Collisions and expansion of an ultracold dilute Fermi gasB. Jackson1, P. Pedri1, 2 and S. Stringari1
1 Dipartimento di Fisica, Università di Trento and BEC-INFM I-38050 Povo (TN) Italy
2 Institut für Theoretische Physik, Universität Hannover D-30167 Hannover, Germany
(Received 8 April 2004; accepted 2 June 2004)
We discuss the effects of collisions on the expansion of a degenerate normal Fermi gas, following the sudden removal of the confining trap. Using a Boltzmann equation approach, we calculate the time dependence of the aspect ratio and the entropy increase of the expanding atomic cloud taking into account the collisional effects due to the deformation of the distribution function in momentum space. We find that in dilute gases the aspect ratio does not deviate significantly from the predictions of ballistic expansion. Conversely, if the trap is sufficiently elongated, the thermal broadening of the density distribution due to the entropy increase can be sizeable, revealing that even at zero temperature collisions are effective in a Fermi gas.
03.75.-b - Matter waves.
03.75.Ss - Degenerate Fermi gases.
© EDP Sciences 2004