203 research outputs found
Interplay of Density and Phase Fluctuations in Ultracold One-dimensional Bose Gases
The relative importance of density and phase fluctuations in ultracold one
dimensional atomic Bose gases is investigated. By defining appropriate
characteristic temperatures for their respective onset, a broad experimental
regime is found, where density fluctuations set in at a lower temperature than
phase fluctuations. This is in stark contrast to the usual experimental regime
explored up to now, in which phase fluctuations are largely decoupled from
density fluctuations, a regime also recovered in this work as a limiting case.
Observation of the novel regime of dominant density fluctuations is shown to be
well within current experimental capabilities for both and ,
requiring relatively low temperatures, small atom numbers and moderate aspect
ratios.Comment: Expanded experimental discussion, modified Fig.
Basis-dependent dynamics of trapped Bose-Einstein condensates and analogies with semi-classical laser theory
We present a consistent second order perturbation theory for the lowest-lying
condensed modes of very small, weakly-interacting Bose-Einstein condensates in
terms of bare particle eigenstates in a harmonic trap. After presenting our
general approach, we focus on explicit expressions for a simple three-level
system, mainly in order to discuss the analogy of a single condensate occupying
two modes of a trap with the semi-classical theory for two-mode photon lasers.
A subsequent renormalization of the single-particle energies to include the
dressing imposed by mean fields demonstrates clearly the consistency of our
treatment with other kinetic approaches.Comment: 2 Modified Sections: (i) Analogy between 2-mode BEC and
Semi-classical laser theory (ii) Links to other kinetic theories made more
explicit. European Physical Journal D (accepted for publication): Laser
Cooling and Quantum Gas Sectio
Phase coherence in quasicondensate experiments: an ab initio analysis via the stochastic Gross-Pitaevskii equation
We perform an ab initio analysis of the temperature dependence of the phase
coherence length of finite temperature, quasi-one-dimensional Bose gases
measured in the experiments of Richard et al. (Phys. Rev. Lett. 91, 010405
(2003)) and Hugbart et al. (Eur. Phys. J. D 35, 155-163 (2005)), finding very
good agreement across the entire observed temperature range
(). Our analysis is based on the one-dimensional stochastic
Gross-Pitaevskii equation, modified to self-consistently account for
transverse, quasi-one-dimensional effects, thus making it a valid model in the
regime . We also numerically implement an
alternative identification of , based on direct analysis of the
distribution of phases in a stochastic treatment.Comment: Amended manuscript with improved agreement to experiment, following
some additional clarifications by Mathilde Hugbart and Fabrice Gerbier and
useful comments by the reviewer; accepted for publication in Physical Review
Crossover dark soliton dynamics in ultracold one-dimensional Bose gases
Ultracold confined one-dimensional atomic gases are predicted to support dark
soliton solutions arising from a nonlinear Schr\"{o}dinger equation of suitable
nonlinearity. In weakly-interacting (high density) gases, the nonlinearity is
cubic, whereas an approximate model for describing the behaviour of strongly -
interacting (low density) gases is one characterized by a quintic nonlinearity.
We use an approximate analytical expression for the form of the nonlinearity in
the intermediate regimes to show that, near the crossover between the two
different regimes, the soliton is predicted and numerically confirmed to
oscillate at a frequency of , where is the harmonic
trap frequency.Comment: To appear in Phys. Lett.
Theory of Bose-Einstein condensation for trapped atoms
We outline the general features of the conventional mean-field theory for the
description of Bose-Einstein condensates at near zero temperatures. This
approach, based on a phenomenological model, appears to give excellent
agreement with experimental data. We argue, however, that such an approach is
not rigorous and cannot contain the full effect of collisional dynamics due to
the presence of the mean-field. We thus discuss an alternative microscopic
approach and explain, within our new formalism, the physical origin of these
effects. Furthermore, we discuss the potential formulation of a consistent
finite-temperature mean-field theory, which we claim necessiates an analysis
beyond the conventional treatment.Comment: 12 pages. To appear in Phil. Trans. R. Soc. Lond. A 355 (1997
Spatial Correlation Functions of one-dimensional Bose gases at Equilibrium
The dependence of the three lowest order spatial correlation functions of a
harmonically confined Bose gas on temperature and interaction strength is
presented at equilibrium. Our analysis is based on a stochastic Langevin
equation for the order parameter of a weakly-interacting gas. Comparison of the
predicted first order correlation functions to those of appropriate mean field
theories demonstrates the potentially crucial role of density fluctuations on
the equilibrium coherence length. Furthermore,the change in both coherence
length and shape of the correlation function, from gaussian to exponential,
with increasing temperature is quantified. Moreover, the presented results for
higher order correlation functions are shown to be in agreeement with existing
predictions. Appropriate consideration of density-density correlations is shown
to facilitate a precise determination of quasi-condensate density profiles,
providing an alternative approach to the bimodal density fits typically used
experimentally
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