Expansion of the non-condensed trapped Bose gas in Bose-Einstein condensation

Unità Istituto Nazionale di Fisica della Materia, Dipartimento di Fisica, Università di Pisa - 56126 Pisa, Italy

Received: 12 February 1998
Accepted: 22 May 1998

Abstract

The anisotropic density distribution of Bose particles after expansion from a magnetic trap has been taken as an important signature of the formation of Bose-Einstein condensate. In this letter we show that even for a non-condensed trapped gas one can get an anisotropic distribution due to anisotropic collision rates along radial and axial directions. We demonstrate that the mean energies along those directions, different from the energy in the trap, have opposite evolution tendencies with the expansion. This difference is particularly important for the case where a large number of bosons is trapped. We also show that the velocity distribution of the non-condensed gas in the trap, assumed initially isotropic and Gaussian, becomes anisotropic and with linewidth dependent on the expansion direction.