perpendicularly
Magnetized Thin Films: A nonlinear model of spin wave excitation using a point contact in a thin
ferromagnetic film is introduced. Large-amplitude magnetic solitary waves are
computed, which help explain recent spin-torque experiments. Numerical
simulations of the fully nonlinear model predict excitation frequencies in
excess of 0.2 THz for contact diameters smaller than 6 nm. Simulations also
predict a saturation and red shift of the frequency at currents large enough to
invert the magnetization under the point contact. The theory is approximated by
a cubic complex Ginzburg-Landau type equation. The mode's nonlinear frequency
shift is found by use of perturbation techniques, whose results agree with
those of direct numerical simulations. | cond-mat_other |
Magnetic vortices induced by a moving tip: A two-dimensional easy-plane ferromagnetic substrate, interacting with a
dipolar tip which is magnetised perpendicular with respect to the easy plane is
studied numerically by solving the Landau-Lifshitz Gilbert equation. Due to the
symmetry of the dipolar field of the tip, in addition to the collinear
structure a magnetic vortex structure becomes stable. It is robust against
excitations caused by the motion of the tip. We show that for high excitations
the system may perform a transition between the two states. The influence of
domain walls, which may also induce this transition, is examined. | cond-mat_other |
Harmonic oscillator model for current- and field-driven magnetic
vortices: In experiments the distinction between spin-torque and Oersted-field driven
magnetization dynamics is still an open problem. Here, the gyroscopic motion of
current- and field-driven magnetic vortices in small thin-film elements is
investigated by analytical calculations and by numerical simulations. It is
found that for small harmonic excitations the vortex core performs an
elliptical rotation around its equilibrium position. The global phase of the
rotation and the ratio between the semi-axes are determined by the frequency
and the amplitude of the Oersted field and the spin torque. | cond-mat_other |
Simulating hyperbolic space on a circuit board: The Laplace operator encodes the behavior of physical systems at vastly
different scales, describing heat flow, fluids, as well as electric,
gravitational, and quantum fields. A key input for the Laplace equation is the
curvature of space. Here we discuss and experimentally demonstrate that the
spectral ordering of Laplacian eigenstates for hyperbolic (negatively curved)
and flat two-dimensional spaces has a universally different structure. We use a
lattice regularization of hyperbolic space in an electric-circuit network to
measure the eigenstates of a "hyperbolic drum", and in a time-resolved
experiment we verify signal propagation along the curved geodesics. Our
experiments showcase both a versatile platform to emulate hyperbolic lattices
in tabletop experiments, and a set of methods to verify the effective
hyperbolic metric in this and other platforms. The presented techniques can be
utilized to explore novel aspects of both classical and quantum dynamics in
negatively curved spaces, and to realise the emerging models of topological
hyperbolic matter. | cond-mat_other |
Fluctuation spectroscopy of surface melting of ice without, and with
impurities: Water, in its three phases, is ubiquitous, and the surface properties of ice
is important to clarifying the process of melting, as well as to various other
fields, including geophysics. As such, the subject has been studied both
theoretically and experimentally, for over a hundred years, while being an
active field of research today. It has been established that surface melting,
or premelting, exists below the melting point, and a `liquid-like layer' (LLL)
exists on the surface of ice. Here, we use the surface thermal fluctuation
spectra to study the properties of LLL, including its thickness, for pure ice,
and for ice with impurities. We find that the properties of LLL are consistent
with those of bulk liquid water, and for layers thicker than 10\,nm, their
properties are experimentally indistinguishable from those of liquid water.
Measured thicknesses are found to be much smaller than the previous
experimental measurements close to the bulk melting temperature. We find that
the additions of impurities at ppm levels cause LLL to be thicker, as well to
be quite inhomogeneous, with properties depending on the dopant. This is
revealed by scanning the surface at $\mu$m level resolution, and can contribute
to the slipperiness of ice in natural settings. | cond-mat_other |
Intense slow beams of bosonic potassium isotopes: We report on an experimental realization of a two-dimensional magneto-optical
trap (2D-MOT) that allows the generation of cold atomic beams of 39K and 41K
bosonic potassium isotopes. The high measured fluxes up to 1.0x10^11 atoms/s
and low atomic velocities around 33 m/s are well suited for a fast and reliable
3D-MOT loading, a basilar feature for new generation experiments on
Bose-Einstein condensation of dilute atomic samples. We also present a simple
multilevel theoretical model for the calculation of the light-induced force
acting on an atom moving in a MOT. The model gives a good agreement between
predicted and measured flux and velocity values for our 2D-MOT. | cond-mat_other |
Collective excitations of trapped Fermi or Bose gases: A new method is developed to calculate all excitations of trapped gases using
hydrodynamics at zero temperature for any equation of state $\mu=\mu(n)$ and
for any trapping potential. It is shown that a natural scalar product can be
defined for the mode functions, by which the wave operator is hermitian and the
mode functions are orthogonal. It is also shown that the Kohn-modes are exact
for harmonic trapping in hydrodynamic theory. Excitations for fermions are
calculated in the BCS-BEC transition region using the equation of state of the
mean-field Leggett-model for isotrop harmonic trap potential. | cond-mat_other |
Detecting the tunneling rates for strongly interacting fermions on
optical lattices: Strongly interacting fermionic atoms on optical lattices are studied through
a Hubbard-like model Hamiltonian, in which tunneling rates of atoms and
molecules between neighboring sites are assumed to be different. In the limit
of large onsite repulsion U, the model is shown to reproduce the t-J
Hamiltonian, in which the J coefficient of the Heisenberg term depends on the
particle-assisted tunneling rate g: explicitly, $J=4 g^2/U$. At half-filling, g
drives a crossover from a Brinkman-Rice paramagnetic insulator of fully
localized atoms (g=0) to the antiferromagnetic Mott insulator of the standard
Hubbard case (g=t). This is observed already at the intermediate coupling
regime in the number of doubly occupied sites, thus providing a criterion to
extract from measurements the effective value of g. | cond-mat_other |
Disordered Supersolids in the Extended Bose-Hubbard Model: The extended Bose-Hubbard model captures the essential properties of a wide
variety of physical systems including ultracold atoms and molecules in optical
lattices, Josephson junction arrays, and certain narrow band superconductors.
It exhibits a rich phase diagram including a supersolid phase where a lattice
solid coexists with a superfluid. We use quantum Monte Carlo to study the
supersolid part of the phase diagram of the extended Bose-Hubbard model on the
simple cubic lattice. We add disorder to the extended Bose-Hubbard model and
find that the maximum critical temperature for the supersolid phase tends to be
suppressed by disorder. But we also find a narrow parameter window in which the
supersolid critical temperature is enhanced by disorder. Our results show that
supersolids survive a moderate amount of spatial disorder and thermal
fluctuations in the simple cubic lattice. | cond-mat_other |
Vortex shedding from a microsphere oscillating in superfluid ^4He at mK
temperatures and from a laser beam moving in a Bose-Einstein condensate: Turbulent drag of an oscillating microsphere, that is levitating in
superfluid $^4$He at mK temperatures, is unstable slightly above a critical
velocity amplitude $v_c$. The lifetime $\tau$ of the turbulent state is
determined by the number $n$ of vortices shed per half-period. It is found that
this number is identical to the superfluid Reynolds number. The possibility of
moving a levitating sphere through superfluid $^3$He at microkelvin
temperatures is considered. A laser beam moving through a Bose-Einstein
condensate (BEC) (as observed by other authors) also produces vortices in the
BEC. In particular, in either case a linear dependence of the shedding
frequency $f_v$ on $\Delta v = v - v_c$ is observed, where $v$ is the velocity
amplitude of the sphere or the constant velocity of the laser beam above $v_c$
for the onset of turbulent flow: $f_v = a \Delta v$, where the coefficient $a$
is proportional to the oscillation frequency $ \omega $ above some
characteristic frequency $\omega_k$ and assumes a finite value for steady
motion $\omega \rightarrow 0$. A relation between the superfluid Reynolds
number and the superfluid Strouhal number is presented that is different from
classical turbulence. | cond-mat_other |
Ultrafast amplification and non-linear magneto-elastic coupling of
coherent magnon modes in an antiferromagnet: We investigate the role of domain walls in the ultrafast magnon dynamics of
an antiferromagnetic NiO single crystal in a pump-probe experiment with
variable pump photon energy. Analysing the amplitude of the energy-dependent
photo-induced ultrafast spin dynamics, we detect a yet unreported coupling
between the material's characteristic THz- and a GHz-magnon modes. We explain
this unexpected coupling between two orthogonal eigenstates of the
corresponding Hamiltonian by modelling the magneto-elastic interaction between
spins in different domains. We find that such interaction, in the non-linear
regime, couples the two different magnon modes via the domain walls and it can
be optically exploited via the exciton-magnon resonance. | cond-mat_other |
Quasiclassical frustration: We study the dissipative properties of a harmonic oscillator subject to two
independent heat baths, one of which couples to its position and the other one
to its momentum. This model describes a large spin impurity in a ferromagnet.
We find that some effects of the two heat baths partially cancel each other.
Most notably, oscillations may remain underdamped for arbitrarily strong
coupling. This effect is a direct consequence of the mutually conjugate
character of position and momentum. For a single dissipative bath coupled
linearly to both position and momentum, no underdamped regime is possible for
strong coupling. The dynamics of purity loss for one and two wave packets is
also investigated. | cond-mat_other |
Fermion mediated long-range interactions of bosons in the 1D
Bose-Fermi-Hubbard model: The ground-state phase diagram of mixtures of spin polarized fermions and
bosons in a 1D periodic lattice is discussed in the limit of large fermion
hopping and half filling of the fermions. Numerical simulations performed with
the density matrix renormalization group (DMRG) show besides bosonic Mott
insulating (MI), superfluid (SF), and charge density-wave phases (CDW) a novel
phase with spatial separation of MI and CDW regions. We derive an effective
bosonic theory which allows for a complete understanding and quantitative
prediction of the bosonic phase diagram. In particular the origin of CDW phase
and the MI-CDW phase separation is revealed as the interplay between
fermion-induced mean-field potential and long range interaction with
alternating sign. | cond-mat_other |
Old wine in new bottles: Onsager's reciprocity relations for the coefficients of transport equations
are now 87 years old. Sometimes these relations are called the Fourth Law of
Thermodynamics. Among others they provide an effective criterion for the
existence of local equilibrium and of microscopic reversibility. Since the
beginning of the century Onsager's relations have seen a revival in the field
of spincaloritronics. There the relations are very helpful in judging the
utility of modern devices for electronic data processing. | 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 |
Su(3) Algebraic Structure of the Cuprate Superconductors Model based on
the Analogy with Atomic Nuclei: A cuprate superconductor model based on the analogy with atomic nuclei was
shown by Iachello to have an $su(3)$ structure. The mean-field approximation
Hamiltonian can be written as a linear function of the generators of $su(3)$
algebra. Using algebraic method, we derive the eigenvalues of the reduced
Hamiltonian beyond the subalgebras $u(1)\bigotimes u(2)$ and $so(3)$ of $su(3)$
algebra. In particular, by considering the coherence between s- and d-wave
pairs as perturbation, the effects of coherent term upon the energy spectrum
are investigated. | cond-mat_other |
Study of superfluid $^3$He under nanoscale confinement. A new approach
to the investigation of superfluid $^3$He films: We review recent experiments in which superfluid $^3$He has been studied
under highly controlled confinement in nanofluidic sample chambers. We discuss
the experimental challenges and their resolution. These methods open the way to
a systematic investigation of the superfluidity of $^3$He films, and the
surface and edge excitations of topological superfluids. | cond-mat_other |
A Computational Study of Rotating Spiral Waves and Spatio-Temporal
Transient Chaos in a Deterministic Three-Level Active System: Spatio-temporal dynamics of a deterministic three-level cellular automaton
(TLCA) of Zykov-Mikhailov type (Sov. Phys. - Dokl., 1986, Vol.31, No.1, P.51)
is studied numerically. Evolution of spatial structures is investigated both
for the original Zykov-Mikhailov model (which is applicable to, for example,
Belousov-Zhabotinskii chemical reactions) and for proposed by us TLCA, which is
a generalization of Zykov-Mikhailov model for the case of two-channel
diffusion. Such the TLCA is a minimal model for an excitable medium of
microwave phonon laser, called phaser (D. N. Makovetskii, Tech. Phys., 2004,
Vol.49, No.2, P.224; cond-mat/0402640). The most interesting observed forms of
TLCA dynamics are as follows: (a) spatio-temporal transient chaos in form of
highly bottlenecked collective evolution of excitations by rotating spiral
waves (RSW) with variable topological charges; (b) competition of left-handed
and right-handed RSW with unexpected features, including self-induced
alteration of integral effective topological charge; (c) transient chimera
states, i.e. coexistence of regular and chaotic domains in TLCA patterns; (d)
branching of TLCA states with different symmetry which may lead to full
restoring of symmetry of imperfect starting pattern. Phenomena (a) and (c) are
directly related to phaser dynamics features observed earlier in real
experiments at liquid helium temperatures on corundum crystals doped by
iron-group ions. ACM: F.1.1, I.6, J.2; PACS:05.65.+b, 07.05.Tp, 82.20.Wt | cond-mat_other |
Dynamical Exchange Interaction From Time-Dependent Spin Density
Functional Theory: We report on {\it ab initio} time-dependent spin dynamics simulations for a
two-center magnetic molecular complex based on time-dependent non-collinear
spin density functional theory. In particular, we discuss how the dynamical
behavior of the {\it ab initio} spin-density in the time-domain can be mapped
onto a model Hamiltonian based on the classical Heisenberg spin-spin
interaction $J\vcr{S}_1\cdot \vcr{S}_2$. By analyzing individual localized-spin
trajectories, extracted from the spin-density evolution, we demonstrate a novel
method for evaluating the effective Heisenberg exchange coupling constant, $J$,
from first principles simulations. We find that $J$, extracted in such a new
dynamical way, agrees quantitatively to that calculated by the standard density
functional theory broken-symmetry scheme. | cond-mat_other |
Probing pairing gap in Fermi atoms by light scattering: We study stimulated scattering of polarized light in a two-component Fermi
gas of atoms at zero temperature. Within the framework of Nambu-Gorkov
formalism, we calculate the response function of superfluid gas taking into
account the final state interactions. The dynamic structure factor deduced from
the response function provides information about the pairing gap and the
momentum distributions of atoms. Model calculations using local density
approximation indicates that the pairing gap of trapped Fermi gas may be
detectable by Bragg spectroscopy due to stimulated scattering. | 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 |
Nonlinear Landau-Zener Processes in a Periodic Driving Field: Effects of a periodic driving field on Landau-Zener processes are studied
using a nonlinear two-mode model that describes the mean-field dynamics of a
many-body system. A variety of different dynamical phenomena in different
parameter regimes of the driving field are discussed and analyzed. These
include shifted, weakened, or enhanced phase dependence of nonlinear
Landau-Zener processes, nonlinearity-enhanced population transfer in the
adiabatic limit, and Hamiltonian chaos on the mean field level. The emphasis of
this work is placed on how the impact of a periodic driving field on
Landau-Zener processes with self-interaction differs from those without
self-interaction. Aside from gaining understandings of driven nonlinear
Landau-Zener processes, our findings can be used to gauge the strength of
nonlinearity and for efficient manipulation of the mean-field dynamics of
many-body systems. | cond-mat_other |
Collisional and molecular spectroscopy in an ultracold Bose-Bose mixture: The route toward a Bose-Einstein condensate of dipolar molecules requires the
ability to efficiently associate dimers of different chemical species and
transfer them to the stable rovibrational ground state. Here, we report on
recent spectroscopic measurements of two weakly bound molecular levels and
newly observed narrow d-wave Feshbach resonances. The data are used to improve
the collisional model for the Bose-Bose mixture 41K87Rb, among the most
promising candidates to create a molecular dipolar BEC. | cond-mat_other |
Parametrically excited "Scars" in Bose-Einstein condensates: Parametric excitation of a Bose-Einstein condensate (BEC) can be realized by
periodically changing the interaction strength between the atoms. Above some
threshold strength, this excitation modulates the condensate density. We show
that when the condensate is trapped in a potential well of irregular shape,
density waves can be strongly concentrated ("scarred") along the shortest
periodic orbits of a classical particle moving within the confining potential.
While single-particle wave functions of systems whose classical counterpart is
chaotic may exhibit rich scarring patterns, in BEC, we show that nonlinear
effects select mainly those scars that are locally described by stripes.
Typically, these are the scars associated with self retracing periodic orbits
that do not cross themselves in real space. Dephasing enhances this behavior by
reducing the nonlocal effect of interference. | cond-mat_other |
Condensation of phonons in an ultracold Bose gas: We consider the generation of longitudinal phonons in an elongated
Bose-condensed gas at zero temperature due to parametric resonance as a result
of the modulation of the transverse trap frequency. The nonlinear temporal
evolution with account of the phonon-phonon interaction leads self-consistently
to the formation of the stationary state with the macroscopic occupation of a
single phonon quantum state. | cond-mat_other |
Diagnostics for the ground state phase of a spin-2 Bose-Einstein
condensate: We propose a method to determine the singlet-pair energy of a spin-2
Bose-Einstein condensate (BEC). By preparing the initial populations in the
magnetic sublevels 0, 2, -2 with appropriate relative phases, we can obtain the
coefficient of the spin singlet-pair term from the spin exchange dynamics. This
method is suitable for hyperfine states with short lifetimes, since only the
initial change in the population of each magnetic sublevel is needed. This
method therefore enables the determination of the ground state phase of a
spin-2 87Rb BEC at zero magnetic field, which is considered to lie in the
immediate vicinity of the boundary between the antiferromagnetic and cyclic
phases. We also show that the initial state in which relative phases are
controlled can be prepared by Raman processes. | cond-mat_other |
The strong form of the Levinson theorem for a distorted KP potential: We present a heuristic derivation of the strong form of the Levinson theorem
for one-dimensional quasi-periodic potentials. The particular potential chosen
is a distorted Kronig-Penney model. This theorem relates the phase shifts of
the states at each band edge to the number of states crossing that edge, as the
system evolves from a simple periodic potential to a distorted one. By applying
this relationship to the two edges of each energy band, the modified Levinson
theorem for quasi-periodic potentials is derived. These two theorems differ
from the usual ones for isolated potentials in non-relativistic and
relativistic quantum mechanics by a crucial alternating sign factor $(-1)^{s}$,
where $s$ refers to the adjacent gap or band index, as explained in the text.
We also relate the total number of bound states present in each energy gap due
to the distortion to the phase shifts at its edges. At the end we present an
overall relationship between all of the phase shifts at the band edges and the
total number of bound states present. | cond-mat_other |
Comment on pressure driven flow of superfluid $^4$He through a nanopipe
(Botimer and Taborek 2016): Botimer and Taborek (2016) measured the mass flux of superfluid $^4$He
through a capillary into an evacuated chamber for various temperatures and
pressures of the reservoir chamber. They found a sharp transition from low flux
at low pressures to high flux at large pressures. Here it is shown that the
superfluid condition of chemical potential equality predicts the induced
temperature and also the transition pressure, which is attributed to the
transition from a semispherical cap to a pool of $^4$He at the exit of the
capillary. The results show that the two-fluid equations of superfluid flow,
Landau's phonon-roton theory, and Feynman's critical vortex theory are
unnecessary for a quantitative account of the measured transition pressure. | cond-mat_other |
CHEERS: A tool for Correlated Hole-Electron Evolution from Real-time
Simulations: We put forward a practical nonequilibrium Green's function (NEGF) scheme to
perform real-time evolutions of many-body interacting systems driven out of
equilibrium by external fields. CHEERS is a computational tool to solve the
NEGF equation of motion in the so called generalized Kadanoff-Baym ansatz and
it can be used for model systems as well as first-principles Hamiltonians.
Dynamical correlation (or memory) effects are added to the Hartree-Fock
dynamics through a many-body self-energy. Applications to time-dependent
quantum transport, time-resolved photoabsorption and other ultrafast phenomena
are discussed. | cond-mat_other |
Bose-Einstein condensation in an optical lattice: In this paper we develop an analytic expression for the critical temperature
for a gas of ideal bosons in a combined harmonic lattice potential, relevant to
current experiments using optical lattices. We give corrections to the critical
temperature arising from effective mass modifications of the low energy
spectrum, finite size effects and excited band states. We compute the critical
temperature using numerical methods and compare to our analytic result. We
study condensation in an optical lattice over a wide parameter regime and
demonstrate that the critical temperature can be increased or reduced relative
to the purely harmonic case by adjusting the harmonic trap frequency. We show
that a simple numerical procedure based on a piecewise analytic density of
states provides an accurate prediction for the critical temperature. | cond-mat_other |
Dynamical equations for time-ordered Green's functions: from the Keldysh
time-loop contour to equilibrium at finite and zero temperature: We study the dynamical equation of the time-ordered Green's function at
finite temperature. We show that the time-ordered Green's function obeys a
conventional Dyson equation only at equilibrium and in the limit of
zero-temperature. In all other cases, i.e. finite-temperature at equilibrium or
non-equilibrium, the time-ordered Green's function obeys instead a modified
Dyson equation. The derivation of this result is obtained from the general
formalism of the non-equilibrium Green's functions on the Keldysh time-loop
contour. At equilibrium, our result is fully consistent with the Matsubara
temperature Green's function formalism and also justifies rigorously the
correction terms introduced in an ad hoc way with Hedin and Lundqvist. Our
results show that one should use the appropriate dynamical equation for the
time-ordered Green's function when working beyond the equilibrium
zero-temperature limit. | cond-mat_other |
Highly sensitive and broadband carbon nanotube radio-frequency
single-electron transistor: We have investigated radio-frequency single-electron transistor (RF-SET)
operation of single-walled carbon nanotube quantum dots in the strong tunneling
regime. At 4.2 K and carrier frequency 754.2 MHz, we reach a charge sensitivity
of 2.3e-6 e/Hz^(1/2) over a bandwidth of 85 MHz. Our results indicate a
gain-bandwidth product of 3.7e13 Hz^(3/2)/e, which is by one order of magnitude
better than for typical RF-SETs. | cond-mat_other |
Intrinsic leakage and adsorption currents associated with the
electrocaloric effect in multilayer capacitors: During the last few years, the increasing demand of energy for refrigeration
applications has relived the interest of the scientific community in the study
of alternative methods to the traditional gas-based refrigeration. Within this
framework, the use of solid state refrigeration based on the electrocaloric
effect reveals itself as one of the most promising technologies. In this work,
we analyze how the temperature change associated with the electrocaloric effect
shows a correlation with the electrical properties of a commercial multilayer
capacitor. In that sense we established a clear relation between the adsorption
currents and the temperature change produced by the electrocaloric effect.
Additionally, intrinsic leakage currents are responsible for the sample heating
due to the Joule effect. These well distinguished contributions can be useful
during the design of solid state refrigeration devices based on the
electrocaloric effect. | cond-mat_other |
Quantum effects in the H-bond symmetrization and in the thermodynamic
properties of high pressure ice: We investigate the structural and thermodynamic properties of high-pressure
ice by incorporating quantum anharmonicity at a non-perturbative level. Quantum
fluctuations reduce the critical pressure of the phase transition between phase
VIII (with asymmetric H-bonds) and phase X (with symmetric H-bonds) by 65 GPa
from its classical value of 116 GPa at 0K. Moreover, quantum effects make it
temperature-independent over a wide temperature range (0K-300K), in agreement
with experimental estimates obtained through vibrational spectroscopy and in
striking contrast to the strong temperature dependence found in the classical
approximation. The equation of state shows fingerprints of the transition in
accordance with experimental evidence. Additionally, we demonstrate that,
within our approach, proton disorder in phase VII has a negligible impact on
the occurrence of phase X. Finally, we reproduce with high accuracy the 10 GPa
isotope shift due to the hydrogen-to-deuterium substitution. | cond-mat_other |
Simplified feedback control system for Scanning Tunneling Microscopy: A Scanning Tunneling Microscope (STM) is one of the most important scanning
probe tools available to study and manipulate matter at the nanoscale. In a
STM, a tip is scanned on top of a surface with a separation of a few \AA.
Often, the tunneling current between tip and sample is maintained constant by
modifying the distance between the tip apex and the surface through a feedback
mechanism acting on a piezoelectric transducer. This produces very detailed
images of the electronic properties of the surface. The feedback mechanism is
nearly always made using a digital processing circuit separate from the user
computer. Here we discuss another approach, using a computer and data
acquisition through the USB port. We find that it allows succesful ultra low
noise studies of surfaces at cryogenic temperatures. We show results on
different compounds, a type II Weyl semimetal (WTe$_2$), a quasi
two-dimensional dichalcogenide superconductor (2H-NbSe$_2$), a magnetic Weyl
semimetal (Co$_3$Sn$_2$S$_2$) and an iron pnictide superconductor (FeSe). | cond-mat_other |
Observation of an Efimov-like resonance in ultracold atom-dimer
scattering: The field of few-body physics has originally been motivated by understanding
nuclear matter. New model systems to experimentally explore few-body quantum
systems can now be realized in ultracold gases with tunable interactions.
Albeit the vastly different energy regimes of ultracold and nuclear matter (peV
as compared to MeV), few-body phenomena are universal for near-resonant
two-body interactions. Efimov states represent a paradigm for universal
three-body states, and evidence for their existence has been obtained in
measurements of three-body recombination in an ultracold gas of caesium atoms.
Interacting samples of halo dimers can provide further information on universal
few-body phenomena. Here we study interactions in an optically trapped mixture
of such halo dimers with atoms, realized in a caesium gas at nanokelvin
temperatures. We observe an atom-dimer scattering resonance, which we interpret
as being due to a trimer state hitting the atom-dimer threshold. We discuss the
close relation of this observation to Efimov's scenario, and in particular to
atom-dimer Efimov resonances. | cond-mat_other |
Atoms in boxes: from confined atoms to electron-atom scattering: We show that both confined atoms and electron-atom scattering can be
described by a unified basis set method. The central idea behind this method is
to place the atom inside a hard potential sphere, enforced by a standard Slater
type basis set multiplied by a cutoff factor. For confined atoms, where the
wall is placed close to the atomic nucleus, we show how the energy of the
highest occupied atomic orbital and the static polarizability of helium and
neon atoms evolve with the confinement radius. To our knowledge, these are the
first confined atom polarizability calculations that include correlation,
through the use of time-dependent density-functional theory. By placing the
atom in a large spherical box, with a wall outside the electron density, we
obtain scattering phase shifts using a recently developed method [M. van
Faassen, A. Wasserman, E. Engel, F. Zhang, and K. Burke, Phys. Rev. Lett. {\bf
99}, 043005 (2007)]. We show that the basis set method gives identical results
to previously obtained phase shifts for $e$-H and $e$-He${}^{+}$ scattering. | cond-mat_other |
Localization by entanglement: We study the localization of bosonic atoms in an optical lattice, which
interact in a spatially confined region. The classical theory predicts that
there is no localization below a threshold value for the strength of
interaction that is inversely proportional to the number of participating
atoms. In a full quantum treatment, however, we find that localized states
exist for arbitrarily weak attractive or repulsive interactions for any number
($>1$) of atoms. We further show, using an explicit solution of the
two-particle bound state and an appropriate measure of entanglement, that the
entanglement tends to a finite value in the limit of weak interactions. Coupled
with the non-existence of localization in an optimized quantum product state,
we conclude that the localization exists by virtue of entanglement. | cond-mat_other |
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