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arXiv:physics/0209083

Role of Molecular Dissociation in Feshbach-Interacting 85Rb Condensates

Matt Mackie, Kalle‐Antti Suominen, Juha Javanainen

Published Sep 23, 2002• 0 citations
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Abstract

Recent Feshbach-resonance experiments with 85Rb Bose-Einstein condensates have led to a host of unexplained results: dramatic losses of condensate atoms for an across-resonance sweep of the magnetic field, a collapsing condensate with a burst of atoms emanating from the remnant condensate, increased losses for decreasing interaction times--until very short times are reached, and coherent oscillations between remnant and burst atoms. In particular, the amplitude of the remnant-burst oscillations, and the corresponding missing atoms, have prompted speculation as to the formation of a molecular condensate. Using a minimal mean-field model, we find that rogue dissociation, molecular dissociation to noncondensate atom pairs, is qualitatively implicated as the physical mechanism responsible for these observations, although very little molecular condensate is formed. Refining the model provides excellent quantitative agreement with the experimental remnant-burst oscillations, and the fraction of molecular condensate accounts almost entirely for the measured atom loss.

DOI
10.48550/arxiv.physics/0209083

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Recent Feshbach-resonance experiments with 85Rb Bose-Einstein condensates have led to a host of unexplained results: dramatic losses of condensate atoms for an across-resonance sweep of the magnetic field, a collapsing condensate with a burst of atoms emanating from the remnant condensate, increased losses for decreasing interaction times--until very short times are reached, and coherent oscillations between remnant and burst atoms. In particular, the amplitude of the remnant-burst oscillations, and the corresponding missing atoms, have prompted speculation as to the formation of a molecular condensate. Using a minimal mean-field model, we find that rogue dissociation, molecular dissociation to noncondensate atom pairs, is qualitatively implicated as the physical mechanism responsible for these observations, although very little molecular condensate is formed. Refining the model provides excellent quantitative agreement with the experimental remnant-burst oscillations, and the fraction of molecular condensate accounts almost entirely for the measured atom loss.
Recent Feshbach-resonance experiments with 85Rb Bose-Einstein condensates have led to a host of unexplained results: dramatic losses of condensate atoms for an across-resonance sweep of the magnetic field, a collapsing condensate with a burst of atoms emanating from the remnant condensate, increased losses for decreasing interaction times, and coherent oscillations between remnant and burst atoms. Using a simple yet realistic mean-field model, we find that rogue dissociation, molecular dissociation to noncondensate atom pairs, is strongly implicated as the physical mechanism responsible for these observations.
We consider the dissociation of a molecular Bose-Einstein condensate due to a magnetic-field sweep that starts on the molecular side of a Feshbach resonance and ends either on the atomic or on the molecular side. We determine both the time-dependent response of the molecular condensate wave function and the energy distribution of the atoms produced after the sweep. In the cases where experiments have been performed, our analytic and numerical calculations provide good agreement with experimental atomic energy distributions.
In experiments conducted recently at MIT on Na Bose-Einstein condensates [S. Inouye et al., Nature (London) 392, 151 (1998); J. Stenger et al., Phys. Rev. Lett. 82, 2422 (1999)], large loss rates were observed when a time-varying magnetic field was used to tune a molecular Feshbach resonance state near the state of a pair of atoms in the condensate. A collisional deactivation mechanism affecting a temporarily formed molecular condensate [see V. A. Yurovsky, A. Ben-Reuven, P. S. Julienne and C. J. Williams, Phys. Rev. A 60, R765 (1999)], studied here in more detail, accounts for the results of the slow-sweep experiments. A best fit to the MIT data yields a rate coefficient for deactivating atom-molecule collisions of $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10} {\mathrm{cm}}^{3}/\mathrm{s}.$ In the case of the fast-sweep experiment, a study is carried out of the combined effect of two competing mechanisms, the three-atom (atom-molecule) or four-atom (molecule-molecule) collisional deactivation versus a process of two-atom trap-state excitation by curve crossing [F. H. Mies, P. S. Julienne, and E. Tiesinga, Phys. Rev. A 61, 022721 (2000)]. It is shown that both mechanisms contribute to the loss comparably and nonadditively.
Recent experiments with Feshbach-resonant cesium Bose-Einstein condensates have led to unexplained molecule formation: a sudden switch of the magnetic field to its resonance value, followed by a finite hold time and another sudden switch to magnetic field values below threshold, converts about a third of the initial condensate atoms into molecules. Based on a model of coherent conversion between an atomic condensate, a molecular condensate, and magnetodissociated noncondensate atom pairs of equal and opposite momentum, we find that population trapping is strongly implicated as the physical mechanism responsible for molecule formation in switch experiments.
We examine collective magnetoassociation of a Bose-Einstein condensate (BEC), focusing on rapid adiabatic passage from atoms to molecules induced by a sweep of the magnetic field across a wide (>~ 10 G) Feshbach resonance in 85Rb. This problem raises an interest because strong magnetoassociation is expected to favor the creation of molecular-dissociated atom pairs over the formation of molecular BEC [Javanainen and Mackie, Phys. Rev. Lett. 88, 090403 (2002)]. Nevertheless, the conversion to atom pairs is found to depend on the direction of the sweep, so that a system initially above threshold (open dissociation channel) may in fact give near-complete conversion to molecules.
In recent experiments on Na Bose-Einstein condensates [S. Inouye et al., Nature 392, 151 (1998); J. Stenger et al., Phys. Rev. Lett. 82, 2422 (1999)], large loss rates were observed when a time-varying magnetic field was used to tune a molecular Feshbach resonance state near the state of atom pairs that belong to the condensate many-body wave function. A mechanism is offered here to account for the observed losses. It is based on the deactivation of the resonant molecular state by interaction with a third condensate atom, with a deactivation rate coefficient of magnitude $\ensuremath{\sim}{10}^{\ensuremath{-}10} {\mathrm{cm}}^{3}/\mathrm{s}.$