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Collective Excitations of Bose-Einstein Condensates in a Double-Well
Potential: We investigate collective excitations of Bose-Einstein condensates at
absolute zero in a double-well trap. We solve the Bogoliubov equations with a
double-well trap, and show that the crossover from the dipole mode to the
Josephson plasma mode occurs in the lowest energy excitation. It is found that
the anomalous tunneling property of low energy excitations is crucial to the
crossover. | cond-mat_other |
Hydrodynamic theory of transport in doped graphene: We study non-linear dc transport in graphene using a hydrodynamic approach
and conclude that in clean samples the drift velocity saturates at a weakly
density-dependent value v_{sat} ~ 10^7 cm/s. We show that saturation results
from the interactions between graphene's Dirac quasi-particles and both
acoustic and optical phonons. Saturation is accompanied by substantial electron
heating and is not reached at realistic driving fields in moderately or
strongly disordered samples. We find that it is essential to account for
interactions among graphene's Dirac quasi-particles, which increase the linear
response resistivity at high temperatures or low densities. | cond-mat_other |
Axicon Lens for Coherent Matter Waves: We have realized a conical matter wave lens. The repulsive potential of a
focused laser beam was used to launch a Bose-Einstein condensate into a
radially expanding wavepacket whose perfect ring shape was ensured by energy
conservation. In spite of significant interactions between atoms, the spatial
and velocity widths of the ring along its radial dimension remained extremely
narrow, as also confirmed by numerical simulations. Our results open the
possibility for cylindrical atom optics without the perturbing effect of
mean-field interactions. | cond-mat_other |
Analytical solution of the equation of motion for a rigid domain wall in
a magnetic material with perpendicular anisotropy: This paper reports the solution of the equation of motion for a domain wall
in a magnetic material which exhibits high magneto-crystalline anisotropy.
Starting from the Landau-Lifschitz-Gilbert equation for field-induced motion,
we solve the equation to give an analytical expression, which specifies the
domain wall position as a function of time. Taking parameters from a Co/Pt
multilayer system, we find good quantitative agreement between calculated and
experimentally determined wall velocities, and show that high field uniform
wall motion occurs when wall rigidity is assumed. | cond-mat_other |
Giant planar Hall effect in colossal magnetoresistive
La(0.84)Sr(0.16)MnO(3) thin films: The transverse resistivity in thin films of La(0.84)Sr(0.16)MnO(3)
(LSMO)exhibits sharp field-symmetric jumps below Tc. We show that a likely
source of this behavior is the giant planar Hall effect (GPHE) combined with
biaxial magnetic anisotropy. The effect is comparable in magnitude to that
observed recently in the magnetic semiconductor Ga(Mn)As. It can be potentially
used in applications such as magnetic sensors and non-volatile memory devices. | cond-mat_other |
Coherent backscattering of Bose-Einstein condensates in two-dimensional
disorder potentials: We study quantum transport of an interacting Bose-Einstein condensate in a
two-dimensional disorder potential. In the limit of vanishing atom-atom
interaction, a sharp cone in the angle-resolved density of the scattered matter
wave is observed, arising from constructive interference between amplitudes
propagating along reversed scattering paths. Weak interaction transforms this
coherent backscattering peak into a pronounced dip, indicating destructive
instead of constructive interference. We reproduce this result, obtained from
the numerical integration of the Gross-Pitaevskii equation, by a diagrammatic
theory of weak localization in presence of a nonlinearity. | cond-mat_other |
Long-lived Feshbach molecules in a 3D optical lattice: We have created and trapped a pure sample of 87Rb2 Feshbach molecules in a
three-dimensional optical lattice. Compared to previous experiments without a
lattice we find dramatic improvements such as long lifetimes of up to 700 ms
and a near unit efficiency for converting tightly confined atom pairs into
molecules. The lattice shields the trapped molecules from collisions and thus
overcomes the problem of inelastic decay by vibrational quenching. Furthermore,
we have developed a novel purification scheme that removes residual atoms,
resulting in a lattice in which individual sites are either empty or filled
with a single molecule in the vibrational ground state of the lattice. | cond-mat_other |
Auger mediated quantum sticking of positrons to surfaces: Evidence for
single step transition from a scattering state to a surface image potential
bound state: We present the observation of an efficient mechanism for positron sticking to
surfaces termed here Auger mediated quantum sticking. In this process the
energy associated with the positrons transition from an unbound scattering
state to a bound image potential state is coupled to a valence electron which
can then have sufficient energy to leave the surface. Compelling evidence for
this mechanism is found in a narrow secondary electron peak observed at
incident positron kinetic energies well below the electron work function. | cond-mat_other |
Expansions of the interatomic potential for different boundary
conditions and the transition to the thermodynamic limit: We analyze the possible expansions of the interatomic potential
$U(|\textbf{r}_{1}-\textbf{r}_{2}|)$ in a Fourier series for a cyclic system
and a system with boundaries. We also study the transition from exact
expansions for a finite system to the expansion that is commonly used in the
thermodynamic limit. The analysis shows that such a transition distorts the
potential of a bounded system by making it cyclic. | cond-mat_other |
Improvements in 3D Automated Shimming Techniques in High-Resolution NMR: Traditionally the improvement of static magnetic field homogeneity of the
magnet in Nuclear Magnetic Resonance (NMR) spectroscopy is performed manually,
which has many limitations. However, in recent years a number of automated
shimming techniques based on Fourier Imaging Technique have been proposed.
Existing 3D automated shimming methods require special, Pulsed Field Gradient
(PFG) hardware, which is not available on majority of high-resolution NMR
spectrometers. The modified technique, presented in this thesis uses the normal
NMR hardware provided with the majority of high-resolution NMR spectrometers.
The 3D shimming technique described was optimised for use with Varian UNITY
INOVA spectrometers and successfully tested with both protonated and deuterated
solvents. A method for calibrating linear transverse shim field gradients and
correcting any non-orthogonality and imbalance of strengths is proposed. The
effect of thermal convection on field mapping was observed and is reported here
for the first time. | cond-mat_other |
Semiconductor-metal nanoparticle molecules: hybrid excitons and
non-linear Fano effect: Modern nanotechnology opens the possibility of combining nanocrystals of
various materials with very different characteristics in one superstructure.
The resultant superstructure may provide new physical properties not
encountered in homogeneous systems. Here we study theoretically the optical
properties of hybrid molecules composed of semiconductor and metal
nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly
coupled and demonstrate novel properties. At low incident light intensity, the
exciton peak in the absorption spectrum is broadened and shifted due to
incoherent and coherent interactions between metal and semiconductor
nanoparticles. At high light intensity, the absorption spectrum demonstrates a
surprising, strongly asymmetric shape. This shape originates from the coherent
inter-nanoparticle Coulomb interaction and can be viewed as a non-linear Fano
effect which is quite different from the usual linear Fano resonance. | cond-mat_other |
Observation of deviations from ideal gas thermodynamics in a trapped
Bose-Einstein condensed gas: We have investigated experimentally the finite-temperature properties of a
Bose-Einstein condensed cloud of $^{87}$Rb atoms in a harmonic trap. Focusing
primarily on condensed fraction and expansion energy, we measure unambiguous
deviations from ideal-gas thermodynamics, and obtain good agreement with a
Hartree-Fock description of the mixed cloud. Our results offer for the first
time clear evidence of the mutual interaction between the condensed and thermal
components. To probe the low-temperature region unaccessible to the usual
time-of-flight technique, we use coherent Bragg scattering as a filtering
technique for the condensate. This allows us to separate spatially the
condensed and normal components in time of flight, and to measure reliably
temperatures as low as $0.2 T_{\rm c}^0$ and thermal fractions as low as
10%.Finally, we observe evidence for the limitations of the usual image
analysis procedure, pointing out to the need for a more elaborate model of the
expansion of the mixed cloud. | cond-mat_other |
Transport of Atom Packets in a Train of Ioffe-Pritchard Traps: We demonstrate transport and evaporative cooling of several atomic clouds in
a chain of magnetic Ioffe-Pritchard traps moving at a low speed ($<1$~m/s). The
trapping scheme relies on the use of a magnetic guide for transverse
confinement and of magnets fixed on a conveyor belt for longitudinal trapping.
This experiment introduces a new approach for parallelizing the production of
Bose-Einstein condensates as well as for the realization of a continuous atom
laser. | cond-mat_other |
Superfluid Bose-Fermi mixture from weak-coupling to unitarity: We investigate the zero-temperature properties of a superfluid Bose-Fermi
mixture by introducing a set of coupled Galilei-invariant nonlinear
Schr\"odinger equations valid from weak-coupling to unitarity. The Bose
dynamics is described by a Gross-Pitaevskii-type equation including
beyond-mean-field corrections possessing the correct weak-coupling and
unitarity limits. The dynamics of the two-component Fermi superfluid is
described by a density-functional equation including beyond-mean-field terms
with correct weak-coupling and unitarity limits. The present set of equations
is equivalent to the equations of generalized superfluid hydrodynamics, which
take into account also surface effects. The equations describe the mixture
properly as the Bose-Bose repulsive (positive) and Fermi-Fermi attractive
(negative) scattering lengths are varied from zero to infinity in the presence
of a Bose-Fermi interaction. The present model is tested numerically as the
Bose-Bose and Fermi-Fermi scattering lengths are varied over wide ranges
covering the weak-coupling to unitarity transition. | cond-mat_other |
Dynamics of localized spins coupled to the conduction electrons with
charge/spin currents: The effects of the charge/spin currents of conduction electrons on the
dynamics of the localized spins are studied in terms of the perturbation in the
exchange coupling $J_{K}$ between them. The equations of motion for the
localized spins are derived exactly up to $O(J_{K}^2)$, and the equations for
the two-spin system is solved numerically. It is found that the dynamics
depends sensitively upon the relative magnitude of the charge and spin
currents, i.e., it shows steady state, periodic motion, and even chaotic
behavior. Extension to the multi-spin system and its implications including
possible ``spin current detector'' are also discussed. | cond-mat_other |
Radio-frequency operation of a double-island single-electron transistor: We present results on a double-island single-electron transistor (DISET)
operated at radio-frequency (rf) for fast and highly sensitive detection of
charge motion in the solid state. Using an intuitive definition for the charge
sensitivity, we compare a DISET to a conventional single-electron transistor
(SET). We find that a DISET can be more sensitive than a SET for identical,
minimum device resistances in the Coulomb blockade regime. This is of
particular importance for rf operation where ideal impedance matching to 50 Ohm
transmission lines is only possible for a limited range of device resistances.
We report a charge sensitivity of 5.6E-6 e/sqrt(Hz) for a rf-DISET, together
with a demonstration of single-shot detection of small (<=0.1e) charge signals
on microsecond timescales. | cond-mat_other |
Exciton and biexciton energies in bilayer systems: We report calculations of the energies of excitons and biexcitons in ideal
two-dimensional bilayer systems within the effective-mass approximation with
isotropic electron and hole masses. The exciton energies are obtained by a
simple numerical integration technique, while the biexciton energies are
obtained from diffusion quantum Monte Carlo calculations. The exciton binding
energy decays as the inverse of the separation of the layers, while the binding
energy of the biexciton with respect to dissociation into two separate excitons
decays exponentially. | 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 |
Faraday waves in Bose-Einstein condensates: Motivated by recent experiments on Faraday waves in Bose-Einstein condensates
we investigate both analytically and numerically the dynamics of cigar-shaped
Bose-condensed gases subject to periodic modulation of the strength of the
transverse confinement. We offer a fully analytical explanation of the observed
parametric resonance, based on a Mathieu-type analysis of the non-polynomial
Schr{\"o}dinger equation. The theoretical prediction for the pattern
periodicity versus the driving frequency is directly compared with the
experimental data, yielding good qualitative and quantitative agreement between
the two. These results are corroborated by direct numerical simulations of both
the one-dimensional non-polynomial Schr{\"o}dinger equation and of the fully
three-dimensional Gross-Pitaevskii equation. | cond-mat_other |
Structural phase transitions in epitaxial perovskite films: Three different film systems have been systematically investigated to
understand the effects of strain and substrate constraint on the phase
transitions of perovskite films. In SrTiO$_3$ films, the phase transition
temperature T$_C$ was determined by monitoring the superlattice peaks
associated with rotations of TiO$_6$ octahedra. It is found that T$_C$ depends
on both SrTiO$_3$ film thickness and SrRuO$_3$ buffer layer thickness. However,
lattice parameter measurements showed no sign of the phase transitions,
indicating that the tetragonality of the SrTiO$_3$ unit cells was no longer a
good order parameter. This signals a change in the nature of this phase
transition, the internal degree of freedom is decoupled from the external
degree of freedom. The phase transitions occur even without lattice relaxation
through domain formation. In NdNiO$_3$ thin films, it is found that the
in-plane lattice parameters were clamped by the substrate, while out-of-plane
lattice constant varied to accommodate the volume change across the phase
transition. This shows that substrate constraint is an important parameter for
epitaxial film systems, and is responsible for the suppression of external
structural change in SrTiO$_3$ and NdNiO$_3$ films. However, in SrRuO$_3$ films
we observed domain formation at elevated temperature through x-ray reciprocal
space mapping. This indicated that internal strain energy within films also
played an important role, and may dominate in some film systems. The final
strain states within epitaxial films were the result of competition between
multiple mechanisms and may not be described by a single parameter. | cond-mat_other |
Theory of magnon-polaritons in quantum Ising materials: We present a theory of magnon-polaritons in quantum Ising materials, and
develop a formalism describing the coupling between light and matter as an
Ising system is tuned through its quantum critical point. The theory is applied
to Ising materials having multilevel single-site Hamiltonians, in which
multiple magnon modes are present, such as the insulating Ising magnet
LiHoF$_4$ . We find that the magnon-photon coupling strengths may be tuned by
the applied transverse field, with the coupling between the soft mode present
in the quantum Ising material and a photonic resonator mode diverging at the
quantum critical point of the material. A fixed system of spins will not
exhibit the diamagnetic response expected when light is coupled to mobile spins
or atoms. Without the diamagnetic response, one expects a divergent
magnon-photon coupling strength to lead to a superradiant quantum phase
transition. However, this neglects the effects of damping and decoherence
present in any real system. We show that damping and decoherence may block the
superradiant quantum phase transition, and lead to weak coupling between the
soft magnon mode and the resonator mode. The results of the theory are applied
to experimental data on the model system LiHoF$_4$ in a microwave resonator. | 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 |
Interference of spin states in photoemission from Sb/Ag(111): Using a three-dimensional spin polarimeter we have gathered evidence for the
interference of spin states in photoemission from the surface alloy Sb/Ag(111).
This system features a small Rashba-type spin-splitting of a size comparable to
the linewidth of the quasiparticles, thus causing an intrinsic overlap between
states with orthogonal spinors. Besides a small spin polarization caused by the
spin-splitting, we observe a large spin polarization component in the plane
normal to the quantization axis provided by the Rashba effect. Strongly
suggestive of coherent spin rotation, this effect is largely independent of the
photon energy and photon polarization. | cond-mat_other |
Non-radiative exciton energy transfer in hybrid organic-inorganic
heterostructures: Non-radiative optical energy transfer from a GaAs quantum well to a thin
overlayer of an infrared organic semiconductor dye is unambiguously
demonstrated. The dynamics of exciton transfer are studied in the time-domain
using pump-probe spectroscopy at the donor site and fluorescence spectroscopy
at the acceptor site. The effect is observed as simultaneous increase of the
population decay rate at the donor and of the rise time of optical emission at
the acceptor sites. The hybrid configuration under investigation provides an
alternative non-radiative, non-contact pumping route to electrical carrier
injection that overcomes the losses imposed by the associated low carrier
mobility of organic emitters. | cond-mat_other |
Light scattering in Cooper-paired Fermi atoms: We present a detailed theoretical study of light scattering off superfluid
trapped Fermi gas of atoms at zero temperature. We apply Nambu-Gorkov formalism
of superconductivity to calculate the response function of superfluid gas due
to stimulated light scattering taking into account the final state
interactions. The polarization of light has been shown to play a significant
role in response of Cooper-pairs in the presence of a magnetic field.
Particularly important is a scheme of polarization-selective light scattering
by either spin-component of the Cooper-pairs leading to the single-particle
excitations of one spin-component only. These excitations have a threshold of
$2\Delta$ where $\Delta$ is the superfluid gap energy. Furthermore,
polarization-selective light scattering allows for unequal energy and momentum
transfer to the two partner atoms of a Cooper-pair. In the regime of low energy
($<< 2\Delta$) and low momentum ($<2\Delta/(\hbar v_F)$, $v_F$ being the Fermi
velocity) transfer, a small difference in momentum transfers to the two
spin-components may be useful in exciting Bogoliubov-Anderson phonon mode. We
present detailed results on the dynamic structure factor (DSF) deduced from the
response function making use of generalized fluctuation-dissipation theorem.
Model calculations using local density approximation for trapped superfluid
Fermi gas shows that when the energy transfer is less than $2\Delta_0$, where
$\Delta_0$ refers to the gap at the trap center, DSF as a function of energy
transfer has reduced gradient compared to that of normal Fermi gas. | cond-mat_other |
Epitaxial self-organization: from surfaces to magnetic materials: Self-organization of magnetic materials is an emerging and active field. An
overview of the use of self-organization for magnetic purposes is given, with a
view to illustrate aspects that cannot be covered by lithography. A first set
of issues concerns the quantitative study of low-dimensional magnetic phenomena
(1D and 0D). Such effects also occur in microstructured and
lithographically-patterned materials but cannot be studied in these because of
the complexity of such materials. This includes magnetic ordering, magnetic
anisotropy and superparamagnetism. A second set of issues concerns the
possibility to directly use self-organization in devices. Two sets of examples
are given: first, how superparamagnetism can be fought by fabricating thick
self-organized structures, and second, what new or improved functionalities can
be expected from self-organized magnetic systems, like the tailoring of
magnetic anisotropy or controlled dispersion of properties. | cond-mat_other |
Characterization of the Shell Structure in Coupled Quantum Dots through
Resonant Optical Probing: Excited states in single quantum dots (QDs) have been shown to be useful for
spin state initialization and manipulation. For scalable quantum information
processing it is necessary to have multiple spins interacting. Therefore, we
present initial results from photoluminescence excitation studies of excited
states in coupled quantum dots (CQDs). Due to the rich set of possible
excitation and recombination possibilities, a technique for visualizing
photoluminescence excitation in coupled quantum dots is discussed, by which
both the interaction between the dots and the type of absorption and emission
that generated the photoluminescence is easily and clearly revealed. As an
example, this technique is applied to characterize the shell structure of the
hole in the top dot and the results are compared with those using Level
Anti-Crossing Spectroscopy (LACS). | cond-mat_other |
Drift mobility of long-living excitons in coupled GaAs quantum wells: We observe high-mobility transport of indirect excitons in coupled GaAs
quantum wells. A voltage-tunable in-plane potential gradient is defined for
excitons by exploiting the quantum confined Stark effect in combination with a
lithographically designed resistive top gate. Excitonic photoluminescence
resolved in space, energy, and time provides insight into the in-plane drift
dynamics. Across several hundreds of microns an excitonic mobility of >10^5
cm2/eVs is observed for temperatures below 10 K. With increasing temperature
the excitonic mobility decreases due to exciton-phonon scattering. | cond-mat_other |
The two-site Bose--Hubbard model: The two-site Bose--Hubbard model is a simple model used to study Josephson
tunneling between two Bose--Einstein condensates. In this work we give an
overview of some mathematical aspects of this model. Using a classical
analysis, we study the equations of motion and the level curves of the
Hamiltonian. Then, the quantum dynamics of the model is investigated using
direct diagonalisation of the Hamiltonian. In both of these analyses, the
existence of a threshold coupling between a delocalised and a self-trapped
phase is evident, in qualitative agreement with experiments. We end with a
discussion of the exact solvability of the model via the algebraic Bethe
ansatz. | cond-mat_other |
Suppressing the Kibble-Zurek mechanism by a symmetry-violating bias: The formation of topological defects in continuous phase transitions is
driven by the Kibble-Zurek mechanism. Here we study the formation of single-
and half-quantum vortices during transition to the polar phase of $^3$He in the
presence of a symmetry-breaking bias provided by the applied magnetic field. We
find that vortex formation is suppressed exponentially when the length scale
associated with the bias field becomes smaller than the Kibble-Zurek length. We
thus demontrate an experimentally feasible shortcut to adiabaticity -- an
important aspect for further understanding of phase transitions as well as for
engineering applications such as quantum computers or simulators. | cond-mat_other |
Electromagnetic properties of graphene junctions: A resonant chiral tunneling (CT) across a graphene junction (GJ) induced by
an external electromagnetic field (EF) is studied. Modulation of the electron
and hole wavefunction phases $\varphi$ by the external EF during the CT
processes strongly impacts the CT directional diagram. Therefore the a.c.
transport characteristics of GJs depend on the EF polarization and frequency
considerably. The GJ shows great promises for various nanoelectronic
applications working in the THz diapason. | cond-mat_other |
Effective field theory for He-IV: We introduce an effective scalar field theory to describe the He-IV phase
diagram, which can be considered as a generalization of the XY model which
gives the usual lambda-transition. This theory results from a Ginzburg-Landau
Hamiltonian with higher order derivatives, which allow to produce transitions
between the superfluid, normal liquid and solid phases of He-IV. Mean field and
Monte Carlo analyses suggest that this model is able to reproduce the main
qualitative features of He-IV phase transitions. | cond-mat_other |
Polarisation rotation of slow light with orbital angular momentum in
ultracold atomic gases: We consider the propagation of slow light with an orbital angular momentum
(OAM) in a moving atomic medium. We have derived a general equation of motion
and applied it in analysing propagation of slow light with an OAM in a rotating
medium, such as a vortex lattice. We have shown that the OAM of slow light
manifests itself in a rotation of the polarisation plane of linearly polarised
light. To extract a pure rotational phase shift, we suggest to measure a
difference in the angle of the polarisation plane rotation by two consecutive
light beams with opposite OAM. The differential angle $\Delta\alpha_{\ell}$ is
proportional to the rotation frequency of the medium $\omega_{\mathrm{rot}}$
and the winding number $\ell$ of light, and is inversely proportional to the
group velocity of light. For slow light the angle $\Delta\alpha_{\ell}$ should
be large enough to be detectable. The effect can be used as a tool for
measuring the rotation frequency $\omega_{\mathrm{rot}}$ of the medium. | cond-mat_other |
Piezo-Magneto-Electric Effects in p-Doped Semiconductors: We predict the appearance of a uniform magnetization in strained three
dimensional p-doped semiconductors with inversion symmetry breaking subject to
an external electric field. We compute the magnetization response to the
electric field as a function of the direction and magnitude of the applied
strain. This effect could be used to manipulate the collective magnetic moment
of hole mediated ferromagnetism of magnetically doped semiconductors. | cond-mat_other |
Radiative annihilation of a soliton and an antisoliton in the coupled
sine-Gordon equation: In the sine-Gordon equation solitons and antisolitons in the absence of
perturbations do not annihilate. Here I present numerical analysis of
soliton-antisoliton collisions in the coupled sine-Gordon equation. It is shown
that in such a system soliton-antisoliton pairs (breathers) do annihilate even
in the absence of perturbations. The annihilation occurs via a
logarithmic-in-time decay of a breather caused by emission of plasma waves in
every period of breather oscillations. This also leads to a significant
coupling between breathers and propagating waves, which may lead to
self-oscillations at the geometrical resonance conditions in a dc-driven
system. The phenomenon may be useful for achieving superradiant emission from
coupled oscillators. | cond-mat_other |
Scattering of surface plasmon polaritons by one-dimensional
inhomogeneities: The scattering of surface plasmons polaritons by a one-dimensional defect of
the surface is theoretically studied, by means of both Rayleigh and modal
expansions. The considered defects are either relief perturbations or
variations in the permittivity of the metal. The dependence of transmission,
reflection and out-of-plane scattering on parameters defining the defect is
presented. We find that the radiated energy is forwardly directed (with respect
to the surface plasmon propagation) in the case of an impedance defect.
However, for relief defects, the radiated energy may be directed into backward
or forward (or both) directions, depending on the defect width. | cond-mat_other |
Pseudo-potential of a power-law decaying interaction in two-dimensional
systems: We analytically derive the general pseudo-potential operator of an arbitrary
isotropic interaction for particles confined in two-dimensional (2D) systems,
using the frame work developed by Huang and Yang for 3D scattering. We also
analytically derive the low energy dependence of the scattering phase-shift for
an arbitrary interaction with a power-law decaying tail, $V_{\rm
2D}(\rho)\propto \rho^{-\alpha}$ (for $\alpha>2$). We apply our results to the
2D dipolar gases ($\alpha=3$) as an example, calculating the momentum and
dipole moment dependence of the pseudo-potential for both $s$- and p-wave
scattering channels if the two scattering particles are in the same 2D layer.
Results for the s-wave scattering between particles in two different (parallel)
layers are also investigated. Our results can be directly applied to the
systems of dipolar atoms and/or polar molecules in a general 2D geometry. | cond-mat_other |
Stochastic Dynamical Structure (SDS) of Nonequilibrium Processes in the
Absence of Detailed Balance. IV: Emerging of Stochastic Dynamical Equalities
and Steady State Thermodynamics from Darwinian Dynamics: This is the fourth paper, the last one, on solution to the problem of absence
of detailed balance in nonequilibrium processes. It is an approach based on
another known universal dynamics: The evolutionary dynamics first conceived by
Darwin and Wallace, referring to as Darwinian dynamics in the present paper,
has been found to be universally valid in biology; The statistical mechanics
and thermodynamics, while enormously successful in physics, have been in an
awkward situation of wanting a consistent dynamical understanding; Here we
present from a formal point of view an exploration of the connection between
thermodynamics and Darwinian dynamics and a few related topics. We first show
that the stochasticity in Darwinian dynamics implies the existence temperature,
hence the canonical distribution of Boltzmann-Gibbs type. In term of relative
entropy the Second Law of thermodynamics is dynamically demonstrated without
detailed balance condition, and is valid regardless of size of the system. In
particular, the dynamical component responsible for breaking detailed balance
condition does not contribute to the change of the relative entropy. Two types
of stochastic dynamical equalities of current interest are explicitly discussed
in the present approach: One is based on Feynman-Kac formula and another is a
generalization of Einstein relation. Both are directly accessible to
experimental tests. Our demonstration indicates that Darwinian dynamics
represents logically a simple and straightforward starting point for
statistical mechanics and thermodynamics and is complementary to and consistent
with conservative dynamics that dominates the physical sciences. Present
exploration suggests the existence of a unified stochastic dynamical framework
both near and far from equilibrium. | cond-mat_other |
Two-particle binding energy of interacting Bose gases: The pole of the two-particle T-matrix including the influence of the
surrounding medium is analyzed for an interacting Bose gas. The phase diagram
of the Bose -Einstein condensation (BEC) depending on the temperature, density,
scattering length, and momentum is derived from this pole. The critical
momentum for the occurrence of superfluidity is obtained in this way. As a new
observation a two- particle binding energy is reported intimately connected
with the occurrence of the BEC. It is suggested that this might have
cosmological consequences on the dark energy problem. | cond-mat_other |
Hard Core Bosons on the Triangular Lattice at Zero Temperature: A Series
Expansion Study: We use high order linked cluster series to investigate the hard core boson
model on the triangular lattice, at zero temperature. Our expansions, in powers
of the hopping parameter $t$, probe the spatially ordered `solid' phase and the
transition to a uniform superfluid phase. At the commensurate fillings $n=1/3,
2/3$ we locate a quantum phase transition point at $(t/V)_c\simeq 0.208(1)$, in
good agreement with recent Monte Carlo studies. At half-filling ($n=1/2$) we
find evidence for a solid phase, which persists to $t/V\simeq 0.06$. | cond-mat_other |
Electron-spin beat susceptibility of excitons in semiconductor quantum
wells: Recent time-resolved differential transmission and Faraday rotation
measurements of long-lived electron spin coherence in quantum wells displayed
intriguing parametric dependencies. For their understanding we formulate a
microscopic theory of the optical response of a gas of optically incoherent
excitons whose constituent electrons retain spin coherence, under a weak
magnetic field applied in the quantum well's plane. We define a spin beat
susceptibility and evaluate it in linear order of the exciton density. Our
results explain the many-body physics underlying the basic features observed in
the experimental measurements. | cond-mat_other |
On the Energy-Based Variational Model for Vector Magnetic Hysteresis: We consider the quasi-static magnetic hysteresis model based on a
dry-friction like representation of magnetization. The model has a consistent
energy interpretation, is intrinsically vectorial, and ensures a direct
calculation of the stored and dissipated energies at any moment in time, and
hence not only on the completion of a closed hysteresis loop. We discuss the
variational formulation of this model and derive an efficient numerical scheme,
avoiding the usually employed approximation which can be inaccurate in the
vectorial case. The parameters of this model for a nonoriented steel are
identified using a set of first order reversal curves. Finally, the model is
incorporated as a local constitutive relation into a 2D finite element
simulation accounting for both the magnetic hysteresis and the eddy current. | cond-mat_other |
Energy flow of moving dissipative topological solitons: We study the energy flow due to the motion of topological solitons in
nonlinear extended systems in the presence of damping and driving. The total
field momentum contribution to the energy flux, which reduces the soliton
motion to that of a point particle, is insufficient. We identify an additional
exchange energy flux channel mediated by the spatial and temporal inhomogeneity
of the system state. In the well-known case of a DC external force the
corresponding exchange current is shown to be small but non-zero. For the case
of AC driving forces, which lead to a soliton ratchet, the exchange energy flux
mediates the complete energy flow of the system. We also consider the case of
combination of AC and DC external forces, as well as spatial discretization
effects. | cond-mat_other |
Nucleation at quantized vortices and the heterogeneous phase separation
in supersaturated superfluid 3He-4He liquid mixtures: Supersaturated superfluid 3He-4He liquid mixture, separating into the
3He-concentrated c-phase and 3He-diluted d-phase, represents a unique
possibility for studying macroscopic quantum nucleation and quantum
phase-separation kinetics in binary mixtures at low temperatures down to
absolute zero. One of possible heterogeneous mechanisms for the phase
separation of supersaturated d-phase is associated with superfluidity of this
phase and with a possible existence of quantized vortices playing a role of
nucleation sites for the c-phase of liquid mixture. We analyze the growth
dynamics of vortex core filled with the c-phase and determine the temperature
behavior of c-phase nucleation rate and the crossover temperature between the
classical and quantum nucleation mechanisms. | cond-mat_other |
Logarithmic velocity profile of quantum turbulence of superfluid $^4$He: The logarithmic velocity profile is the most important statistical law of
classical turbulence affected by channel walls. This paper demonstrates
numerically that the logarithmic velocity profile of a superfluid flow appears
in quantum turbulence under pure normal flow in a channel. We investigated the
configuration and dynamics of an inhomogeneous vortex tangle affected by the
walls, and found the characteristic behavior of the log-law. | cond-mat_other |
Rayleigh-Taylor instability of crystallization waves at the
superfluid-solid 4He interface: At the superfluid-solid 4He interface there exist crystallization waves
having much in common with gravitational-capillary waves at the interface
between two normal fluids. The Rayleigh-Taylor instability is an instability of
the interface which can be realized when the lighter fluid is propelling the
heavier one. We investigate here the analogues of the Rayleigh-Taylor
instability for the superfluid-solid 4He interface. In the case of a uniformly
accelerated interface the instability occurs only for a growing solid phase
when the magnitude of the acceleration exceeds some critical value independent
of the surface stiffness. For the Richtmyer-Meshkov limiting case of an
impulsively accelerated interface, the onset of instability does not depend on
the sign of the interface acceleration. In both cases the effect of
crystallization wave damping is to reduce the perturbation growth-rate of the
Taylor unstable interface. | cond-mat_other |
Tunneling and Resonant Conductance in One-Dimensional Molecular
Structures: We present a theory of tunneling and resonant transitions in one-dimensional
molecular systems which is based on Green's function theory of electron
sub-barrier scattering off the structural units (or functional groups) of a
molecular chain. We show that the many-electron effects are of paramount
importance in electron transport and they are effectively treated using a
formalism of sub-barrier scattering operators. The method which calculates the
total scattering amplitude of the bridge molecule not only predicts the
enhancement of the amplitude of tunneling transitions in course of tunneling
electron transfer through one-dimensional molecular structures but also allows
us to interpret conductance mechanisms by calculating the bound energy spectrum
of the tunneling electron, the energies being obtained as poles of the total
scattering amplitude of the bridge molecule. We found that the resonant
tunneling via bound states of the tunneling electron is the major mechanism of
electron conductivity in relatively long organic molecules. The sub-barrier
scattering technique naturally includes a description of tunneling in applied
electric fields which allows us to calculate I-V curves at finite bias. The
developed theory is applied to explain experimental findings such as bridge
effect due to tunneling through organic molecules, and threshold versus Ohmic
behavior of the conductance due to resonant electron transfer. | cond-mat_other |
Electronic Structure and Dynamics of Quantum-Well States in thin
Yb-Metal Films: Quantum-well states above the Fermi energy in thin Yb(111)-metal films
deposited on a W(110) single crystal were studied by low-temperature scanning
tunneling spectroscopy. These states are laterally highly localized and give
rise to sharp peaks in the tunneling spectra. A quantitative analysis of the
spectra yields the bulk-band dispersion in Gamma - L direction as well as
quasi-particle lifetimes. The quadratic energy dependence of the lifetimes is
in quantitative agreement with Fermi-liquid theory. | cond-mat_other |
Crystal truncation rods in kinematical and dynamical x-ray diffraction
theories: Crystal truncation rods calculated in the kinematical approximation are shown
to quantitatively agree with the sum of the diffracted waves obtained in the
two-beam dynamical calculations for different reflections along the rod. The
choice and the number of these reflections are specified. The agreement extends
down to at least $\sim 10^{-7}$ of the peak intensity. For lower intensities,
the accuracy of dynamical calculations is limited by truncation of the electron
density at a mathematically planar surface, arising from the Fourier series
expansion of the crystal polarizability. | cond-mat_other |
Structure and magnetic properties of the Ho2Ge2O7 pyrogermanate: We report the anisotropic magnetic properties of Ho2Ge2O7 determined from dc
and ac magnetization, specific heat and powder neutron diffraction experiments.
The magnetic lanthanide sublattice, seen in our refinement of the tetragonal
pyrogermanate crystal structure, is a right-handed spiral of edge-sharing and
corner-sharing triangles; the local Ho-O coordination indicates that the
crystal field is anisotropic. Susceptibility and magnetization data indeed show
that the magnetism is highly anisotropic, and the magnetic structure has the Ho
moments confined to the plane perpendicular to the structural spiral. The
ordered moment of Ho3+, as determined from refinement of the neutron
diffraction data, is 9.0 mu_B. Magnetic ordering occurs around 1.6 K.
Temperature and field dependent ac susceptibility measurements show that this
compound displays spin relaxation phenomena analogous to what is seen in the
spin ice pyrochlore system Ho2Ti2O7. | cond-mat_other |
Bragg spectroscopy of a strongly interacting Fermi gas: We present a comprehensive study of the Bose-Einstein condensate to
Bardeen-Cooper-Schrieffer (BEC-BCS) crossover in fermionic $^6$Li using Bragg
spectroscopy. A smooth transition from molecular to atomic spectra is observed
with a clear signature of pairing at and above unitarity. These spectra probe
the dynamic and static structure factors of the gas and provide a direct link
to two-body correlations. We have characterised these correlations and measured
their density dependence across the broad Feshbach resonance at 834 G. | cond-mat_other |
Critical temperature of a trapped Bose gas: comparison of theory and
experiment: We apply the Projected Gross-Pitaevskii equation (PGPE) formalism to the
experimental problem of the shift in critical temperature $T_c$ of a
harmonically confined Bose gas as reported in Gerbier \emph{et al.} [Phys. Rev.
Lett. \textbf{92}, 030405 (2004)]. The PGPE method includes critical
fluctuations and we find the results differ from various mean-field theories,
and are in best agreement with experimental data. To unequivocally observe
beyond mean-field effects, however, the experimental precision must either
improve by an order of magnitude, or consider more strongly interacting
systems. This is the first application of a classical field method to make
quantitative comparison with experiment. | cond-mat_other |
Entanglement between particle partitions in itinerant many-particle
states: We review `particle partitioning entanglement' for itinerant many-particle
systems. This is defined as the entanglement between two subsets of particles
making up the system. We identify generic features and mechanisms of particle
entanglement that are valid over whole classes of itinerant quantum systems. We
formulate the general structure of particle entanglement in many-fermion ground
states, analogous to the `area law' for the more usually studied entanglement
between spatial regions. Basic properties of particle entanglement are first
elucidated by considering relatively simple itinerant models. We then review
particle-partitioning entanglement in quantum states with more intricate
physics, such as anyonic models and quantum Hall states. | cond-mat_other |
Interference of a variable number of coherent atomic sources: We have studied the interference of a variable number of independently
created $m_F=0$ microcondensates in a CO$_{2}$-laser optical lattice. The
observed average interference contrast decreases with condensate number N. Our
experimental results agree well with the predictions of a random walk model.
While the exact result can be given in terms of Kluyver's formula, for a large
number of sources a $1/\sqrt{N}$ scaling of the average fringe contrast is
obtained. This scaling law is found to be of more general applicability when
quantifying the decay of coherence of an ensemble with N independently phased
sources. | cond-mat_other |
Synchronized and Desynchronized Phases of Exciton-Polariton Condensates
in the Presence of Disorder: Condensation of exciton-polaritons in semiconductor microcavities takes place
despite in plane disorder. Below the critical density the inhomogeneity of the
potential seen by the polaritons strongly limits the spatial extension of the
ground state. Above the critical density, in presence of weak disorder, this
limitation is spontaneously overcome by the non linear interaction, resulting
in an extended synchronized phase. This mechanism is clearly evidenced by
spatial and spectral studies, coupled to interferometric measurements. In case
of strong disorder, several non phase-locked (independent) condensates can be
evidenced. The transition from synchronized phase to desynchronized phase is
addressed considering multiple realizations of the disorder. | cond-mat_other |
Observation of Mass Transport through Solid 4He: By use of a novel experimental design, one that provides for superfluid
helium in contact with bulk hcp 4He off the melting curve, we have observed the
DC transport of mass through a cell filled with solid 4He in the hcp region of
the phase diagram. Flow, which shows characteristics of a superflow, is seen to
be independent of the method used to grow the solid, but depends on pressure
and temperature. The temperature dependence suggests the possibility of
hysteresis. | cond-mat_other |
Spin Transfer Switching and Spin Polarization in Magnetic Tunnel
Junctions with Mgo and Alox Barriers: We present spin transfer switching results for MgO based magnetic tunneling
junctions (MTJs)with large tunneling magnetoresistance (TMR) ratio of up to
150% and low intrinsic switching current density of 2-3 x 10 MA/cm2. The
switching data are compared to those obtained on similar MTJ nanostructures
with AlOx barrier. It is observed that the switching current density for MgO
based MTJs is 3-4 times smaller than that for AlOx based MTJs, and that can be
attributed to higher tunneling spin polarization (TSP) in MgO based MTJs. In
addition, we report a qualitative study of TSP for a set of samples, ranging
from 0.22 for AlOx to 0.46 for MgO based MTJs, and that shows the TSP (at
finite bias) responsible for the current-driven magnetization switching is
suppressed as compared to zero-bias tunneling spin polarization determined from
TMR. | cond-mat_other |
Rotational Analog of the Hall Effect: Coriolis Contribution to Electric
Current: A galvanogyroscopic effect which is the rotational analog of the
gravitomagnetic Hall effect has been proposed. As a consequence of Ohm's law in
the rotating frame, the effect of the Coriolis force on the conduction current
is predicted to give rise to an azimuthal potential difference $V_{gg}$ about
$10^{-3}V$ in a spinning rotor carrying radial electric current $i_r$. The
potential difference developed by the galvanogyroscopic effect is proportional
both to angular velocity ${\mathbf \Omega}$ and to the electric current. | cond-mat_other |
Phase diagrams of the Bose-Hubbard model at finite temperature: The phase transitions in the Bose-Hubbard model are investigated. A
single-particle Green's function is calculated in the random phase
approximation and the formalism of the Hubbard operators is used. The regions
of existence of the superfluid and Mott insulator phases are established and
the $(\mu,t)$ (the chemical potential -- transfer parameter) phase diagrams are
built. The influence of temperature change on this transition is analyzed and
the phase diagram in the $(T,\mu)$ plane is constructed. The role of thermal
activation of the ion hopping is investigated by taking into account the
temperature dependence of the transfer parameter. The reconstruction of the
Mott-insulator lobes due to this effect is analyzed. | cond-mat_other |
Charge qubit entanglement in double quantum dots: We study entanglement of charge qubits in a vertical tunnel-coupled double
quantum dot containing two interacting electrons. Exact diagonalization is used
to compute the negativity characterizing entanglement. We find that
entanglement can be efficiently generated and controlled by sidegate voltages,
and describe how it can be detected. For large enough tunnel coupling, the
negativity shows a pronounced maximum at an intermediate interaction strength
within the Wigner molecule regime. | cond-mat_other |
Unusual magnetic behavior in ferrite hollow nanospheres: We report unusual magnetic behavior in iron oxide hollow nanospheres of 9.3
$nm$ in diameter. The large fraction of atoms existing at the inner and outer
surfaces gives rise to a high magnetic disorder. The overall magnetic behavior
can be explained considering the coexistence of a soft superparamagnetic phase
and a hard phase corresponding to the highly frustrated cluster-glass like
phase at the surface regions. | cond-mat_other |
Dynamics of the quantum Duffing oscillator in the driving induced
bistable regime: We investigate the nonlinear response of an anharmonic monostable quantum
mechanical resonator to strong external periodic driving. The driving thereby
induces an effective bistability in which resonant tunneling can be identified.
Within the framework of a Floquet analysis, an effective Floquet-Born-Markovian
master equation with time-independent coefficients can be established which can
be solved straightforwardly. Various effects including resonant tunneling and
multi-photon transitions will be described. Our model finds applications in
nano-electromechanical devices such as vibrating suspended nano-wires as well
as in non-destructive read-out procedures for superconducting quantum bits
involving the nonlinear response of the read-out SQUID. | cond-mat_other |
Precision measurement of spin-dependent interaction strengths for spin-1
and spin-2 87Rb atoms: We report on precision measurements of spin-dependent interaction-strengths
in the 87Rb spin-1 and spin-2 hyperfine ground states. Our method is based on
the recent observation of coherence in the collisionally driven spin-dynamics
of ultracold atom pairs trapped in optical lattices. Analysis of the Rabi-type
oscillations between two spin states of an atom pair allows a direct
determination of the coupling parameters in the interaction hamiltonian. We
deduce differences in scattering lengths from our data that can directly be
compared to theoretical predictions in order to test interatomic potentials.
Our measurements agree with the predictions within 20%. The knowledge of these
coupling parameters allows one to determine the nature of the magnetic ground
state. Our data imply a ferromagnetic ground state for 87Rb in the f=1
manifold, in agreement with earlier experiments performed without the optical
lattice. For 87Rb in the f=2 manifold the data points towards an
antiferromagnetic ground state, however our error bars do not exclude a
possible cyclic phase. | cond-mat_other |
Plasma mechanisms of resonant terahertz detection in two-dimensional
electron channel with split gates: We analyze the operation of a resonant detector of terahertz (THz) radiation
based on a two-dimensional electron gas (2DEG) channel with split gates. The
side gates are used for the excitation of plasma oscillations by incoming THz
radiation and control of the resonant plasma frequencies. The central gate
provides the potential barrier separating the source and drain portions of the
2DEG channel. Two possible mechanisms of the detection are considered: (1)
modulation of the ac potential drop across the barrier and (2) heating of the
2DEG due to the resonant plasma-assisted absorption of THz radiation followed
by an increase in thermionic dc current through the barrier. Using the device
model we calculate the frequency and temperature dependences of the detector
responsivity associated with both dynamic and heating (bolometric) mechanisms.
It is shown that the dynamic mechanisms dominates at elevated temperatures,
whereas the heating mechanism provides larger contribution at low temperatures,
T=35-40 K. | cond-mat_other |
Kramers-Kronig constrained variational analysis of optical spectra: A universal method of extraction of the complex dielectric function
$\epsilon(\omega)=\epsilon_{1}(\omega)+i\epsilon_{2}(\omega)$ from
experimentally accessible optical quantities is developed. The central idea is
that $\epsilon_{2}(\omega)$ is parameterized independently at each node of a
properly chosen anchor frequency mesh, while $\epsilon_{1}(\omega)$ is
dynamically coupled to $\epsilon_{2}(\omega)$ by the Kramers-Kronig (KK)
transformation. This approach can be regarded as a limiting case of the
multi-oscillator fitting of spectra, when the number of oscillators is of the
order of the number of experimental points. In the case of the normal-incidence
reflectivity from a semi-infinite isotropic sample the new method gives
essentially the same result as the conventional KK transformation of
reflectivity. In contrast to the conventional approaches, the proposed
technique is applicable, without readaptation, to virtually all types of
linear-response optical measurements, or arbitrary combinations of
measurements, such as reflectivity, transmission, ellipsometry {\it etc.}, done
on different types of samples, including thin films and anisotropic crystals. | cond-mat_other |
Quantum confinement corrections to the capacitance of gated
one-dimensional nanostructures: With the help of a multi-configurational Green's function approach we
simulate single-electron Coulomb charging effects in gated ultimately scaled
nanostructures which are beyond the scope of a selfconsistent mean-field
description. From the simulated Coulomb-blockade characteristics we derive
effective system capacitances and demonstrate how quantum confinement effects
give rise to corrections. Such deviations are crucial for the interpretation of
experimentally determined capacitances and the extraction of
application-relevant system parameters. | cond-mat_other |
Comment on "Magnetic quantum oscillations of the conductivity in layered
conductors": We discuss the recent theory of Gvozdikov [Phys. Rev. B 70, 085113 (2004)]
which aims at explaining the Shubnikov-de Haas oscillations of the longitudinal
resistivity \rho_zz observed in the quasi-two-dimensional organic compound
\beta''-(BEDT-TTF)_2SF_5CH_2CF_2SO_3.
We point out that the self-consistent equations of the theory yielding the
longitudinal resistivity and the magnetic field dependence of the chemical
potential have been incorrectly solved. We show that the consideration of the
self-consistent Born approximation (which determines the relaxation rate in
Gvozdikov's paper) leads in fact to the complete absence of the longitudinal
conductivity \sigma_{zz} at leading order in high magnetic fields. | cond-mat_other |
Derivation of phenomenological expressions for transition matrix
elements for electron-phonon scattering: In the literature on electron-phonon scatterings very often a
phenomenological expression for the transition matrix element is used which was
derived in the textbooks of Ashcroft/Mermin and of Czycholl. There are various
steps in the derivation of this expression. In the textbooks in part different
arguments have been used in these steps, but the final result is the same. In
the present paper again slightly different arguments are used which motivate
the procedure in a more intuitive way. Furthermore, we generalize the
phenomenological expression to describe the dependence of the matrix elements
on the spin state of the initial and final electron state. | cond-mat_other |
Dislocation Mobility and Anomalous Shear Modulus Effect in $^4$He
Crystals: We calculate the dislocation glide mobility in solid $^4$He within a model
that assumes the existence of a superfluid field associated with dislocation
lines. Prompted by the results of this mobility calculation, we study within
this model the role that such a superfluid field may play in the motion of the
dislocation line when a stress is applied to the crystal. To do this, we relate
the damping of dislocation motion, calculated in the presence of the assumed
superfluid field, to the shear modulus of the crystal. As the temperature
increases, we find that a sharp drop in the shear modulus will occur at the
temperature where the superfluid field disappears. We compare the drop in shear
modulus of the crystal arising from the temperature dependence of the damping
contribution due to the superfluid field, to the experimental observation of
the same phenomena in solid $^4$He and find quantitative agreement. Our results
indicate that such a superfluid field plays an important role in dislocation
pinning in a clean solid $^4$He at low temperatures and in this regime may
provide an alternative source for the unusual elastic phenomena observed in
solid $^4$He. | cond-mat_other |
Structural phase transitions in epitaxial perovskite films: Three different film systems have been systematically investigated to
understand the effects of strain and substrate constraint on the phase
transitions of perovskite films. In SrTiO$_3$ films, the phase transition
temperature T$_C$ was determined by monitoring the superlattice peaks
associated with rotations of TiO$_6$ octahedra. It is found that T$_C$ depends
on both SrTiO$_3$ film thickness and SrRuO$_3$ buffer layer thickness. However,
lattice parameter measurements showed no sign of the phase transitions,
indicating that the tetragonality of the SrTiO$_3$ unit cells was no longer a
good order parameter. This signals a change in the nature of this phase
transition, the internal degree of freedom is decoupled from the external
degree of freedom. The phase transitions occur even without lattice relaxation
through domain formation. In NdNiO$_3$ thin films, it is found that the
in-plane lattice parameters were clamped by the substrate, while out-of-plane
lattice constant varied to accommodate the volume change across the phase
transition. This shows that substrate constraint is an important parameter for
epitaxial film systems, and is responsible for the suppression of external
structural change in SrTiO$_3$ and NdNiO$_3$ films. However, in SrRuO$_3$ films
we observed domain formation at elevated temperature through x-ray reciprocal
space mapping. This indicated that internal strain energy within films also
played an important role, and may dominate in some film systems. The final
strain states within epitaxial films were the result of competition between
multiple mechanisms and may not be described by a single parameter. | cond-mat_other |
Magnetic monopoles in a charged two-condensate Bose-Einstein system: We propose that a charged two-condensate Bose system possesses point-like
topological defects which can be interpreted as magnetic monopoles. By making
use of the $\phi$-mapping theory, the topological charges of these magnetic
monopoles can be expressed in terms of the Hopf indices and Brouwer degree of
the $\phi$-mapping. | 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 |
Influence of s-d scattering on the electron density of states in
ferromagnet/superconductor bilayer: We study the dependence of the electronic density of states (DOS) on the
distance from the boundary for a ferromagnet/superconductor bilayer. We
calculate the electron density of states in such structure taking into account
the two-band model of the ferromagnet (FM) with conducting s and localized d
electrons and a simple s-wave superconductor (SC). It is demonstrated that due
to the electron s-d scattering in the ferromagnetic layer in the third order of
s-d scattering parameter the oscillation of the density of states has larger
period and more drastic decrease in comparison with the oscillation period for
the electron density of states in the zero order. | cond-mat_other |
Entanglement area law in superfluid $^4$He: Area laws were first discovered by Bekenstein and Hawking, who found that the
entropy of a black hole grows proportional to its surface area, and not its
volume. Entropy area laws have since become a fundamental part of modern
physics, from the holographic principle in quantum gravity to ground state
wavefunctions of quantum matter, where entanglement entropy is generically
found to obey area law scaling. As no experiments are currently capable of
directly probing the entanglement area law in naturally occurring many-body
systems, evidence of its existence is based on studies of simplified theories.
Using new exact microscopic numerical simulations of superfluid $^4$He, we
demonstrate for the first time an area law scaling of entanglement entropy in a
real quantum liquid in three dimensions. We validate the fundamental principles
underlying its physical origin, and present an "entanglement equation of state"
showing how it depends on the density of the superfluid. | cond-mat_other |
Ground and excited-state fermions in a 1D double-well, exact and
time-dependent density-functional solutions: Two of the most popular quantum mechanical models of interacting fermions are
compared to each other and to potentially exact solutions for a pair of
contact-interacting fermions trapped in a 1D double-well potential, a model of
atoms in a quasi-1D optical lattice or electrons of a Hydrogen molecule in a
strong magnetic field. An exact few-body Hamiltonian is solved numerically in
momentum space yielding a highly-correlated eigenspectrum. Additionally,
approximate ground-state energies are obtained using both density functional
theory (DFT) functional and 2-site Hubbard models. A 1D adiabatic LDA kernel is
constructed for use in time-dependent density functional theory (TDDFT), and
the resulting excited-state spectrum is compared to the exact and Hubbard
results. DFT is shown to give accurate results for wells with small separations
but fails to describe localization of opposite spin fermions to different
sites. A locally cognizant (LC) density functional based on an effective local
fermion number would provide a solution to this problem, and an approximate
treatment presented here compares favorably with the exact and Hubbard results.
The TDDFT excited-state spectrum is accurate in the small parameter regime with
non-adiabatic effects accounting for any deviations. As expected, the
ground-state Hubbard model outperforms DFT at large separations but breaks down
at intermediate separations due to improper scaling to the united-atom limit.
At strong coupling, both Hubbard and TDDFT methods fail to capture the
appropriate energetics. | cond-mat_other |
Predicting scattering properties of ultracold atoms: adiabatic
accumulated phase method and mass scaling: Ultracold atoms are increasingly used for high precision experiments that can
be utilized to extract accurate scattering properties. This calls for a
stronger need to improve on the accuracy of interatomic potentials, and in
particular the usually rather inaccurate inner-range potentials. A boundary
condition for this inner range can be conveniently given via the accumulated
phase method. However, in this approach one should satisfy two conditions,
which are in principle conflicting, and the validity of these approximations
comes under stress when higher precision is required. We show that a better
compromise between the two is possible by allowing for an adiabatic change of
the hyperfine mixing of singlet and triplet states for interatomic distances
smaller than the separation radius. A mass scaling approach to relate
accumulated phase parameters in a combined analysis of isotopically related
atom pairs is described in detail and its accuracy is estimated, taking into
account both Born-Oppenheimer and WKB breakdown. We demonstrate how numbers of
singlet and triplet bound states follow from the mass scaling. | cond-mat_other |
Optimal design of fast topological pumping: Utilizing synthetic dimensions generated by spatial or temporal modulation,
topological pumping enables the exploration of higher-dimensional topological
phenomena through lower-dimensional physical systems. In this letter, we
propose a rational design paradigm of fast topological pumping based on 1D and
2D time-modulated discrete elastic lattices for the first time. Firstly, the
realization of topological pumping is ensured by introducing quantitative
indicators to drive a transition of the edge or corner state in the lattice
spectrum. Meanwhile, with the help of limiting speed for adiabaticity to
calculate the modulation time, a mathematical formulation of designing
topological pumping with the fastest modulation speed is presented. By applying
the proposed design paradigm, topological edge-bulk-edge and corner-bulk-corner
energy transport are successfully achieved, with 11.2 and 4.0 times of
improvement in modulation speed compared to classical pumping systems in the
literature. In addition, applying to 1D and 2D space-modulated systems, the
optimized modulation schemes can reduce the number of stacks to 5.3% and 26.8%
of the classical systems while ensuring highly concentrated energy transport.
This design paradigm is expected to be extended to the rational design of fast
topological pumping in other physical fields. | cond-mat_other |
Mapping Approach for Quantum-Classical Time Correlation Functions: The calculation of quantum canonical time correlation functions is considered
in this paper. Transport properties, such as diffusion and reaction rate
coefficients, can be determined from time integrals of these correlation
functions. Approximate, quantum-classical expressions for correlation
functions, which are amenable to simulation, are derived. These expressions
incorporate the full quantum equilibrium structure of the system but
approximate the dynamics by quantum-classical evolution where a quantum
subsystem is coupled to a classical environment. The main feature of the
formulation is the use of a mapping basis where the subsystem quantum states
are represented by fictitious harmonic oscillator states. This leads to a full
phase space representation of the dynamics that can be simulated without appeal
to surface-hopping methods. The results in this paper form the basis for new
simulation algorithms for the computation of quantum transport properties of
large many-body systems. | cond-mat_other |
Threshold behavior of bosonic two-dimensional few-body systems: Bosonic two-dimensional self-bound clusters consisting of $N$ atoms
interacting through additive van der Waals potentials become unbound at a
critical mass m*(N); m*(N) has been predicted to be independent of the size of
the system. Furthermore, it has been predicted that the ground state energy
E(N) of the N-atom system varies exponentially as the atomic mass approaches
m*. This paper reports accurate numerical many-body calculations that allow
these predictions to be tested. We confirm the existence of a universal
critical mass m*, and show that the near-threshold behavior can only be
described properly if a previously neglected term is included. We comment on
the universality of the energy ratio E(N+1)/E(N) near threshold. | cond-mat_other |
An Advice about Shimming in High-Resolution Nuclear Magnetic Resonance: Three methods of active shimming in high-resolution NMR in existence (manual
shimming, lock optimization and gradient shimming) are briefly discussed and
their advantages and shortcomings are compared and also an advice on their use
is given. | cond-mat_other |
Atom-to-molecule conversion efficiency and adiabatic fidelity: The efficiency of converting two-species fermionic atoms into bosonic
molecules is investigated in terms of mean-field Lagrangian density. We find
that the STIRAP technique aided by Feshbach resonance is more effective than
the bare Fechbach resonance for $^6$Li atoms rather than $^{40}$K atoms. We
also make general consideration on the symmetry and its relevant conservation
law, which enable us to introduce a natural definition of adiabatic fidelity
for CPT state. The calculated values of the fidelity then provide an
interpretation on why the conversion efficiencies for $^{40}$K and $^6$Li are
distinctly different. | cond-mat_other |
Exact BCS stochastic schemes for a time dependent many-body fermionic
system: The exact quantum state evolution of a fermionic gas with binary interactions
is obtained as the stochastic average of BCS-state trajectories. We find the
most general Ito stochastic equations which reproduce exactly the dynamics of
the system and we obtain some conditions to minimize the stochastic spreading
of the trajectories in the Hilbert space. The relation between the optimized
equations and mean-field equations is analyzed. The method is applied to a
simple two-site model. The simulations display effects that cannot be obtained
in the mean-field approximation. | cond-mat_other |
Pairing mean-field theory for the dynamics of dissociation of molecular
Bose-Einstein condensates: We develop a pairing mean-field theory to describe the quantum dynamics of
the dissociation of molecular Bose-Einstein condensates into their constituent
bosonic or fermionic atoms. We apply the theory to one, two, and
three-dimensional geometries and analyze the role of dimensionality on the atom
production rate as a function of the dissociation energy. As well as
determining the populations and coherences of the atoms, we calculate the
correlations that exist between atoms of opposite momenta, including the column
density correlations in 3D systems. We compare the results with those of the
undepleted molecular field approximation and argue that the latter is most
reliable in fermionic systems and in lower dimensions. In the bosonic case we
compare the pairing mean-field results with exact calculations using the
positive-$P$ stochastic method and estimate the range of validity of the
pairing mean-field theory. Comparisons with similar first-principle simulations
in the fermionic case are currently not available, however, we argue that the
range of validity of the present approach should be broader for fermions than
for bosons in the regime where Pauli blocking prevents complete depletion of
the molecular condensate. | cond-mat_other |
A random matrix approach to detect defects in a strongly scattering
polycrystal: how the memory effect can help overcome multiple scattering: We report on ultrasonic imaging in a random heterogeneous medium. The goal is
to detect flaws embedded deeply into a polycrystalline material. A 64-element
array of piezoelectric transmitters/receivers at a central frequency of 5 MHz
is used to capture the Green's matrix in a backscattering configuration.
Because of multiple scattering, conventional imaging completely fails to detect
the deepest flaws. We utilize a random matrix approach, taking advantage of the
deterministic coherence of the backscattered wave-field which is characteristic
of single scattering and related to the memory effect. This allows us to
separate single and multiple scattering contributions. As a consequence, we
show that flaws are detected beyond the conventional limit, as if multiple
scattering had been overcome. | cond-mat_other |
Spinor condensates with a laser-induced quadratic Zeeman effect: We show that an effective quadratic Zeeman effect can be generated in
$^{52}$Cr by proper laser configurations, and in particular by the dipole trap
itself. The induced quadratic Zeeman effect leads to a rich ground-state phase
diagram, can be used to induce topological defects by controllably quenching
across transitions between phases of different symmetries, allows for the
observability of the Einstein-de Haas effect for relatively large magnetic
fields, and may be employed to create $S=1/2$ systems with spinor dynamics.
Similar ideas could be explored in other atomic species opening an exciting new
control tool in spinor systems. | 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 |
X-ray Coherent diffraction interpreted through the fractional Fourier
transform: Diffraction of coherent x-ray beams is treated through the Fractionnal
Fourier transform. The transformation allow us to deal with coherent
diffraction experiments from the Fresnel to the Fraunhofer regime. The analogy
with the Huygens-Fresnel theory is first discussed and a generalized
uncertainty principle is introduced. | cond-mat_other |
Hall Effect of Light: We derive the semiclassical equation of motion for the wave-packet of light
taking into account the Berry curvature in the momentum space. This equation
naturally describes the interplay between the orbital and spin angular momenta,
i.e., the conservation of the total angular momentum of light. This leads to
the shift of the wave-packet motion perpendicular to the gradient of the
dielectric constant, i.e., the polarization-dependent Hall effect of light. An
enhancement of this effect in the photonic crystal is also proposed. | 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 |
Nonintegrable Schrodinger Discrete Breathers: In an extensive numerical investigation of nonintegrable translational motion
of discrete breathers in nonlinear Schrodinger lattices, we have used a
regularized Newton algorithm to continue these solutions from the limit of the
integrable Ablowitz-Ladik lattice. These solutions are shown to be a
superposition of a localized moving core and an excited extended state
(background) to which the localized moving pulse is spatially asymptotic. The
background is a linear combination of small amplitude nonlinear resonant plane
waves and it plays an essential role in the energy balance governing the
translational motion of the localized core. Perturbative collective variable
theory predictions are critically analyzed in the light of the numerical
results. | cond-mat_other |
Edge-localized states in quantum one-dimensional lattices: In one-dimensional quantum lattice models with open boundaries, we find and
study localization at the lattice edge. We show that edge-localized eigenstates
can be found in both bosonic and fermionic systems, specifically, in the
Bose-Hubbard model with on-site interactions and in the spinless fermion model
with nearest-neighbor interactions. We characterize the localization through
spectral studies via numerical diagonalization and perturbation theory, through
considerations of the eigenfunctions, and through the study of explicit time
evolution. We concentrate on few-particle systems, showing how more complicated
edge states appear as the number of particles is increased. | cond-mat_other |
Dynamical Aspects of Analogue Gravity: The Backreaction of Quantum
Fluctuations in Dilute Bose-Einstein Condensates: We discuss the backreaction force exerted by quantum fluctuations in dilute
Bose-Einstein condensates onto the motion of the classical background, derived
by an ab initio approach from microscopic physics. It is shown that the
effective-action method, widely employed in semiclassical quantum gravity,
fails to give the full backreaction force. The failure of the effective-action
method is traced back, inter alia, to the problem of the correct choice of the
fundamental variables and the related operator ordering issues. | cond-mat_other |
Stability of Inhomogeneous Multi-Component Fermi Gases: Two-component equal-mass Fermi gases, in which unlike atoms interact through
a short-range two-body potential and like atoms do not interact, are stable
even when the interspecies s-wave scattering length becomes infinitely large.
Solving the many-body Schroedinger equation within a hyperspherical framework
and by Monte Carlo techniques, this paper investigates how the properties of
trapped two-component gases change if a third or fourth component are added. If
all interspecies scattering lengths are equal and negative, our calculations
suggest that both three- and four-component Fermi gases become unstable for a
certain critical set of parameters. The relevant length scale associated with
the collapse is set by the interspecies scattering length and we argue that the
collapse is, similar to the collapse of an attractive trapped Bose gas, a
many-body phenomenon. Furthermore, we consider a three-component Fermi gas in
which two interspecies scattering lengths are negative while the other
interspecies scattering length is zero. In this case, the stability of the
Fermi system is predicted to depend appreciably on the range of the underlying
two-body potential. We find parameter combinations for which the system appears
to become unstable for a finite negative scattering length and parameter
combinations for which the system appears to be made up of weakly-bound trimers
that consist of one fermion of each species. | cond-mat_other |
Interferences in the density of two Bose-Einstein condensates consisting
of identical or different atoms: The density of two {\it initially independent} condensates which are allowed
to expand and overlap can show interferences as a function of time due to
interparticle interaction. Two situations are separately discussed and
compared: (1) all atoms are identical and (2) each condensate consists of a
different kind of atoms. Illustrative examples are presented. | cond-mat_other |
Adiabatic Transport of Bose-Einstein Condensate in Double- and
Triple-Well Traps: By using a close similarity between multi-photon and tunneling population
transfer schemes, we propose robust adiabatic methods for the transport of
Bose-Einstein condensate (BEC) in double- and triple-well traps. The
calculations within the mean-field approximation (Gross-Pitaevskii equation)
show that irreversible and complete transport takes place even in the presence
of the non-linear effects caused by interaction between BEC atoms. The transfer
is driven by adiabatic time-dependent monitoring the barriers and well depths.
The proposed methods are universal and can be applied to a variety of systems
and scenarios. | cond-mat_other |
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 |
Beyond the locality approximation in the standard diffusion Monte Carlo
method: We present a way to include non local potentials in the standard Diffusion
Monte Carlo method without using the locality approximation. We define a
stochastic projection based on a fixed node effective Hamiltonian, whose lowest
energy is an upper bound of the true ground state energy, even in the presence
of non local operators in the Hamiltonian. The variational property of the
resulting algorithm provides a stable diffusion process, even in the case of
divergent non local potentials, like the hard-core pseudopotentials. It turns
out that the modification required to improve the standard Diffusion Monte
Carlo algorithm is simple. | cond-mat_other |
On applicability of differential mixing rules for statistically
homogeneous and isotropic dispersions: The classical differential mixing rules are assumed to be independent
effective-medium approaches, applicable to certain classes of systems. In the
present work, the inconsistency of differential models for macroscopically
homogeneous and isotropic systems is illustrated with a model for the effective
permittivity of simple dielectric systems of impenetrable balls. The analysis
is carried out in terms of the compact group approach reformulated in a way
that allows one to analyze the role of different contributions to the
permittivity distribution in the system. It is shown that the asymmetrical
Bruggeman model (ABM) is physically inconsistent since the electromagnetic
interaction between previously added constituents and those being added is
replaced by the interaction of the latter with recursively formed effective
medium. The overall changes in the effective permittivity due to addition of
one constituent include the contributions from both constituents and depend on
the system structure before the addition. Ignoring the contribution from one of
the constituents, we obtain generalized versions of the original ABM mixing
rules. They still remain applicable only in a certain concentration ranges, as
is shown with the Hashin-Shtrikman bounds. The results obtained can be
generalized to macroscopically homogeneous and isotropic systems with complex
permittivities of constituents. | 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 |
Joule expansion of a pure many-body state: We derive the Joule expansion of an isolated perfect gas from the principles
of quantum mechanics. Contrary to most studies of irreversible processes which
consider composite systems, the gas many-body Hilbert space cannot be
factorised into Hilbert spaces corresponding to interesting and ignored degrees
of freedom. Moreover, the expansion of the gas into the entire accessible
volume is obtained for pure states. Still, the number particle density is
characterised by a chemical potential and a temperature. We discuss the special
case of a boson gas below the Bose condensation temperature. | cond-mat_other |
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