114 research outputs found
Finite-temperature properties of quasi-2D Bose-Einstein condensates
Using the finite-temperature path integral Monte Carlo method, we investigate
dilute, trapped Bose gases in a quasi-two dimensional geometry. The quantum
particles have short-range, s-wave interactions described by a hard-sphere
potential whose core radius equals its corresponding scattering length. The
effect of both the temperature and the interparticle interaction on the
equilibrium properties such as the total energy, the density profile, and the
superfluid fraction is discussed. We compare our accurate results with both the
semi-classical approximation and the exact results of an ideal Bose gas. Our
results show that for repulsive interactions, (i) the minimum value of the
aspect ratio, where the system starts to behave quasi-two dimensionally,
increases as the two-body interaction strength increases, (ii) the superfluid
fraction for a quasi-2D Bose gas is distinctly different from that for both a
quasi-1D Bose gas and a true 3D system, i.e., the superfluid fraction for a
quasi-2D Bose gas decreases faster than that for a quasi-1D system and a true
3D system with increasing temperature, and shows a stronger dependence on the
interaction strength, (iii) the superfluid fraction for a quasi-2D Bose gas
lies well below the values calculated from the semi-classical approximation,
and (iv) the Kosterlitz-Thouless transition temperature decreases as the
strength of the interaction increases.Comment: 6 pages, 5 figure
Low-energy behavior of strongly interacting bosons on a flat-band lattice above the critical filling factor
Phase separation and vortex states in binary mixture of Bose-Einstein condensates in the trapping potentials with displaced centers
The system of two simultaneously trapped codensates consisting of
atoms in two different hyperfine states is investigated theoretically in the
case when the minima of the trapping potentials are displaced with respect to
each other. It is shown that the small shift of the minima of the trapping
potentials leads to the considerable displacement of the centers of mass of the
condensates, in agreement with the experiment. It is also shown that the
critical angular velocities of the vortex states of the system drastically
depend on the shift and the relative number of particles in the condensates,
and there is a possibility to exchange the vortex states between condensates by
shifting the centers of the trapping potentials.Comment: 4 pages, 2 figure
Excitation of a Dipole Topological Mode in a Strongly Coupled Two-Component Bose-Einstein Condensate
Two internal hyperfine states of a Bose-Einstein condensate in a dilute
magnetically trapped gas of Rb atoms are strongly coupled by an
external field that drives Rabi oscillations between the internal states. Due
to their different magnetic moments and the force of gravity, the trapping
potentials for the two states are offset along the vertical axis, so that the
dynamics of the internal and external degrees of freedom are inseparable. The
rapid cycling between internal atomic states in the displaced traps results in
an adiabatic transfer of population from the condensate ground state to its
first antisymmetric topological mode. This has a pronounced effect on the
internal Rabi oscillations, modulating the fringe visibility in a manner
reminiscent of collapses and revivals. We present a detailed theoretical
description based on zero-temperature mean-field theory.Comment: 10 pages, 8 eps figures included; submitted to PR
Collective ferromagnetism in two-component Fermi-degenerate gas trapped in finite potential
Spin asymmetry of the ground states is studied for the trapped
spin-degenerate (two-component) gases of the fermionic atoms with the repulsive
interaction between different components, and, for large particle number, the
asymmetric (collective ferromagnetic) states are shown to be stable because it
can be energetically favorable to increase the fermi energy of one component
rather than the increase of the interaction energy between up-down components.
We formulate the Thomas-Fermi equations and show the algebraic methods to solve
them. From the Thomas-Fermi solutions, we find three kinds of ground states in
finite system: 1) paramagnetic (spin-symmetric), 2) ferromagnetic (equilibrium)
and 3) ferromagnetic (nonequilibrium) states. We show the density profiles and
the critical atom numbers for these states obtained analytically, and, in
ferromagnetic states, the spin-asymmetries are shown to occur in the central
regions of the trapped gas, and grows up with increasing particle number. Based
on the obtained results, we discuss the experimental conditions and current
difficulties to realize the ferromagnetic states of the trapped atom gas, which
should be overcome.Comment: submit to PR
Dynamics of Bose condensed gases in highly deformed traps
We provide a unified investigation of normal modes and sound propagation at
zero temperature in Bose condensed gases confined in highly asymmetric harmonic
traps and interacting with repulsive forces. By using hydrodynamic theory for
superfluids we obtain explicit analytic results for the dispersion law of the
low energy discretized modes for both cigar and disk shaped geometries,
including the regime of large quantum numbers where discrete modes can be
identified with phonons. The correspondence with sound propagation in
cylindrical traps and the one-dimensional nature of cigar type configurations
are explicitly discussed.Comment: 12 pages Revtex, no figure
Dielectric formalism and damping of collective modes in trapped Bose-Einstein condensed gases
We present the general dielectric formalism for Bose-Einstein condensed
systems in external potential at finite temperatures. On the basis of a model
arising within this framework as a first approximation in an intermediate
temperature region for large condensate we calculate the damping of low-energy
excitations in the collisionless regime.Comment: 4 pages, no figures, RevTe
Finite Temperature Perturbation Theory for a Spatially Inhomogeneous Bose-condensed Gas
We develop a finite temperature perturbation theory (beyond the mean field)
for a Bose-condensed gas and calculate temperature-dependent damping rates and
energy shifts for Bogolyubov excitations of any energy. The theory is
generalized for the case of excitations in a spatially inhomogeneous (trapped)
Bose-condensed gas, where we emphasize the principal importance of
inhomogeneouty of the condensate density profile and develop the method of
calculating the self-energy functions. The use of the theory is demonstrated by
calculating the damping rates and energy shifts of low-energy quasiclassical
excitations, i.e. the quasiclassical excitations with energies much smaller
than the mean field interaction between particles. In this case the boundary
region of the condensate plays a crucial role, and the result for the damping
rates and energy shifts is completely different from that in spatially
homogeneous gases. We also analyze the frequency shifts and damping of sound
waves in cylindrical Bose condensates and discuss the role of damping in the
recent MIT experiment on the sound propagation.Comment: 16 pages, 3 figures, Revtex, uses epsfi
Does inter-vertebral range of motion increase after spinal manipulation? A prospective cohort study.
Background: Spinal manipulation for nonspecific neck pain is thought to work in part by improving inter-vertebral range of motion (IV-RoM), but it is difficult to measure this or determine whether it is related to clinical outcomes.
Objectives: This study undertook to determine whether cervical spine flexion and extension IV-RoM increases after a course of spinal manipulation, to explore relationships between any IV-RoM increases and clinical outcomes and to compare palpation with objective measurement in the detection of hypo-mobile segments.
Method: Thirty patients with nonspecific neck pain and 30 healthy controls matched for age and gender received quantitative fluoroscopy (QF) screenings to measure flexion and extension IV-RoM (C1-C6) at baseline and 4-week follow-up between September 2012-13. Patients received up to 12 neck manipulations and completed NRS, NDI
and Euroqol 5D-5L at baseline, plus PGIC and satisfaction questionnaires at follow-up. IV-RoM accuracy, repeatability and hypo-mobility cut-offs were determined. Minimal detectable changes (MDC) over 4 weeks were calculated
from controls. Patients and control IV-RoMs were compared at baseline as well as changes in patients over 4 weeks. Correlations between outcomes and the number of manipulations received and the agreement (Kappa) between palpated and QF-detected of hypo-mobile segments were calculated.
Results: QF had high accuracy (worst RMS error 0.5o) and repeatability (highest SEM 1.1o, lowest ICC 0.90) for
IV-RoM measurement. Hypo-mobility cut offs ranged from 0.8o to 3.5o. No outcome was significantly correlated with increased IV-RoM above MDC and there was no significant difference between the number of hypo-mobile segments in patients and controls at baseline or significant increases in IV-RoMs in patients. However, there was a modest and significant correlation between the number of manipulations received and the number of levels and directions whose IV-RoM increased beyond MDC (Rho=0.39, p=0.043). There was also no agreement between palpation and QF in identifying hypo-mobile segments (Kappa 0.04-0.06).
Conclusions: This study found no differences in cervical sagittal IV-RoM between patients with non-specific neck pain and matched controls. There was a modest dose-response relationship between the number of manipulations given and number of levels increasing IV-RoM - providing evidence that neck manipulation has a mechanical effect at segmental levels. However, patient-reported outcomes were not related to this
Collisionless modes of a trapped Bose gas
We calculate the excitation frequencies of the m=0 monopole and m=2
quadrupole modes in the collisionless regime by solving a non-linear
Schroedinger equation for the condensate, coupled to a collisionless Boltzmann
equation for the quasiparticles. Since the dynamics of the noncondensate cloud
is also taken into account, the theory satisfies the Kohn theorem. The spectrum
turns out to be strongly temperature dependent and we compare our results with
experiment.Comment: 6 pages of LaTeX and 1 postscript figure. Contribution to the meeting
`New directions in atomic phycics' held in Cambridge this mont
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