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arXiv:cond-mat/0110498

Growth-Collapse Cycles of a Bose-Einstein Condensate with Attractive Interactions

Kerson Huang

Published Oct 23, 2001• 1 citation
arXivHTMLPDF

Abstract

A Bose-Einstein condensate of atoms with attractive interactions exhibits growth and collapse cycles, when it is fed by a thermal cloud. Recently this phenomenon has been directly observed in a trapped Li-7 gas. We offer a quantitative explanation of the data, on the basis of a model proposed earlier. It is shown that the condensate wave function acquires a chaotic component after the first collapse, indicating superfluid turbulence.

DOI
10.48550/arxiv.cond-mat/0110498

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•Nov 1, 1999•0 citations
A Bose-Einstein condensate of atoms with attractive interactions exhibits growth and collapse cycles, when it is fed by a thermal cloud. Recently this phenomenon has been directly observed in a trapped Li-7 gas. We offer a quantitative explanation of the data, on the basis of a model proposed earlier. It is shown that the condensate wave function acquires a chaotic component after the first collapse, indicating superfluid turbulence.
We consider the dynamics of a quantum degenerate trapped gas of 7Li{}^{7}\mathrm{Li}7Li atoms. Because the atoms have a negative sss-wave scattering length, a Bose condensate of 7Li{}^{7}\mathrm{Li}7Li becomes mechanically unstable when the number of condensate atoms approaches a maximum value. We calculate the dynamics of the collapse that occurs when the unstable point is reached. In addition, we use the quantum Boltzmann equation to investigate the nonequilibrium kinetics of the atomic distribution during and after evaporative cooling. The condensate is found to undergo many cycles of growth and collapse before a stationary state is reached.
Summary form only given. Quantum theory predicts that Bose-Einstein condensation (BEC) of a spatially homogeneous gas with attractive interactions is precluded by a conventional phase transition into either a liquid or solid. When confined to a trap, however, such a condensate can form provided that its occupation number No does not exceed a limiting value. The stability limit is determined by a balance between self-attraction and a repulsion arising from position-momentum uncertainty under conditions of spatial confinement. Near the stability limit, self-attraction can overwhelm the repulsion, causing the condensate to collapse. Growth of the condensate, therefore, is punctuated by intermittent collapses, which are triggered either by macroscopic quantum tunneling or thermal fluctuation. The data presented here are the first direct observations of the growth and collapse of Bose Einstein condensates with attractive interactions.
The nature of Bose-Einstein condensation (BEC) in atomic vapours depends critically on the mean-field interaction of the atoms. When this interaction is attractive, the number of atoms in the condensate is limited. The interplay of this limit and the natural growth of the condensate during BEC leads to complicated dynamical behaviour. We develop a model for this behaviour based on the nonlinear Schrödinger equation and the quantum Boltzmann equation. The condensate occupation number is predicted to oscillate rapidly as the condensate alternately fills and collapses, and the oscillations are seen to persist for many cycles before the gas comes to equilibrium. Experimental evidence for these oscillations in the case of 7 Li is presented, along with details of the experimental apparatus and methods.
Bose-Einstein condensation (BEC) of atoms with attractive interactions is profoundly different from BEC of atoms with repulsive interactions. We describe experiments with Bose condensates of 7Li atoms, which are weakly attracting at ultralow temperature. We measure the distribution of condensate occupation numbers occurring in the gas using a near-resonant optical probe to image the atom cloud. This data shows that the number is limited and demonstrates the dynamics of condensate growth and collapse due to the attractive interactions.
In trapped Bose-Einstein condensates (BECs), \emph{condensate growth} refers to the process in which an increasing number of quasi-particles are immediately transferred from the non-condensate state (the thermal cloud) into the condensate state following the initial formation of the BEC. Despite its physical significance, this phenomenon has not yet been studied rigorously from a mathematical standpoint. In this work, we investigate a kinetic equation whose collision operator includes three types of wave interactions: one corresponding to a 3-wave process, and two classified as 4-wave processes. This wave kinetic equation models the evolution of the density function of the thermal cloud. We establish the immediate formation of condensation in solutions to this equation, thus providing a rigorous demonstration of the condensate growth phenomenon.
Bose-Einstein condensation (BEC) of 7Li has been investigated. Because the effective interaction between 7Li atoms is attractive, the condensate occupation number N 0 is limited to ∼ 1250 atoms, and when this limit is exceeded, the condensate becomes unstable with respect to mechanical collapse. The interplay of this limit and the natural growth of the condensate during BEC leads to complicated dynamical behavior, which has been studied both theoretically and experimentally. It has been modeled by solving the quantum Boltzmann equation, in conjunction with results from the nonlinear Schrodinger equation. It is found that N0 oscillates rapidly as the condensate alternately fills and collapses, and that the oscillations can persist for many cycles before the gas comes to equilibrium. Experimental evidence for these oscillations was obtained by repeatedly producing a condensate and measuring N 0. The results were seen to vary randomly from one measurement to the next, which is to be expected as the timing of the oscillations is intrinsically stochastic. The distribution of N0 values occurring was measured, and provides quantitative information on the condensate growth and collapse. The equilibration process itself was also observed, by quenching the gas into degeneracy and observing its relaxation.\nIn order to carry out these experiments, sensitive measurement and analysis techniques were developed which enabled N0 to be determined in situ with an accuracy of +/-20% and a precision of +/-60 atoms. The theoretical tools used to study quantum degenerate gases were also applied to the important experimental technique of evaporative cooling, which led to substantial optimization and improvements. As part of this study, the rate constant for dipolar relaxation was measured to be 1.05 +/- 0.1 x 10-14 cm3/s, in agreement with theoretical predictions.