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arXiv:cond-mat/0306514

No-soliton--soliton phase transition in a trapped one-dimensional Bose gas

Vanja Dunjko, Christopher P. Herzog, Yvan Castin, Maxim Olshanii

Published Jun 19, 2003• 1 citation
arXivHTMLPDF

Abstract

Following the experimental observation of bright matter-wave solitons [L. Khaykovich et al., Science v. 296, 1290 (2002); K. E. Strecker et al., Nature (London) v. 417, 150 (2002)], we develop a semi-phenomenological theory for soliton thermodynamics and find the condensation temperature. Under a modified thermodynamic limit, the condensate occupation at the critical temperature undergoes a sudden jump to a nonzero value, indicating a discontinuous phase transition. Treating the condensation as a diffusion over a barrier shows that the condensation time is exponentially long as one approaches the thermodynamic limit, and the longest near the critical temperature.

DOI
10.48550/arxiv.cond-mat/0306514

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Following the experimental observation of bright matter-wave solitons [L. Khaykovich et al., Science v. 296, 1290 (2002); K. E. Strecker et al., Nature (London) v. 417, 150 (2002)], we develop a semi-phenomenological theory for soliton thermodynamics and find the condensation temperature. Under a modified thermodynamic limit, the condensate occupation at the critical temperature undergoes a sudden jump to a nonzero value, indicating a discontinuous phase transition. Treating the condensation as a diffusion over a barrier shows that the condensation time is exponentially long as one approaches the thermodynamic limit, and the longest near the critical temperature.
Critical behavior of solitonic wave forms of Bose-Einstein condensates in optical lattices (OL's) has been studied in the framework of the continuous mean-field equation. In two and three-dimensions OL's bright matter-wave solitons undergo abrupt delocalization as the strength of the OL is decreased below some critical value. A similar delocalizing transition happens when the coefficient of nonlinearity crosses the critical value. As opposed, bright solitons in one-dimensional OL's retain their integrity over the whole range of parameter variations. An interpretation of this phenomenon in terms of quantum bound states in the effective potential is proposed.
We investigate the thermodynamic properties of a Bose-Einstein condensate with negative scattering length confined in a toroidal trapping potential. By numerically solving the coupled Gross-Pitaevskii and Bogoliubov--de Gennes equations, we study the phase transition from the uniform state to the symmetry-breaking state characterized by a bright-soliton condensate and a localized thermal cloud. In the localized regime, three states with a finite condensate fraction are present: the thermodynamically stable localized state, a metastable localized state, and also a metastable uniform state. Remarkably, the presence of the stable localized state strongly increases the critical temperature of Bose-Einstein condensation.
Contrary to many other translationally invariant one-dimensional models, the low-temperature phase for an attractively interacting one-dimensional Bose-gas (a quantum bright soliton) is stable against thermal fluctuations. However, treating the thermal properties of quantum bright solitons within the canonical ensemble leads to anomalous fluctuations of the total energy that indicate that canonical and micro-canonical ensembles are not equivalent. State-of-the-art experiments are best described by the micro-canonical ensemble, within which we predict a co-existence between single atoms and solitons even in the thermodynamic limit - contrary to strong predictions based on both the Landau hypothesis and the canonical ensemble. This questions the use of temperatures to describe state-of-the-art bright soliton experiments that currently load Bose-Einstein condensates into quasi-one-dimensional wave guides without adding contact to a heat bath.
Bright solitary-waves may be created in dilute Bose-Einstein condensates of at tractively interacting atoms in one dimensional regimes. In integrable systems, such solitary waves are particle-like objects called solitons. We investigate the consequences of non-integrability on the solitary waves in trapped Bose-Einstein condensates caused by an axial harmonic trap, and non-integrability caused by three dimensional effects. To analyse the soliton-like nature of the solitary-waves in an axial harmonic trap, a particle analogy for the solitary-waves is formulated. Exact Soliton solutions exist in the absence of an external trapping potential, which behave in a particle-like manner, and we find the particle analogy we employ to be a good model also when a harmonic trapping potential is present up to a gradual shift in the trajectories when the harmonic trap period is short compared with the, collision time of the solitons. We find that the collision time of the solions is dependent on the relative phase of the solitons as they collide. In the case of two solitons, the particle model is integrable, and the dynamics are completely regular. In the case of a system of two solitary waves of equal norm, the solitons are shown to retain their phase difference for repeated collisions. The extension to three particles supports both regular and chaotic regimes. The trajectory shift observed for two solitons carrier over to the case of three solitons. This shift aside, the agreement between the particle model and the wave dynamics remains good, even in chaotic regimes. We predict that these chaotic regimes will be an indicator of rapid depletion of the condensate due to quantum transitions of the condensate particles into non-condensate modes. To analyse the residual effects of the three dimensional nature of the solitary waves, we use a nonlinear Schrödinger equation with an additional quintic term. We perform variational calculations, and confirm the collapse of a soliton when the number of particles contained therein is increased past a critical number. We investigate the effects of varying the axial trap frequency and scattering length on the critical number. We propose a method to model particle exchange between solitons by extending the variational treatment to two solitons.
We investigate the dynamics of Bose-Einstein condensate bright solitons made of alkali-metal atoms with negative scattering length and under harmonic confinement in the transverse direction. Contrary to the one-dimensional (1D) case, the 3D bright soliton exists only below a critical attractive interaction that depends on the extent of confinement. Such a behavior is also found in multisoliton condensates with box boundary conditions. We obtain numerical and analytical estimates of the critical strength beyond which the solitons do not exist. By using an effective 1D nonpolynomial nonlinear Schr\"odinger equation, which accurately takes into account the transverse dynamics of cigarlike condensates, we numerically simulate the dynamics of the ``soliton train'' reported in a recent experiment [Nature (London) 417, 150 (2002)]. Then, analyzing the macroscopic quantum tunneling of the bright soliton on a Gaussian barrier, we find that its interference in the tunneling region is strongly suppressed with respect to nonsolitonic case; moreover, the tunneling through a barrier breaks the shape invariance of the matter wave. Finally, we show that the collapse of the soliton is induced by the scattering on the barrier or by the collision with another matter wave when the density reaches a critical value, for which we derive an accurate analytical formula.
We consider a Bose gas confined by a harmonic potential. We define an appropriate thermodynamic limit and analyze the properties of the phases and phase transition in this limit. Critical properties in the presence of the potential are found to be different from, though simply related to, those in the usual translationally invariant case. We argue that the properties of magnetically trapped rubidium and sodium gases (in which Bose-Einstein condensation has recently been observed) are well approximated by our thermodynamic limit except in a narrow window of temperature around the critical temperature. We also consider the effect of the confining potential on the non-equilibrium dynamics following a rapid quench to the ordered side and give a scaling description of the late-time universal dynamics.