Anomalous rotational properties of Bose-Einstein condensates in asymmetric traps

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Abstract
We study the rotational properties of a Bose-Einstein condensate confined in a rotating harmonic trap for different trap anisotropies. With simple arguments, we obtain the velocity field of the quantum fluid for condensates with or without a centered vortex. While the bosons describe open spiraling trajectories, on the frame of reference of the rotating trap, the fluid moves against the trap’s rotation. We also find expressions for the angular momentum of linear and Thomas-Fermi solutions for a vortexless state. In these two limits we find the same analytic relation between the shape of the cloud and the rotation speed. Our predictions are supported by numerical simulations of the mean-field Gross-Pitaevskii model and by current experiments with rotating Bose-Einstein condensates (K. W. Madison, F. Chevy, V. Bretin, and J. Dalibard, e-print cond-mat/0101051).