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A Time-Orbiting Potential Trap for Bose-Einstein Condensate
Interferometry: We describe a novel atom trap for Bose-Einstein condensates of 87Rb to be
used in atom interferometry experiments. The trap is based on a time-orbiting
potential waveguide. It supports the atoms against gravity while providing weak
confinement to minimize interaction effects. We observe harmonic oscillation
frequencies omega_x, omega_y, omega_z as low as 2 pi times (6.0,1.2,3.3) Hz. Up
to 2 times 10^4 condensate atoms have been loaded into the trap, at estimated
temperatures as low as 850 pK. We anticipate that interferometer measurement
times of 1 s or more should be achievable in this device. | cond-mat_other |
Diamagnetism and the dispersion of the magnetic permeability: It is well known that the usual Kramers--Kronig relations for the relative
permeability function $\mu(\omega)$ are not compatible with diamagnetism
($\mu(0)<1$) and a positive imaginary part ($\text{Im}\,\mu(\omega)>0$ for
$\omega>0$). We demonstrate that a certain physical meaning can be attributed
to $\mu$ for all frequencies, and that in the presence of spatial dispersion,
$\mu$ does not necessarily tend to 1 for high frequencies $\omega$ and fixed
wavenumber $\mathbf k$. Taking the asymptotic behavior into account,
diamagnetism can be compatible with Kramers--Kronig relations even if the
imaginary part of the permeability is positive. We provide several examples of
diamagnetic media and metamaterials for which $\mu(\omega,\mathbf k)\not\to 1$
as $\omega\to\infty$. | cond-mat_other |
Influence of topography and Co domain walls on the magnetization
reversal of the FeNi layer in FeNi/Al$\_2$O$\_3$/Co magnetic tunnel junctions: We have studied the magnetization reversal dynamics of FeNi/Al$\_2$O$\_3$/Co
magnetic tunnel junctions deposited on step-bunched Si substrates using
magneto-optical Kerr effect and time-resolved x-ray photoelectron emission
microscopy combined with x-ray magnetic circular dichroism (XMCD-PEEM).
Different reversal mechanisms have been found depending on the substrate miscut
angle. Larger terraces (smaller miscut angles) lead to a higher nucleation
density and stronger domain wall pinning. The width of domain walls with
respect to the size of the terraces seems to play an important role in the
reversal. We used the element selectivity of XMCD-PEEM to reveal the strong
influence of the stray field of domain walls in the hard magnetic layer on the
magnetic switching of the soft magnetic layer. | cond-mat_other |
Spin-Exchange Interaction in ZnO-based Quantum Wells: Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit
unprecedented tunability of the short-range spin exchange interaction. In the
context of large exciton binding energies and electron-hole exchange
interaction in ZnO, this tunability results from the competition between
quantum confinement and giant quantum confined Stark effect. By using
time-resolved photoluminescence we identify, for well widths under 3 nm, the
redistribution of oscillator strengths between the A and B excitonic
transitions, due to the enhancement of the exchange interaction. Conversely,
for wider wells, the redistribution is cancelled by the dominant effect of
internal electric fields, which dramatically reduce the exchange energy. | cond-mat_other |
A basis-set based Fortran program to solve the Gross-Pitaevskii Equation
for dilute Bose gases in harmonic and anharmonic traps: Inhomogeneous boson systems, such as the dilute gases of integral spin atoms
in low-temperature magnetic traps, are believed to be well described by the
Gross-Pitaevskii equation (GPE). GPE is a nonlinear Schroedinger equation which
describes the order parameter of such systems at the mean field level. In the
present work, we describe a Fortran 90 computer program developed by us, which
solves the GPE using a basis set expansion technique. In this technique, the
condensate wave function (order parameter) is expanded in terms of the
solutions of the simple-harmonic oscillator (SHO) characterizing the atomic
trap. Additionally, the same approach is also used to solve the problems in
which the trap is weakly anharmonic, and the anharmonic potential can be
expressed as a polynomial in the position operators x, y, and z. The resulting
eigenvalue problem is solved iteratively using either the self-consistent-field
(SCF) approach, or the imaginary time steepest-descent (SD) approach. Our
results for harmonic traps are also compared with those published by other
authors using different numerical approaches, and excellent agreement is
obtained. GPE is also solved for a few anharmonic potentials, and the influence
of anharmonicity on the condensate is discussed. Additionally, the notion of
Shannon entropy for the condensate wave function is defined and studied as a
function of the number of particles in the trap. It is demonstrated numerically
that the entropy increases with the particle number in a monotonic way. | cond-mat_other |
Fully three dimensional breather solitons can be created using Feshbach
resonance: We investigate the stability properties of breather solitons in a
three-dimensional Bose-Einstein Condensate with Feshbach Resonance Management
of the scattering length and con ned only by a one dimensional optical lattice.
We compare regions of stability in parameter space obtained from a fully 3D
analysis with those from a quasi two-dimensional treatment. For moderate con
nement we discover a new island of stability in the 3D case, not present in the
quasi 2D treatment. Stable solutions from this region have nontrivial dynamics
in the lattice direction, hence they describe fully 3D breather solitons. We
demonstrate these solutions in direct numerical simulations and outline a
possible way of creating robust 3D solitons in experiments in a Bose Einstein
Condensate in a one-dimensional lattice. We point other possible applications. | cond-mat_other |
Dynamical Instability of a Doubly Quantized Vortex in a Bose-Einstein
condensate: Doubly quantized vortices were topologically imprinted in $|F=1>$ $^{23}$Na
condensates, and their time evolution was observed using a tomographic imaging
technique. The decay into two singly quantized vortices was characterized and
attributed to dynamical instability. The time scale of the splitting process
was found to be longer at higher atom density. | cond-mat_other |
Rabi switch of condensate wavefunctions in a multicomponent Bose gas: Using a time-dependent linear (Rabi) coupling between the components of a
weakly interacting multicomponent Bose-Einstein condensate (BEC), we propose a
protocol for transferring the wavefunction of one component to the other. This
"Rabi switch" can be generated in a binary BEC mixture by an electromagnetic
field between the two components, typically two hyperfine states. When the
wavefunction to be transfered is - at a given time - a stationary state of the
multicomponent Hamiltonian, then, after a time delay (depending on the Rabi
frequency), it is possible to have the same wavefunction on the other
condensate. The Rabi switch can be used to transfer also moving bright
matter-wave solitons, as well as vortices and vortex lattices in
two-dimensional condensates. The efficiency of the proposed switch is shown to
be 100% when inter-species and intra-species interaction strengths are equal.
The deviations from equal interaction strengths are analyzed within a two-mode
model and the dependence of the efficiency on the interaction strengths and on
the presence of external potentials is examined in both 1D and 2D settings. | cond-mat_other |
Confinement-induced resonances for a two-component ultracold atom gas in
arbitrary quasi-one-dimensional traps: We solve the two-particle s-wave scattering problem for ultracold atom gases
confined in arbitrary quasi-one-dimensional trapping potentials, allowing for
two different atom species. As a consequence, the center-of-mass and relative
degrees of freedom do not factorize. We derive bound-state solutions and obtain
the general scattering solution, which exhibits several resonances in the 1D
scattering length induced by the confinement. We apply our formalism to two
experimentally relevant cases: (i) interspecies scattering in a two-species
mixture, and (ii) the two-body problem for a single species in a non-parabolic
trap. | cond-mat_other |
Strong coupling theory for the superfluidity of Bose-Fermi mixtures: We develop a strong-coupling theory for the superfluidity of fermion pairing
phase in a Bose-Fermi mixture. Dynamical screening, self-energy
renormalization, and a pairing gap function are included self-consistently
within the adiabatic limit (i.e., the phonon velocity is much smaller than the
Fermi velocity). An analytical solution for the transition temperature (Tc) is
derived within reasonable approximations. Using typical parameters of a
40K-87Rb mixture, we find that the calculated Tc is several times larger than
that obtained in the weak coupling theory, and can be up to several percents of
the Fermi temperature. | cond-mat_other |
Coexisting ordinary elasticity and superfluidity in a model of
defect-free supersolid: We present the mechanics of a model of supersolid in the frame of the
Gross-Pitaevskii equation at $T=0K$ that do not require defects nor vacancies.
A set of coupled nonlinear partial differential equations plus boundary
conditions is derived. The mechanical equilibrium is studied under external
constrains as steady rotation or external stress. Our model displays a
paradoxical behavior: the existence of a non classical rotational inertia
fraction in the limit of small rotation speed and no superflow under small (but
finite) stress nor external force. The only matter flow for finite stress is
due to plasticity. | cond-mat_other |
Spin dynamics for bosons in an optical lattice: We study the internal dynamics of bosonic atoms in an optical lattice. Within
the regime in which the atomic crystal is a Mott insulator with one atom per
well, the atoms behave as localized spins which interact according to some spin
Hamiltonian. The type of Hamiltonian (Heisenberg, Ising), and the sign of
interactions may be tuned by changing the properties of the optical lattice, or
applying external magnetic fields. When, on the other hand, the number of atoms
per lattice site is unknown, we can still use the bosons to perform general
quantum computation. | cond-mat_other |
Extracting spectral density function of a binary composite without
a-priori assumption: The spectral representation separates the contributions of geometrical
arrangement (topology) and intrinsic constituent properties in a composite. The
aim of paper is to present a numerical algorithm based on the Monte Carlo
integration and contrainted-least-squares methods to resolve the spectral
density function for a given system. The numerical method is verified by
comparing the results with those of Maxwell-Garnett effective permittivity
expression. Later, it is applied to a well-studied rock-and-brine system to
instruct its utility. The presented method yields significant microstructural
information in improving our understanding how microstructure influences the
macroscopic behaviour of composites without any intricate mathematics. | cond-mat_other |
STM Studies of TbTe3: Evidence for a fully Incommensurate Charge Density
Wave: We observe unidirectional charge density wave ordering on the cleaved surface
of TbTe3 with a Scanning Tunneling Microscope at ~6 K. The modulation
wave-vector q_{CDW} as determined by Fourier analysis is 0.71 +/- 0.02 * 2
pi/c. (Where c is one edge of the in-plane 3D unit cell.) Images at different
tip-sample voltages show the unit cell doubling effects of dimerization and the
layer below. Our results agree with bulk X-ray measurements, with the addition
of ~(1/3) * 2 pi/a ordering perpendicular to the CDW. Our analysis indicates
that the CDW is incommensurate. | cond-mat_other |
Energy and Vorticity Spectra in Turbulent Superfluid $^4$He from $T=0$
to $T_λ$: We discuss the energy and vorticity spectra of turbulent superfluid $^4$He in
all the temperature range from $T=0$ up to the phase transition "$\lambda$
point", $T_\lambda\simeq 2.17\,$K. Contrary to classical developed turbulence
in which there are only two typical scales, i.e. the energy injection $L$ and
the dissipation scales $\eta$, here the quantization of vorticity introduces
two additional scales, i.e the vortex core radius $a_0$ and the mean vortex
spacing $\ell$. We present these spectra for the super- and normal-fluid
components in the entire range of scales from $L$ to $a_0$ including the
cross-over scale $\ell$ where the hydrodynamic eddy-cascade is replaced by the
cascade of Kelvin waves on individual vortices. At this scale a bottleneck
accumulation of the energy was found earlier at $T=0$.
We show that even very small mutual friction dramatically suppresses the
bottleneck effect due to the dissipation of the Kelvin waves. Using our results
for the spectra we estimate the Vinen "effective viscosity" $\nu'$ in the
entire temperature range and show agreement with numerous experimental
observation for $\nu'(T)$. | cond-mat_other |
Imaging of s and d partial-wave interference in quantum scattering of
identical bosonic atoms: We report on the direct imaging of s and d partial-wave interference in cold
collisions of atoms. Two ultracold clouds of Rb87 atoms were accelerated by
magnetic fields to collide at energies near a d-wave shape resonance. The
resulting halos of scattered particles were imaged using laser absorption. By
scanning across the resonance we observed a marked evolution of the scattering
patterns due to the energy dependent phase shifts for the interfering s and d
waves. Since only two partial wave states are involved in the collision process
the scattering yield and angular distributions have a simple interpretation in
terms of a theoretical model. | cond-mat_other |
Mean-field dynamics of a two-mode Bose-Einstein condensate subject to
noise and dissipation: We discuss the dynamics of an open two-mode Bose-Hubbard system subject to
phase noise and particle dissipation. Starting from the full many-body dynamics
described by a master equation the mean-field limit is derived resulting in an
effective non-hermitian (discrete) Gross-Pitaevskii equation which has been
introduced only phenomenologically up to now. The familiar mean-field phase
space structure is substantially altered by the dissipation. Especially the
character of the fixed points shows an abrupt transition from elliptic or
hyperbolic to attractiv or repulsive, respectively. This reflects the
metastable behaviour of the corresponding many-body system which surprisingly
also leads to a significant increase of the purity of the condensate. A
comparison of the mean-field approximation to simulations of the full master
equation using the Monte Carlo wave function method shows an excellent
agreement for wide parameter ranges. | cond-mat_other |
Probing Strong Correlations with Light Scattering: the Example of the
Quantum Ising model: In this paper we calculate the nonlinear susceptibility and the resonant
Raman cross section for the paramagnetic phase of the ferromagnetic Quantum
Ising model in one dimension. In this region the spectrum of the Ising model
has a gap $m$. The Raman cross section has a strong singularity when the energy
of the outgoing photon is at the spectral gap $\omega_{f} \approx m$ and a
square root threshold when the frequency difference between the incident and
outgoing photons $\omega_{i} -\omega_{f} \approx 2m$. The latter feature
reflects the fermionic nature of the Ising model excitations. | cond-mat_other |
Natural orbits of atomic Cooper pairs in a nonuniform Fermi gas: We examine the basic mode structure of atomic Cooper pairs in an
inhomogeneous Fermi gas. Based on the properties of Bogoliubov quasi-particle
vacuum, the single particle density matrix and the anomalous density matrix
share the same set of eigenfunctions. These eigenfunctions correspond to
natural pairing orbits associated with the BCS ground state. We investigate
these orbits for a Fermi gas in a spherical harmonic trap, and construct the
wave function of a Cooper pair in the form of Schmidt decomposition. The issue
of spatial quantum entanglement between constituent atoms in a pair is
addressed. | cond-mat_other |
Cluster states of Fermions in the single l-shell model: The paper concerns the ground state structure of the partly filled l-shell of
a fermionic gas of atoms of spin s in a spherically symmetric spin independent
trap potential. At particle numbers N=n(2s+1), n=1,2,...,2l+1 the basic
building blocks are clusters consisting of (2s+1) atoms, whose wave functions
are completely symmetric and antisymmetric in space and spin variables,
respectively. The creation operator of a cluster is constructed whose repeated
application to the vacuum leads to the multi-cluster state. Ground state energy
expressions are derived for the n-cluster states at different l,s values and
interpreted in simple terms. | cond-mat_other |
On the Order Parameter of the Continuous Phase Transition in the
Classical and Quantum Mechanical limits: The mean field theory is revisited in the classical and quantum mechanical
limits. Taking into account the boundary conditions at the phase transition and
the third law of the thermodynamics the physical properties of the ordered and
disordered phases were reported. The equation for the order parameter predicts
the occurrence of a saturation of $\Psi^2$ = 1 near $\Theta_S$, the temperature
below the quantum mechanical ground state is reached. The theoretical
predictions are also compared with high resolution thermal expansion data of
SrTiO$_{\text{3}}$ monocrystalline samples and other some previous results. An
excellent agreement has been found suggesting a universal behavior of the
theoretical model to describe continuous structural phase transitions. | cond-mat_other |
Reply to Comment on "Dynamics of the Density of Quantized Vortex-Lines
in Superfluid Turbulence": This is a Reply to Nemirovskii Comment [Phys. Rev. B 94, 146501 (2016)] on
the Khomenko et al, [Phys.Rev. B v.91, 180504(2016)], in which a new form of
the production term in Vinen's equation for the evolution of the vortex-line
density $\cal L$ in the thermal counterflow of superfluid $^4$He in a channel
was suggested. To further substantiate the suggested form which was questioned
in the Comment, we present a physical explanation for the improvement of the
closure suggested in Khomenko et al [Phys.Rev. B v. 91, 180504(2016)] in
comparison to the form proposed by Vinen. We also discuss the closure for the
flux term, which agrees well with the numerical results without any fitting
parameters. | cond-mat_other |
Sufficient conditions for two-dimensional localization by arbitrarily
weak defects in periodic potentials with band gaps: We prove, via an elementary variational method, 1d and 2d localization within
the band gaps of a periodic Schrodinger operator for any mostly negative or
mostly positive defect potential, V, whose depth is not too great compared to
the size of the gap. In a similar way, we also prove sufficient conditions for
1d and 2d localization below the ground state of such an operator. Furthermore,
we extend our results to 1d and 2d localization in d dimensions; for example, a
linear or planar defect in a 3d crystal. For the case of D-fold degenerate band
edges, we also give sufficient conditions for localization of up to D states. | cond-mat_other |
Self-consistent calculation of semiconductor heterojunctions by using
quantum genetic algorithm: In this study, we have investigated the ground state energy level of
electrons in modulation doped GaAs/AlxGa1-xAs heterojunctions. For this
purpose, Schrodinger and Poisson equations are solved self consistently using
quantum genetic algorithm (QGA). Thus, we have found the potential profile, the
ground state subband energy and their corresponding envelope functions, Fermi
level, and the amount of tunneling charge from barrier to channel region. Their
dependence on various device parameters are also examined. | cond-mat_other |
Residual attractive force between superparamagnetic nanoparticles: A superparamagnetic nanoparticle (SPN) is a nanometre-sized piece of a
material that would, in bulk, be a permanent magnet. In the SPN the individual
atomic spins are aligned via Pauli effects into a single giant moment that has
easy orientations set by shape or magnetocrystalline anisotropy. Above a
size-dependent blocking temperature $T_{b}(V,\tau_{obs})$, thermal fluctuations
destroy the average moment by flipping the giant spin between easy orientations
at a rate that is rapid on the scale of the observation time $\tau_{obs}$.
We show that, depite the vanising of the average moment, two SPNs experience
a net attractive force of magnetic origin, analogous to the van der Waals force
between molecules that lack a permanent electric dipole. This could be relevant
for ferrofluids, for the clumping of SPNs used for drug delivery, and for
ultra-dense magnetic recording media. | cond-mat_other |
Spin wave diffraction model for perpendicularly magnetized films: We present a near-field diffraction model for spin waves in perpendicularly
magnetized films applicable in any geometries of excitation fields. This model
relies on Kalinikos-Slavin formalism to express the dynamic susceptibility
tensor in k-space, and calculate the diffraction patterns via inverse
2D-Fourier transform of the response functions. We show an excellent
quantitative agreement between our model and MuMax3 micro-magnetic simulations
on two different geometries of antennas. Our method benchmarks spin wave
diffraction in perpendicularly magnetized films, and is readily applicable for
future designs of magnon beamforming and interferometric devices. | cond-mat_other |
Extraordinary wetting phase diagram for mixtures of Bose-Einstein
condensates: The possibility of wetting phase transitions in Bose-Einstein condensed gases
is predicted on the basis of Gross-Pitaevskii theory. The surface of a binary
mixture of Bose-Einstein condensates can undergo a first-order wetting phase
transition upon varying the interparticle interactions, using, e.g., Feshbach
resonances. Interesting ultralow-temperature effects shape the wetting phase
diagram. The prewetting transition is, contrary to general expectations, not of
first order but critical, and the prewetting line does not meet the bulk phase
coexistence line tangentially. Experimental verification of these extraordinary
results is called for, especially now that it has become possible, using
optical methods, to realize a planar "hard wall" boundary for the condensates. | cond-mat_other |
Three-fermion problems in optical lattices: We present exact results for the spectra of three fermionic atoms in a single
well of an optical lattice. For the three lowest hyperfine states of Li6 atoms,
we find a Borromean state across the region of the distinct pairwise Feshbach
resonances. For K40 atoms, nearby Feshbach resonances are known for two of the
pairs, and a bound three-body state develops towards the positive
scattering-length side. In addition, we study the sensitivity of our results to
atomic details. The predicted few-body phenomena can be realized in optical
lattices in the limit of low tunneling. | cond-mat_other |
Coherent spinor dynamics in a spin-1 Bose condensate: Collisions in a thermal gas are perceived as random or incoherent as a
consequence of the large numbers of initial and final quantum states accessible
to the system. In a quantum gas, e.g. a Bose-Einstein condensate or a
degenerate Fermi gas, the phase space accessible to low energy collisions is so
restricted that collisions be-come coherent and reversible. Here, we report the
observation of coherent spin-changing collisions in a gas of spin-1 bosons.
Starting with condensates occupying two spin states, a condensate in the third
spin state is coherently and reversibly created by atomic collisions. The
observed dynamics are analogous to Josephson oscillations in weakly connected
superconductors and represent a type of matter-wave four-wave mixing. The
spin-dependent scattering length is determined from these oscillations to be
-1.45(18) Bohr. Finally, we demonstrate coherent control of the evolution of
the system by applying differential phase shifts to the spin states using
magnetic fields. | 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 |
Suppression of Superfluidity of $^4$He in a Nanoporous Glass by
Preplating a Kr Layer: Helium in nanoporous media has attracted much interest as a model Bose system
with disorder and confinement. Here we have examined how a change in porous
structure by preplating a monolayer of krypton affects the superfluid
properties of $^4$He adsorbed or confined in a nanoporous Gelsil glass, which
has a three-dimensional interconnected network of nanopores of 5.8 nm in
diameter. Isotherms of adsorption and desorption of nitrogen show that
monolayer preplating of Kr decreases the effective pore diameter to 4.7 nm and
broadens the pore size distribution by about eight times from the sharp
distribution of the bare Gelsil sample. The superfluid properties were studied
by a torsional oscillator for adsorbed film states and pressurized liquid
states, both before and after the monolayer Kr preplating. In the film states,
both the superfluid transition temperature $T_{\mathrm c}$ and the superfluid
density decrease about 10 percent by Kr preplating. The suppression of film
superfluidity is attributed to the quantum localization of $^4$He atoms by the
randomness in the substrate potential, which is caused by the
preplating--induced broadening of the pore size distribution. In the
pressurized liquid states, the superfluid density $\rho_{\mathrm s}$ is found
to increase by 10 percent by Kr preplating, whereas $T_{\mathrm c}$ is
decreased by 2 percent at all pressures. The unexpected enhancement of
$\rho_{\mathrm s}$ might indicate the existence of an unknown disorder effect
for confined $^4$He. | cond-mat_other |
Bounds for the Superfluid Fraction from Exact Quantum Monte Carlo Local
Densities: For solid 4He and solid p-H2, using the flow-energy-minimizing one-body phase
function and exact T=0 K Monte Carlo calculations of the local density, we have
calculated the phase function, the velocity profile and upper bounds for the
superfluid fraction f_s. At the melting pressure for solid 4He we find that f_s
< 0.20-0.21, about ten times what is observed. This strongly indicates that the
theory for the calculation of these upper bounds needs substantial
improvements. | cond-mat_other |
Ultracold Atoms in 1D Optical Lattices: Mean Field, Quantum Field,
Computation, and Soliton Formation: In this work, we highlight the correspondence between two descriptions of a
system of ultracold bosons in a one-dimensional optical lattice potential: (1)
the discrete nonlinear Schr\"{o}dinger equation, a discrete mean-field theory,
and (2) the Bose-Hubbard Hamiltonian, a discrete quantum-field theory. The
former is recovered from the latter in the limit of a product of local coherent
states. Using a truncated form of these mean-field states as initial
conditions, we build quantum analogs to the dark soliton solutions of the
discrete nonlinear Schr\"{o}dinger equation and investigate their dynamical
properties in the Bose-Hubbard Hamiltonian. We also discuss specifics of the
numerical methods employed for both our mean-field and quantum calculations,
where in the latter case we use the time-evolving block decimation algorithm
due to Vidal. | cond-mat_other |
Negative refraction and plano-concave lens focusing in one-dimensional
photonic crystals: Negative refraction is demonstrated in one-dimensional (1D) dielectric
photonic crystals (PCs) at microwave frequencies. Focusing by plano-concave
lens made of 1D PC due to negative refraction is also demonstrated. The
frequency-dependent negative refractive indices, calculated from the
experimental data matches very well with those determined from band structure
calculations. The easy fabrication of one-dimensional photonic crystals may
open the door for new applications. | cond-mat_other |
Exchange coupling in transition metal monoxides: Electronic structure
calculations: An ab initio study of magnetic exchange interactions in antiferromagnetic and
strongly correlated 3d transition metal monoxides is presented. Their
electronic structure is calculated using the local self-interaction correction
approach, implemented within the Korringa-Kohn-Rostoker band structure method,
which is based on multiple scattering theory. The Heisenberg exchange constants
are evaluated with the magnetic force theorem. Based on these the corresponding
Neel temperatures T_N and spin wave dispersions are calculated. The Neel
temperatures are obtained using mean field approximation, random phase
approximation and Monte Carlo simulations. The pressure dependence of T_N is
investigated using exchange constants calculated for different lattice
constants. All the calculated results are compared to experimental data. | cond-mat_other |
Low frequency excitations of C60 chains inserted inside single-walled
carbon nanotubes: The low frequency excitations of C60 chains inserted inside single-walled
carbon nanotubes (SWNTs) have been studied by inelastic neutron scattering
(INS) on a high quality sample of peapods. The comparison of the
neutron-derived generalized phonon density of states (GDOS) of the peapods
sample with that of a raw SWNTs allows the vibrational properties of the C60
chains encapsulated in the hollow core of the SWNTs to be probed. Lattice
dynamical models are used to calculate the GDOS of chains of monomers, dimers
and polymers inserted into SWNTs, which are compared to the experimental data.
The presence of strong interactions between C60 cages inside the nanotube is
clearly demonstrated by an excess of mode density in the frequency range around
10 meV. However, the presence of a quasi-elastic signal indicates that some of
the C60\'s undergo rotational motion. This suggests that peapods are made from
a mixture of C60 monomers and C60 n-mer (dimer, trimer ... polymer) structures. | cond-mat_other |
A Derivation of the Classical Einstein-Dirac-Maxwell Equations From a
Model of an Elastic Medium: Starting from a model of an elastic medium, partial differential equations
with the form of the coupled Einstein-Dirac-Maxwell equations are derived. The
form of these equations describes particles with mass and spin coupled to
electromagnetic and gravitational type of interactions. A two dimensional
version of these equations is obtained by starting with a model in three
dimensions and deriving equations for the dynamics of the lowest fourier modes
assuming one dimension to be periodic. Generalizations to higher dimensions are
discussed. | cond-mat_other |
Plasmonic engineering of metal nanoparticles for enhanced fluorescence
and Raman scattering: We have investigated the effects of tuning the localized surface plasmon
resonances (LSPRs) of silver nanoparticles on the fluorescence intensity,
lifetime, and Raman signal from nearby fluorophores. The presence of a metallic
structure can alter the optical properties of a molecule by increasing the
excitation field, and by modifying radiative and non-radiative decay
mechanisms. By careful choice of experimental parameters we have been able to
decouple these effects. We observe a four-fold increase in fluorescence
enhancement and an almost 30-fold increase in decay rate from arrays of Ag
nanoparticles, when the LSPR is tuned to the emission wavelength of a locally
situated fluorophore. This is consistent with a greatly increased efficiency
for energy transfer from fluorescence to surface plasmons. Additionally,
surface enhanced Raman scattering (SERS) measurements show a maximum
enhancement occurs when both the incident laser light and the Raman signal are
near resonance with the plasmon energy. Spatial mapping of the SERS signal from
a nanoparticle array reveals highly localized differences in the excitation
field resulting from small differences in the LSPR energy. | cond-mat_other |
Effective magnetic fields in degenerate atomic gases induced by light
beams with orbital angular momenta: We investigate the influence of two resonant laser beams on the mechanical
properties of degenerate atomic gases. The control and probe beams of light are
considered to have Orbital Angular Momenta (OAM) and act on the three-level
atoms in the Electromagnetically Induced Transparency (EIT) configuration. The
theory is based on the explicit analysis of the quantum dynamics of cold atoms
coupled with two laser beams. Using the adiabatic approximation, we obtain an
effective equation of motion for the atoms driven to the dark state. The
equation contains a vector potential type interaction as well as an effective
trapping potential. The effective magnetic field is shown to be oriented along
the propagation direction of the control and probe beams containing OAM. Its
spatial profile can be controlled by choosing proper laser beams. We
demonstrate how to generate a constant effective magnetic field, as well as a
field exhibiting a radial distance dependence. The resulting effective magnetic
field can be concentrated within a region where the effective trapping
potential holds the atoms. The estimated magnetic length can be considerably
smaller than the size of the atomic cloud. | cond-mat_other |
Comments on "Mixed Bose-Fermi statistics Kinetic equation and navigation
through network" by S.F. Chekmarev, Phys. Rev. E 82, 026106 (2010): The paper shows that the kinetic equations considered in [1], equilibrium
distribution obtained in [1], and results and conclusions obtained on the basis
of the kinetic equation derived in [1] do not correspond to the mixed
Bose-Fermi statistics. Moreover, it is shown that the kinetic equation
corresponding to the case when the copies of the system are characterized by
different values of the fraction of the Fermi-like moves is incorrect. We
present a correct kinetic equation for the mixture of the Bose and Fermi moves
and obtained the equilibrium distribution for the case when the probability of
the Fermi moves is higher or equal to that of the Bose moves. | cond-mat_other |
Exact Solitonic Solutions of the Gross-Pitaevskii Equation with a Linear
Potential: We derive classes of exact solitonic solutions of the time-dependent
Gross-Pitaevskii equation with repulsive and attractive interatomic
interactions. The solutions correspond to a string of bright solitons with
phase difference between adjacent solitons equal to $\pi$. While the relative
phase, width, and distance between adjacent solitons turn out to be a constant
of the motion, the center of mass of the string moves with a constant
acceleration arising from the inhomogeneity of the background. | cond-mat_other |
Bright-Dark Soliton Complexes in Spinor Bose-Einstein Condensates: We present bright-dark vector solitons in quasi-one-dimensional spinor (F=1)
Bose-Einstein condensates. Using a multiscale expansion technique, we reduce
the corresponding nonintegrable system of three coupled Gross-Pitaevskii
equations (GPEs) to a completely integrable Yajima-Oikawa system. In this way,
we obtain approximate solutions for small-amplitude vector solitons of
dark-dark-bright and bright-bright-dark types, in terms of the $m_{F}=+1,-1,0$
spinor components, respectively. By means of numerical simulations of the full
GPE system, we demonstrate that these states indeed feature soliton properties,
i.e., they propagate undistorted and undergo quasi-elastic collisions. It is
also shown that, in the presence of a parabolic trap of strength $\omega $, the
bright component(s) is (are) guided by the dark one(s), and, as a result, the
small-amplitude vector soliton as a whole performs harmonic oscillations of
frequency $\omega/ \sqrt{2}$ in the shallow soliton limit. We investigate
numerically deviations from this prediction, as the depth of the solitons is
increased, as well as when the strength of the spin-dependent interaction is
modified. | cond-mat_other |
Casimir-like force arising from quantum fluctuations in a slow-moving
dilute Bose-Einstein condensate: We calculate a force due to zero-temperature quantum fluctuations on a
stationary object in a moving superfluid flow. We model the object by a
localized potential varying only in the flow direction and model the flow by a
three-dimensional weakly interacting Bose-Einstein condensate at zero
temperature. We show that this force exists for any arbitrarily small flow
velocity and discuss the implications for the stability of superfluid flow. | cond-mat_other |
Equation of state of an interacting Bose gas at finite temperature: a
Path Integral Monte Carlo study: By using exact Path Integral Monte Carlo methods we calculate the equation of
state of an interacting Bose gas as a function of temperature both below and
above the superfluid transition. The universal character of the equation of
state for dilute systems and low temperatures is investigated by modeling the
interatomic interactions using different repulsive potentials corresponding to
the same s-wave scattering length. The results obtained for the energy and the
pressure are compared to the virial expansion for temperatures larger than the
critical temperature. At very low temperatures we find agreement with the
ground-state energy calculated using the diffusion Monte Carlo method. | cond-mat_other |
Self-localized impurities embedded in a one dimensional Bose-Einstein
condensate and their quantum fluctuations: We consider the self-localization of neutral impurity atoms in a
Bose-Einstein condensate in a 1D model. Within the strong coupling approach, we
show that the self-localized state exhibits parametric soliton behavior. The
corresponding stationary states are analogous to the solitons of non-linear
optics and to the solitonic solutions of the Schroedinger-Newton equation
(which appears in models that consider the connection between quantum mechanics
and gravitation). In addition, we present a Bogoliubov-de-Gennes formalism to
describe the quantum fluctuations around the product state of the strong
coupling description. Our fluctuation calculations yield the excitation
spectrum and reveal considerable corrections to the strong coupling
description. The knowledge of the spectrum allows a spectroscopic detection of
the impurity self-localization phenomenon. | cond-mat_other |
Fundamental limits to nonlinear energy harvesting: Ease of miniaturization, and less or no maintenance, among other advantages,
have pushed towards replacement of conventional batteries with energy
harvesters in particular, vibratory energy harvesters. In the recent years,
nonlinearity has been intentionally introduced into the otherwise linear energy
harvesters in the hope of increasing the frequency bandwidth and power density.
However, fundamental limits on the harvestable energy of a harvester subjected
to an arbitrary excitation force is yet unknown. Understanding of these limits
is not only essential for assessment of the technology potential, but also
provides a broader prospective on the current harvesting mechanisms and
guidance in their improvement. Here we derive the fundamental limits on output
power of an ideal energy harvester, and develop an analysis framework for
simple computation of this limit for more sophisticated set-ups. We show that
the optimal harvester maximizes the harvested energy through a mechanical
analogue of 'buy low-sell high' strategy. Inspired by this strategy we propose
a novel concept of latch-assisted harvesting that is shown to harvest energy
more efficiently than its linear and bistable counterparts over a wider range
of excitation frequencies and amplitudes. | cond-mat_other |
Dynamics of kicked matter-wave solitons in an optical lattice: We investigate effects of the application of a kick to one-dimensional
matter-wave solitons in a self-attractive Bose-Einstein condensate trapped in a
optical lattice. The resulting soliton's dynamics is studied within the
framework of the time-dependent nonpolynomial Schrodinger equation. The
crossover from the pinning to quasi-free motion crucially depends on the size
of the kick, strength of the self-attraction, and parameters of the optical
lattice. | cond-mat_other |
Construction of localized atomic wave packets: It is shown that highly localized solitons can be created in lower
dimensional Bose-Einstein condensates (BEC), trapped in a regular harmonic
trap, by temporally varying the trap frequency. A BEC trapped in such a trap
can be effectively used to construct a pulsed atomic laser emitting coherent
atomic wave packets. It is also shown that one has complete control over the
spatio-temporal dynamics of the solitons. The dynamics of these solitons are
compared with those constructed in a BEC where the trap frequency is constant. | cond-mat_other |
Evidence of Luttinger liquid behavior in one-dimensional dipolar quantum
gases: The ground state and structure of a one-dimensional Bose gas with dipolar
repulsions is investigated at zero temperature by a combined Reptation Quantum
Monte Carlo (RQMC) and bosonization approach. A non trivial Luttinger-liquid
behavior emerges in a wide range of intermediate densities, evolving into a
Tonks-Girardeau gas at low density and into a classical quasi-ordered state at
high density. The density dependence of the Luttinger exponent is extracted
from the numerical data, providing analytical predictions for observable
quantities, such as the structure factor and the momentum distribution. We
discuss the accessibility of such predictions in current experiments with
ultracold atomic and molecular gases. | cond-mat_other |
Symmetry-Protected Topological relationship between $SU(3)$ and
$SU(2)\times{U(1)}$ in Two Dimension: Symmetry-protected topological $\left(SPT\right)$ phases are gapped
short-range entangled states with symmetry $G$, which can be systematically
described by group cohomology theory. $SU(3)$ and $SU(2)\times{U(1)}$ are
considered as the basic groups of Quantum Chromodynamics and
Weak-Electromagnetic unification, respectively. In two dimension $(2D)$,
nonlinear-sigma models with a quantized topological Theta term can be used to
describe nontrivial SPT phases. By coupling the system to a probe field and
integrating out the group variables, the Theta term becomes the effective
action of Chern-Simons theory which can derive the response current density. As
a result, the current shows a spin Hall effect, and the quantized number of the
spin Hall conductance of SPT phases $SU(3)$ and $SU(2)\times{U(1)}$ are same.
In addition, relationships between $SU(3)$ and $SU(2)\times{U(1)}$ which maps
$SU(3)$ to $SU(2)$ with a rotation $U(1)$ will be given. | cond-mat_other |
X-ray Studies of Structure and Defects in Solid 4He from 50 mK to
Melting: Recent measurements have found non-classical rotational inertia (NCRI) in
solid 4He starting at T ~ 200 mK, leading to speculation that a supersolid
state may exist in these materials. Differences in the NCRI fraction due to the
growth method and annealing history imply that defects play an important role
in the effect. Using x-ray synchrotron radiation, we have studied the nature of
the crystals and the properties of the defects in solid 4He at temperatures
down to 50 mK. Measurements of peak intensities and lattice parameters do not
show indications of the supersolid transition. Using growth methods similar to
those of groups measuring the NCRI we find that large crystals form. Scanning
with a small (down to 10 x 10 um2) beam, we resolve a mosaic structure within
these crystals consistent with numerous small angle grain boundaries. The
mosaic shows significant shifts over time even at temperatures far from
melting. We discuss the relevance of these defects to the NCRI observations. | cond-mat_other |
Quantum fluctuation-induced uniaxial and biaxial spin nematics: It is shown that zero point quantum fluctuations (ZPQFs) completely lift the
accidental continuous degeneracy that is found in mean field analysis of
quantum spin nematic phases of hyperfine spin 2 cold atoms. The result is two
distinct ground states which have higher symmetries: a uniaxial spin nematic
and a biaxial spin nematic with dihedral symmetry ${Dih}_4$. There is a novel
first order quantum phase transition between the two phases as atomic
scattering lengths are varied. We find that the ground state of $^{87}Rb$ atoms
should be a uniaxial spin nematic. We note that the energy barrier between the
phases could be observable in dynamical experiments. | cond-mat_other |
Volume element structure and roton-maxon-phonon excitations in
superfluid helium beyond the Gross-Pitaevskii approximation: We propose a theory which deals with the structure and interactions of volume
elements in liquid helium II. The approach consists of two nested models linked
via parametric space. The short-wavelength part describes the interior
structure of the fluid element using a non-perturbative approach based on the
logarithmic wave equation; it suggests the Gaussian-like behaviour of the
element's interior density and interparticle interaction potential. The
long-wavelength part is the quantum many-body theory of such elements which
deals with their dynamics and interactions. Our approach leads to a unified
description of the phonon, maxon and roton excitations, and has noteworthy
agreement with experiment: with one essential parameter to fit we reproduce at
high accuracy not only the roton minimum but also the neighboring local maximum
as well as the sound velocity and structure factor. | cond-mat_other |
Rotating quantum turbulence in superfluid 4He in the T=0 limit: Observations of quantum turbulence in pure superfluid 4He in a rotating
container are reported. New techniques of large-scale forcing (rotational
oscillations of the cubic container) and detecting (monitoring ion transport
along the axis of rotation) turbulence were implemented. Near the axial walls,
with increasing forcing the vortex tangle grows without an observable
threshold. This tangle gradually develops into bulk turbulence at a
characteristic amplitude of forcing that depends on forcing frequency and
rotation rate. At higher amplitudes, the total vortex line length increases
rapidly. Resonances of inertial waves are observed in both laminar and
turbulent bulk states. On such resonances, the turbulence appears at smaller
amplitudes of forcing. | cond-mat_other |
The growth of a Super Stable Heap : an experimental and numerical study: We report experimental and numerical results on the growth of a super stable
heap (SSH). Such a regime appears for flows in a thin channel and for high flow
rate : the flow occurs atop a nearly static heap whose angle is stabilized by
the flowing layer at its top and the side wall friction. The growth of the
static heap is investigated in this paper. A theoretical analysis inspired by
the BRCE formalism predicts the evolution of the growth process, which is
confirmed by both experiments and numerical simulations. The model allows us to
link the characteristic time of the growth to the exchange rate between the
"moving" and "static" grains. We show that this rate is proportional to the
height of the flowing layer even for thick flows. The study of upstream
traveling waves sheds new light on the BCRE model. | cond-mat_other |
Correlation of the angular dependence of spin-transfer torque and giant
magnetoresistance in the limit of diffusive transport in spin valves: Angular variation of giant magnetoresistance and spin-transfer torque in
metallic spin-valve heterostructures is analyzed theoretically in the limit of
diffusive transport. It is shown that the spin-transfer torque in asymmetric
spin valves can vanish in non-collinear magnetic configurations, and such a
non-standard behavior of the torque is generally associated with a
non-monotonic angular dependence of the giant magnetoresistance, with a global
minimum at a non-collinear magnetic configuration. | cond-mat_other |
Basis-Independent Spectral Methods for Non-linear Optical Response in
Arbitrary Tight-binding Models: In this paper, we developed a basis-independent perturbative method for
calculating the non-linear optical response of arbitrary non-interacting
tight-binding models. Our method is based on the non-equilibrium Keldysh
formalism and allows an efficient numerical implementation within the framework
of the Kernel Polynomial Method for systems which are not required to be
translation-invariant. Some proof-of-concept results of the second-order
optical conductivity are presented for the special case of gapped graphene with
vacancies and an on-site Anderson disordered potential. | cond-mat_other |
Effective thermodynamics of strongly coupled qubits: Interactions between a quantum system and its environment at low temperatures
can lead to violations of thermal laws for the system. The source of these
violations is the entanglement between system and environment, which prevents
the system from entering into a thermal state. On the other hand, for two-state
systems, we show that one can define an effective temperature, placing the
system into a `pseudo-thermal' state where effective thermal laws are upheld.
We then numerically explore these assertions for an n-state system inspired by
the spin-boson environment. | cond-mat_other |
Phase separations of bosonic mixtures in optical lattices from
macroscopic to microscopic scales: Mixtures of cold bosonic atoms in optical lattices undergo phase separations
on different length scales with increasing inter-species repulsion. As a
general rule, the stronger the intra-species interactions, the shorter is this
length scale. The wealth of phenomena is documented by illustrative examples on
both superfluids and Mott-insulators. | cond-mat_other |
Artificial gravitation effect on spin-polarized exciton-polaritons: The pseudospin dynamics of long-living exciton-polaritons in a wedged 2D
cavity has been studied theoretically accounting for the external magnetic
field effect. The cavity width variation plays the role of the artificial
gravitational force acting on a massive particle: exciton-polariton. A
semi-classical model of the spin-polarization dynamics of ballistically
propagating exciton-polaritons has been developed. It has been shown that for
the specific choise of the magnetic field magnitude and the initial polariton
wave vector the polariton polarization vector tends to an attractor on the
Poincare sphere. Based on this effect, the switching the polariton polarization
in the ballistic regime has been demonstrated. The self-interference of the
polariton field emitted by a point-like source has been shown to induce the
formation of interference patterns reminiscent of the interference patterns of
cylindrical and plane waves. | cond-mat_other |
Extremal transmission through a microwave photonic crystal and the
observation of edge states in a rectangular Dirac billiard: This article presents experimental results on properties of waves propagating
in an unbounded and a bounded photonic crystal consisting of metallic cylinders
which are arranged in a triangular lattice. First, we present transmission
measurements of plane waves traversing a photonic crystal. The experiments are
performed in the vicinity of a Dirac point, i.e., an isolated conical
singularity of the photonic band structure. There, the transmission shows a
pseudodiffusive 1/L dependence, with $L$ being the thickness of the crystal, a
phenomenon also observed in graphene. Second, eigenmode intensity distributions
measured in a microwave analog of a relativistic Dirac billiard, a rectangular
microwave billiard that contains a photonic crystal, are discussed. Close to
the Dirac point states have been detected which are localized at the straight
edge of the photonic crystal corresponding to a zigzag edge in graphene. | cond-mat_other |
Effective theory for the Goldstone field in the BCS-BEC crossover at T=0: We perform a detailed study of the effective Lagrangian for the Goldstone
mode of a superfluid Fermi gas at zero temperature in the whole BCS-BEC
crossover. By using a derivative expansion of the response functions, we derive
the most general form of this Lagrangian at the next to leading order in the
momentum expansion in terms of four coefficient functions. This involves the
elimination of all the higher order time derivatives by careful use of the
leading order field equations. In the infinite scattering length limit where
conformal invariance is realized, we show that the effective Lagrangian must
contain an unnoticed invariant combination of higher spatial gradients of the
Goldstone mode, while explicit couplings to spatial gradients of the trapping
potential are absent. Across the whole crossover, we determine all the
coefficient functions at the one-loop level, taking into account the dependence
of the gap parameter on the chemical potential in the mean-field approximation.
These results are analytically expressed in terms of elliptic integrals of the
first and second kind. We discuss the form of these coefficients in the extreme
BCS and BEC regimes and around the unitary limit, and compare with recent work
by other authors. | cond-mat_other |
Relaxation of Bose-Einstein Condensates of Magnons in Magneto-Textural
Traps in Superfluid $^3$He-B: In superfluid $^3$He-B externally pumped quantized spin-wave excitations or
magnons spontaneously form a Bose-Einstein condensate in a 3-dimensional trap
created with the order-parameter texture and a shallow minimum in the
polarizing field. The condensation is manifested by coherent precession of the
magnetization with a common frequency in a large volume. The trap shape is
controlled by the profile of the applied magnetic field and by the condensate
itself via the spin-orbit interaction. The trapping potential can be
experimentally determined with the spectroscopy of the magnon levels in the
trap. We have measured the decay of the ground state condensates after
switching off the pumping in the temperature range $(0.14\div
0.2)T_{\mathrm{c}}$. Two contributions to the relaxation are identified: (1)
spin-diffusion with the diffusion coefficient proportional to the density of
thermal quasiparticles and (2) the approximately temperature-independent
radiation damping caused by the losses in the NMR pick-up circuit. The measured
dependence of the relaxation on the shape of the trapping potential is in a
good agreement with our calculations based on the magnetic field profile and
the magnon-modified texture. Our values for the spin diffusion coefficient at
low temperatures agree with the theoretical prediction and earlier measurements
at temperatures above $0.5T_{\mathrm{c}}$. | cond-mat_other |
Stability of Formation of Large Bipolaron: Nonrelativistic Quantum Field
Theory: We are concerned with the stability of formation of large bipolaron in a
3-dimensional (3D) crystal. This problem is considered in the framework of
nonrelativistic quantum field theory. Thus, the Hamiltonian formalism, as
Froehlich introduced, is employed to describe the bipolaron. We approach the
problem by characterizing some sufficient or necessary conditions for the
bipolaron being stable. This paper gives a full detail of the author's talks at
ESI, RIMS, and St. Petersburg State Univ. in 2005. | cond-mat_other |
Proposed definitions of the correlation energy density from a
Hartree-Fock starting point: The two-electron Moshinsky model atom as an
exactly solvable model: In both molecular physics and condensed matter theory, deeper understanding
of the correlation energy density epsilon_c (r) remains a high priority. By
adopting Loewdin's definition of correlation energy as the difference between
the exact and the Hartree-Fock values, here we propose two alternative routes
to define this. One of these involves both exact and Hartree-Fock (HF)
wavefunctions, while the second requires a coupling constant integration. As an
exact analytical example of the first route, we treat the two-electron model
atom of Moshinsky, for which both confinement potential and interactions are
harmonic. Though the correlation energy density epsilon_c (r) is known
analytically, we also investigate numerically its relation to the exact
ground-state density in this example. | cond-mat_other |
Vortices in Atomic Bose-Einstein Condensates in the Large Gas Parameter
Region: In this work we compare the results of the Gross-Pitaevskii and modified
Gross-Pitaevskii equations with ab initio variational Monte Carlo calculations
for Bose-Einstein condensates of atoms in axially symmetric traps. We examine
both the ground state and excited states having a vortex line along the z-axis
at high values of the gas parameter and demonstrate an excellent agreement
between the modified Gross-Pitaevskii and ab initio Monte Carlo methods, both
for the ground and vortex states. | cond-mat_other |
Propagation of thermal excitations in a cluster of vortices in
superfluid 3He-B: We describe the first measurement on Andreev scattering of thermal
excitations from a vortex configuration with known density, spatial extent, and
orientations in 3He-B superfluid. The heat flow from a blackbody radiator in
equilibrium rotation at constant angular velocity is measured with two quartz
tuning fork oscillators. One oscillator creates a controllable density of
excitations at 0.2Tc base temperature and the other records the thermal
response. The results are compared to numerical calculations of ballistic
propagation of thermal quasiparticles through a cluster of rectilinear
vortices. | cond-mat_other |
Pekar's Ansatz and the Strong Coupling Problem in Polaron Theory: A detailed consideration is given to the translation-invariant theory of
Tulub polaron constructed without the use of Pekar ansatz. A fundamental result
of the theory is that the value of the polaron energy is lower than that
obtained on the basis of Pekar ansatz which was considered as an asymptotically
exact solution in the strong coupling limit. In the case of bipolarons the
theory yields the best values of the coupling energy and critical parameters of
their stability. Numerous physical consequences of the existence of
translation-invariant polarons and bipolarons are discussed. | cond-mat_other |
Vortex quantum creation and winding number scaling in a quenched spinor
Bose gas: Motivated by a recent experiment, we study non-equilibrium quantum phenomena
taking place in the quench of a spinor Bose-Einstein condensate through the
zero-temperature phase transition separating the polar paramagnetic and planar
ferromagnetic phases. We derive the typical spin domain structure (correlations
of the effective magnetization) created by the quench arising due to spin-mode
quantum fluctuations, and establish a sample-size scaling law for the creation
of spin vortices, which are topological defects in the transverse
magnetization. | cond-mat_other |
Fully permanent magnet atom chip for Bose-Einstein condensation: We describe a self-biased, fully permanent magnet atom chip used to study
ultracold atoms and to produce a Bose-Einstein condensate (BEC). The magnetic
trap is loaded efficiently by adiabatic transport of a magnetic trap via the
application of uniform external fields. Radio frequency spectroscopy is used
for in-trap analysis and to determine the temperature of the atomic cloud. The
formation of a Bose-Einstein condensate is observed in time of flight images
and as a narrow peak appearing in the radio frequency spectrum. | cond-mat_other |
Bosonic enhancement of spontaneous emission near an interface: We show how the spontaneous emission rate of an excited two-level atom placed
in a trapped Bose-Einstein condensate of ground-state atoms is enhanced by
bosonic stimulation. This stimulation depends on the overlap of the excited
matter-wave packet with the macroscopically occupied condensate wave function,
and provides a probe of the spatial coherence of the Bose gas. The effect can
be used to amplify the distance-dependent decay rate of an excited atom near an
interface. | cond-mat_other |
Strong linewidth variation for spin-torque nano-oscillators as a
function of in-plane magnetic field angle: We measure the microwave signals produced by spin-torque-driven magnetization
dynamics in patterned magnetic multilayer devices at room temperature, as a
function of the angle of a magnetic field applied in the sample plane. We find
strong variations in the frequency linewidth of the signals, with a decrease by
more than a factor of 20 as the field is rotated from the magnetic easy axis to
the in-plane hard axis. Based on micromagnetic simulations, we identify these
variations as due to a transition from spatially incoherent to coherent
precession. | cond-mat_other |
Calculation of Kapitza resistance with kinetic equation: A new method is introduced for calculation of interfacial thermal resistance
in the case of heat transport through the interface by phonons. A unique
feature of the method is taking into account all the consequences of a
non-equilibrium character of phonon distribution functions during the heat
transport. We introduce a model set of transmission and reflection amplitudes
of phonons at the interface based on the most common in the literature
Diffusive Mismatch Model. For the proposed model we derive an exact analytical
solution. The problem is also solved for a set of transmission and reflection
amplitudes characterized by a free parameter. We found that the calculation
results are in a good agreement with the experimental data. | cond-mat_other |
Bound-to-bound and bound-to-continuum optical transitions in combined
quantum dot - superlattice systems: By combining band gap engineering with the self-organized growth of quantum
dots, we present a scheme of adjusting the mid-infrared absorption properties
to desired energy transitions in quantum dot based photodetectors. Embedding
the self organized InAs quantum dots into an AlAs/GaAs superlattice enables us
to tune the optical transition energy by changing the superlattice period as
well as by changing the growth conditions of the dots. Using a one band
envelope function framework we are able, in a fully three dimensional
calculation, to predict the photocurrent spectra of these devices as well as
their polarization properties. The calculations further predict a strong impact
of the dots on the superlattices minibands. The impact of vertical dot
alignment or misalignment on the absorption properties of this dot/superlattice
structure is investigated. The observed photocurrent spectra of vertically
coupled quantum dot stacks show very good agreement with the calculations.In
these experiments, vertically coupled quantum dot stacks show the best
performance in the desired photodetector application. | cond-mat_other |
Chiral structures of lander molecules on Cu(100): Supramolecular assemblies of lander molecules (C$_{90}$H$_{98}$) on Cu(100)
are investigated with low-temperature scanning tunneling microscopy. The
energetically most favourable conformation of the adsorbed molecule is found to
exist in two mirror symmetric enantiomers or conformers. At low coverage, the
molecules align in enantiomerically pure chains along the chiral directions
$[01\bar{2}],[02\bar{1}],[012]$ and $[021]$. The arrangement is proposed to be
mainly governed by intermolecular van-der-Waals interaction. At higher
coverages, the molecular chains arrange into chiral domains, for which a
structural model is presented. | cond-mat_other |
Stone-Wales Transformation Paths in Fullerene C60: The mechanisms of formation of a metastable defect isomer of fullerene C60
due to the Stone-Wales transformation are theoretically studied. It is
demonstrated that the paths of the "dynamic" Stone-Wales transformation at a
high sufficient for overcoming potential barriers) temperature can differ from
the two "adiabatic" transformation paths discussed in the literature. This
behavior is due to the presence of a great near-flat segment of the
potential-energy surface in the neighborhood of metastable states. Besides, the
sequence of rupture and formation of interatomic bonds is other than that in
the case of the adiabatictransformation. | cond-mat_other |
Soliton Analysis of the Electro-Optical Response of Blue Bronze: In recent measurements on the charge-density-wave (CDW) conductor blue bronze
(K0.3MoO3), the electro-transmittance and electro-reflectance spectra were
searched for intragap states that could be associated with solitons created by
injection of electrons into the CDW at the current contacts [Eur. Phys. J. B
16, 295 (2000); ibid 35, 233 (2003)]. In this work, we adapt the model of
soliton absorption in dimerized polyacetylene to the blue bronze results, to
obtain the (order of magnitude) estimate that current induced solitons occur on
less than ~ 10% of the conducting chains. We discuss the implications of these
results on models of soliton lifetimes and motion of CDW phase dislocations. | cond-mat_other |
First-principles investigation of spin polarized conductance in atomic
carbon wire: We analyze spin-dependent energetics and conductance for one dimensional (1D)
atomic carbon wires consisting of terminal magnetic (Co) and interior
nonmagnetic (C) atoms sandwiched between gold electrodes, obtained employing
first-principles gradient corrected density functional theory and Landauer's
formalism for conductance. Wires containing an even number of interior carbon
atoms are found to be acetylenic with sigma-pi bonding patterns, while cumulene
structures are seen in wires containing odd number of interior carbon atoms, as
a result of strong pi-conjugation. Ground states of carbon wires containing up
to 13 C atoms are found to have anti-parallel spin configurations of the two
terminal Co atoms, while the 14 C wire has a parallel Co spin configuration in
the ground state. The stability of the anti-ferromagnetic state in the wires is
ascribed to a super-exchange effect. For the cumulenic wires this effect is
constant for all wire lengths. For the acetylenic wires, the super-exchange
effect diminishes as the wire length increases, going to zero for the atomic
wire containing 14 carbon atoms. Conductance calculations at the zero bias
limit show spin-valve behavior, with the parallel Co spin configuration state
giving higher conductance than the corresponding anti-parallel state, and a
non-monotonic variation of conductance with the length of the wires for both
spin configurations. | cond-mat_other |
Tightly bound gap solitons in a Fermi gas: Within the framework of the mean-field-hydrodynamic model of a degenerate
Fermi gas (DFG), we study, by means of numerical methods and variational
approximation (VA), the formation of fundamental gap solitons (FGSs) in a DFG
(or in a BCS superfluid generated by weak interaction between spin-up and
spin-down fermions), which is trapped in a periodic optical-lattice (OL)
potential. An effectively one-dimensional (1D) configuration is considered,
assuming strong transverse confinement; in parallel, a proper 1D model of the
DFG (which amounts to the known quintic equation for the Tonks-Girardeau gas in
the OL) is considered too. The FGSs found in the first two bandgaps of the
OL-induced spectrum (unless they are very close to edges of the gaps) feature a
tightly-bound shape, being essentially confined to a single cell of the OL. In
the second bandgap, we also find antisymmetric tightly-bound subfundamental
solitons (SFSs), with zero at the midpoint. The SFSs are also confined to a
single cell of the OL, but, unlike the FGSs, they are unstable. The predicted
solitons, consisting of $\sim 10^4 - 10^5$ atoms, can be created by available
experimental techniques in the DFG of $^6$Li atoms. | cond-mat_other |
Quantum viscosity and the Reynolds similitude in quantum liquid He-II: Reynolds similitude, a key concept in hydrodynamics, states that two
phenomena of different length scales with a similar geometry are physically
identical. Flow properties are universally determined in a unified way in terms
of the Reynolds number ${\cal R}$ (dimensionless, ratio of inertial to viscous
forces in incompressible fluids). For example, the drag coefficient $c_D$ of
objects with similar shapes moving in fluids is expressed by a universal
function of ${\cal R}$. Certain studies introduced similar dimensionless
numbers, that is, the superfluid Reynolds number ${\cal R}_s$, to characterize
turbulent flows in superfluids. However, the applicablity of the similitude to
inviscid quantum fluids is nontrivial as the original theory is applicable to
viscous fluids. This study proposed a method to verify the similitude using
current experimental techniques in quantum liquid He-II. A highly precise
relation between $c_D$ and ${\cal R}_s$ was obtained in terms of the terminal
speed of a macroscopic body falling in He-II at finite temperatures across the
Knudsen (ballistic) and hydrodynamic regimes of thermal excitations. Reynolds
similitude in superfluids can facilitate unified mutual development of
classical and quantum hydrodynamics. | cond-mat_other |
Resonant Atom-Dimer Relaxation in Ultracold Atoms: Three-body systems with large scattering length display universal phenomena
associated with a discrete scaling symmetry. These phenomena include resonant
enhancement of three-body loss rates when an Efimov three-body resonance is at
the scattering threshold. In particular, there can be resonant peaks in the
atom-dimer relaxation rate for large positive scattering length. We improve
upon earlier studies and calculate the atom-dimer relaxation rate as a function
of temperature using a Bose-Einstein distribution for the thermal average. As
input, we use calculations of the atom-dimer scattering phase shifts from
effective field theory. | cond-mat_other |
Magneto-optic far-infrared study of Sr$_{14}$Cu$_{24}$O$_{41}$: triplet
excitations in chains: Using far-infrared spectroscopy we have studied the magnetic field and
temperature dependence of the spin gap modes in the chains of
Sr$_{14}$Cu$_{24}$O$_{41}$. Two triplet modes T$_1$ and T$_2$ were found in the
center of the Brillouin zone at $\Delta_1=9.65$ meV and $\Delta_2=10.86$ meV in
zero magnetic field. The T$_1$ mode was excited when the electric field vector
${\bf E}$ of the light was polarized along the b axis (perpendicular to the
planes of chains and ladders) and T$_2$ was excited for ${\bf E}\parallel {\bf
a}$ (perpendicular to the chains and along the rungs). Up to the maximum
magnetic field of 18 T, applied along the chains, the electron $g$ factors of
these two modes were similar, $g_{1c}=2.049$ and $g_{2c}=2.044$. Full linewidth
at half maximum for both modes was 1 cm$^{-1}$ (0.12 meV) at 4K and increased
with $T$. The temperature dependence of mode energies and line intensities was
in agreement with the inelastic neutron scattering results from two groups
[Matsuda {\it et al.}, Phys. Rev. B {\bf 59}, 1060 (1999) and Regnault {\it et
al.}, Phys. Rev. B {\bf 59}, 1055 (1999)]. The T$_1$ mode has not been observed
by inelastic neutron scattering in the points of the $k$-space equivalent to
the center of the Brillouin zone. Our study indicates that the zone structure
model of magnetic excitations of Sr$_{14}$Cu$_{24}$O$_{41}$ must be modified to
include a triplet mode at 9.65 meV in the center of the magnetic Brillouin
zone. | cond-mat_other |
Diffusive Decay of the Vortex Tangle and Kolmogorov turbulence in
quantum fluids: The idea that chaotic set of quantum vortices can mimic classical turbulence,
or at least reproduce many main features, is currently actively being
developed. Appreciating significance of the challenging problem of the
classical turbulence it can be expressed that the idea to study it in terms of
quantized line is indeed very important and may be regarded as a breakthrough.
For this reason, this theory should be carefully scrutinized. One of the basic
arguments supporting this point of view is the fact that vortex tangle decays
at zero temperature, when the apparent mechanism of dissipation (mutual
friction) is absent. Since the all possible mechanisms of dissipation of the
vortex energy, discussed in the literature, are related to the small scales, it
is natural to suggest that the Kolmogorov cascade takes the place with the flow
of the energy, just as in the classical turbulence. In a series of recent
experiment attenuation of vortex line density was observed and authors
attribute this decay to the properties of the Kolmogorov turbulence. In the
present work we discuss alternative possibility of decay of the vortex tangle,
which is not related to dissipation at small scales. This mechanism is just the
diffusive like spreading of the vortex tangle. We discuss a number of the key
experiments, considering them both from the point of view of alternative
explanation and of the theory of Kolmogorov turbulence in quantum fluids. | cond-mat_other |
Measurement and analysis of the Doppler broadened energy spectra of
annihilation gamma radiation originating from clean and adsorbate-covered
surfaces: We present measurements and theoretical modeling demonstrating the capability
of Doppler Broadened annihilation gamma Spectroscopy (DBS) to provide
element-specific information from the topmost atomic layer of surfaces that are
either clean or covered with adsorbates or thin films. Our measurements show
that the energy spectra of Doppler-shifted annihilation gamma photons emitted
following the annihilation of positrons from the topmost atomic layers of clean
gold (Au) and copper (Cu) differ significantly. With the aid of the positron
annihilation-induced Auger electron spectroscopy (PAES) performed
simultaneously with DBS, we show that measurable differences between the
Doppler broadened gamma spectra from Au and Cu surfaces in the high energy
region of the gamma spectra can be used for the quantification of surface
chemical composition. Modeling the measured Doppler spectra from clean Au and
Cu surfaces using gamma spectra obtained from ab initio calculations after
considering the detector energy resolution and surface positronium formation
pointed to an increase in the relative contribution of gamma from positron
annihilation with valence shell electrons. The fit result also suggests that
the surface-trapped positrons predominantly annihilated with the delocalized
valence shell (s and p) electrons that extended into the vacuum as compared to
the highly localized d electrons. Simultaneous DBS and PAES measurements from
adsorbate (sulfur, oxygen, carbon) or thin film (selenium (Se), graphene)
covered Cu surface showed that it is possible to distinguish and quantify the
surface adsorbate and thin-film composition just based on DBS. DBS of elemental
surfaces presents a promising avenue for developing a characterization tool
that can be used to probe external and internal surfaces that are inaccessible
by conventional surface science techniques. | cond-mat_other |
Three-body Recombination of Lithium-6 Atoms with Large Negative
Scattering Lengths: The 3-body recombination rate at threshold for distinguishable atoms with
large negative pair scattering lengths is calculated in the zero-range
approximation. The only parameters in this limit are the 3 scattering lengths
and the Efimov parameter, which can be complex valued. We provide semi-analytic
expressions for the cases of 2 or 3 equal scattering lengths and we obtain
numerical results for the general case of 3 different scattering lengths. Our
general result is applied to the three lowest hyperfine states of Lithium-6
atoms. Comparisons with recent experiments provide indications of loss features
associated with Efimov trimers near the 3-atom threshold. | cond-mat_other |
Repeating head-on collisions in an optical trap and the evaluation of
spin-dependent interactions among neutral particles: A dynamic process of repeating collisions of a pair of trapped neutral
particles with weak spin-dependent interaction is designed and studied. Related
theoretical derivation and numerical calculation have been performed to study
the inherent coordinate-spin and momentum-spin correlation. Due to the
repeating collisions the effect of the weak interaction can be accumulated and
enlarged, and therefore can be eventually detected. Numerical results suggest
that the Cr-Cr interaction, which has not yet been completely clear, could be
thereby determined. The design can be in general used to determine various
interactions among neutral atoms and molecules, in particular for the
determination of very weak forces. | cond-mat_other |
Spin excitations in a monolayer scanned by a magnetic tip: Energy dissipation via spin excitations is investigated for a hard
ferromagnetic tip scanning a soft magnetic monolayer. We use the classical
Heisenberg model with Landau-Lifshitz-Gilbert (LLG)-dynamics including a
stochastic field representing finite temperatures. The friction force depends
linearly on the velocity (provided it is small enough) for all temperatures.
For low temperatures, the corresponding friction coefficient is proportional to
the phenomenological damping constant of the LLG equation. This dependence is
lost at high temperatures, where the friction coefficient decreases
exponentially. These findings can be explained by properties of the spin
polarization cloud dragged along with the tip. | cond-mat_other |
Generation of mesoscopic superpositions of a binary Bose-Einstein
condensate in a slightly asymmetric double well: A previous publication [Europhysics Letters 78, 10009 (2007)] suggested to
coherently generate mesoscopic superpositions of a two-component Bose-Einstein
condensate in a double well under perfectly symmetric conditions. However,
already tiny asymmetries can destroy the entanglement properties of the ground
state. Nevertheless, even under more realistic conditions, the scheme is
demonstrated numerically to generate mesoscopic superpositions. | cond-mat_other |
Influence of interface structure on electronic properties and Schottky
barriers in Fe/GaAs magnetic junctions: The electronic and magnetic properties of Fe/GaAs(001) magnetic junctions are
investigated using first-principles density-functional calculations. Abrupt and
intermixed interfaces are considered, and the dependence of charge transfer,
magnetization profiles, Schottky barrier heights, and spin polarization of
densities of states on interface structure is studied. With As-termination, an
abrupt interface with Fe is favored, while Ga-terminated GaAs favors the
formation of an intermixed layer with Fe. The Schottky barrier heights are
particularly sensitive to the abruptness of the interface. A significant
density of states in the semiconducting gap arises from metal interface states.
These spin-dependent interface states lead to a significant minority spin
polarization of the density of states at the Fermi level that persists well
into the semiconductor, providing a channel for the tunneling of minority spins
through the Schottky barrier. These interface-induced gap states and their
dependence on atomic structure at the interface are discussed in connection
with potential spin-injection applications. | cond-mat_other |
Parity-time symmetry breaking in magnetic systems: The understanding of out-of-equilibrium physics, especially dynamic
instabilities and dynamic phase transitions, is one of the major challenges of
contemporary science, spanning the broadest wealth of research areas that range
from quantum optics to living organisms. Focusing on nonequilibrium dynamics of
an open dissipative spin system, we introduce a non-Hermitian Hamiltonian
approach, in which non-Hermiticity reflects dissipation and deviation from
equilibrium. The imaginary part of the proposed spin Hamiltonian describes the
effects of Gilbert damping and applied Slonczewski spin-transfer torque. In the
classical limit, our approach reproduces Landau-Lifshitz-Gilbert-Slonczewski
dynamics of a large macrospin. We reveal the spin-transfer torque-driven
parity-time symmetry-breaking phase transition corresponding to a transition
from precessional to exponentially damped spin dynamics. Micromagnetic
simulations for nanoscale ferromagnetic disks demonstrate the predicted effect.
Our findings can pave the way to a general quantitative description of
out-of-equilibrium phase transitions driven by spontaneous parity-time symmetry
breaking. | cond-mat_other |
X-ray imaging of the dynamic magnetic vortex core deformation: Magnetic platelets with a vortex configuration are attracting considerable
attention. The discovery that excitation with small in-plane magnetic fields or
spin polarised currents can switch the polarisation of the vortex core did not
only open the possibility of using such systems in magnetic memories, but also
initiated the fundamental investigation of the core switching mechanism itself.
Micromagnetic models predict that the switching is mediated by a
vortex-antivortex pair, nucleated in a dynamically induced vortex core
deformation. In the same theoretical framework, a critical core velocity is
predicted, above which switching occurs. Although these models are extensively
studied and generally accepted, experimental support has been lacking until
now. In this work, we have used high-resolution time-resolved X-ray microscopy
to study the detailed dynamics in vortex structures. We could reveal the
dynamic vortex core deformation preceding the core switching. Also, the
threshold velocity could be measured, giving quantitative comparison with
micromagnetic models. | cond-mat_other |
Ultracold Molecule Production Via a Resonant Oscillating Magnetic Field: A novel atom-molecule conversion technique has been investigated. Ultracold
85Rb atoms sitting in a DC magnetic field near the 155G Feshbach resonance are
associated by applying a small sinusoidal oscillation to the magnetic field.
There is resonant atom to molecule conversion when the modulation frequency
closely matches the molecular binding energy. We observe that the atom to
molecule conversion efficiency depends strongly on the frequency, amplitude,
and duration of the applied modulation and on the initial phase space density
of the sample. This technique offers high conversion efficiencies without the
necessity of crossing or closely approaching the Feshbach resonance and allows
precise spectroscopic measurements. | cond-mat_other |
Stable and unstable vector dark solitons of coupled nonlinear
Schrödinger equations. Application to two-component Bose-Einstein
condensates: Dynamics of vector dark solitons in two-component Bose-Einstein condensates
is studied within the framework of the coupled one-dimensional nonlinear
Schr\"odinger (NLS) equations. We consider the small amplitude limit in which
the coupled NLS equations are reduced to the coupled Korteweg-de Vries (KdV)
equations. For a specific choice of the parameters the obtained coupled KdV
equations are exactly integrable. We find that there exist two branches of
(slow and fast) dark solitons corresponding to the two branches of the sound
waves. Slow solitons, corresponding to the lower branch of the acoustic wave
appear to be unstable and transform during the evolution into the stable fast
solitons (corresponding to the upper branch of the dispersion law). Vector dark
solitons of arbitrary depths are studied numerically. It is shown that
effectively different parabolic traps, to which the two components are
subjected, cause instability of the solitons leading to splitting of their
components and subsequent decay. Simple phenomenological theory, describing
oscillations of vector dark solitons in a magnetic trap is proposed. | 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 |
Exploring correlated 1D Bose gases from the superfluid to the
Mott-insulator state by inelastic light scattering: We report the Bragg spectroscopy of interacting one-dimensional Bose gases
loaded in optical lattices across the superfluid to Mott-insulator phase
transition. Elementary excitations are created with a non-zero momentum and the
response of the correlated 1D gases is in the linear regime. The complexity of
the strongly correlated quantum phases is directly displayed in the spectra
which exhibit novel features. This work paves the way for a precise
characterization of the state of correlated atomic phases in optical lattices. | cond-mat_other |
Modeling of a Cantilever-Based Near-Field Scanning Microwave Microscope: We present a detailed modeling and characterization of our scalable microwave
nanoprobe, which is a micro-fabricated cantilever-based scanning microwave
probe with separated excitation and sensing electrodes. Using finite-element
analysis, the tip-sample interaction is modeled as small impedance changes
between the tip electrode and the ground at our working frequencies near 1GHz.
The equivalent lumped elements of the cantilever can be determined by
transmission line simulation of the matching network, which routes the
cantilever signals to 50 Ohm feed lines. In the microwave electronics, the
background common-mode signal is cancelled before the amplifier stage so that
high sensitivity (below 1 atto-Farad capacitance changes) is obtained.
Experimental characterization of the microwave probes was performed on
ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or
topographical signals can be realized using different reflection modes of the
probe. | cond-mat_other |
Universal Relations of Energy Flow, Acoustic Spin and Torque for
Near-Field Acoustic Tweezers: Acoustic spin, radiation torque, energy flow, and reactive power are of
significant importance from both fundamental and practical aspects, responsible
for flexible tweezer manipulations and near-field sound directionality.
Nevertheless, the intrinsic relations among these physical quantities are far
from clear. Here, we prove the universal geometric relations among them in
acoustics, independent on wave structure details. Particularly, we connect
acoustic spin and torque to the cross product of time-averaged energy flow and
reactive power, as well as to the local vorticity of energy flow. These
relations are universally valid, verified in a variety of different acoustic
systems. We also demonstrate the multipole mechanical torques and forces
generated in three acoustic near-field sources: Janus, Huygens and Spin
sources, applying on small lossy particles. These universal geometric relations
uncover hidden locking relations beyond simple spin-momentum locking of
near-field waves, and show the basic principles between the acoustic spin,
radiation torque, and energy flow, reactive power. | cond-mat_other |
Quantum control and entanglement using periodic driving fields: We propose a scheme for producing directed motion in a lattice system by
applying a periodic driving potential. By controlling the dynamics by means of
the effect known as coherent destruction of tunneling, we demonstrate a novel
ratchet-like effect that enables particles to be coherently manipulated and
steered without requiring local control. Entanglement between particles can
also be controllably generated, which points to the attractive possibility of
using these technique for quantum information processing. | cond-mat_other |
Quantum depletion of collapsing Bose-Einstein condensates: We perform the first numerical three-dimensional studies of quantum field
effects in the Bosenova experiment on collapsing condensates by E. Donley et
al. [Nature 415, 39 (2002)] using the exact experimental geometry. In a
stochastic truncated Wigner simulation of the collapse, the collapse times are
larger than the experimentally measured values. We find that a finite
temperature initial state leads to an increased creation rate of uncondensed
atoms, but not to a reduction of the collapse time. A comparison of the
time-dependent Hartree-Fock-Bogoliubov and Wigner methods for the more
tractable spherical trap shows excellent agreement between the uncondensed
populations. We conclude that the discrepancy between the experimental and
theoretical values of the collapse time cannot be explained by Gaussian quantum
fluctuations or finite temperature effects. | cond-mat_other |
Superfluid behavior of quasi-1D p-H$_2$ inside carbon nanotube: We perform ab-initio Quantum Monte Carlo simulations of para-hydrogen
(pH$_2$) at $T=0$ K confined in carbon nanotubes (CNT) of different radii. The
radial density profiles show a strong layering of the pH$_2$ molecules which
grow, with increasing number of molecules, in solid concentric cylindrical
shells and eventually a central column. The central column can be considered an
effective one-dimensional (1D) fluid whose properties are well captured by the
Tomonaga-Luttinger liquid theory. The Luttinger parameter is explicitly
computed and interestingly it shows a non-monotonic behavior with the linear
density similar to what found for pure 1D $^3$He. Remarkably, for the central
column in a (10,10) CNT, we found an ample linear density range in which the
Luttinger liquid is (i) superfluid and (ii) stable against a weak disordered
external potential, as the one expected inside realistic pores. This superfluid
behavior could be experimentally revealed in bundles of carbon nanotubes, where
deviations from classical inertial values associated with superfluid density
could be measured via torsional oscillator techniques. In summary, our results
suggest that pH$_2$ within carbon nanopores could be a practical realization of
the long sought-after, elusive superfluid phase of parahydrogen. | cond-mat_other |
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