Non-self-similar modes of vibration of a Bose-Einstein condensate

A hydrodynamical version of the time-dependent Gross-Pitaevskii equation is used to describe driven vibrations of a Bose-Einstein condensate of Rb-87 atoms in a magnetic trap. If the trap frequency is suddenly decreased, and later is suddenly returned to its initial value, the response of the conden sate departs from the sell-similar character that is obtained in the Thomas -Fermi approach. We show that the self-similar Thomas-Fermi modes are, in f act, unstable. Thus, the 'quantum pressure' term in the hydrodynamic equati ons of motion can play a significant role in condensate excitation dynamics , even when its effect on ground-state properties is negligible.