Progress in year 1998

Collective excitations are the fingerprints of a system and reveal many of its dynamic properties. We extended earlier work on collective excitations of a Bose-Einstein condensate by studying them at non-zero temperature and at high density where they become analogous to first and second sound in superfluid helium [1]. The existence of such two modes is characteristic of a superfluid system. Our study focused on the lowest-lying shape oscillation. This oscillation was probed above and below the Bose-Einstein condensation temperature. The temperature dependencies of the frequency and damping rates of condensate oscillations indicate significant interactions between the condensate and the thermal cloud. First sound, which constitutes hydrodynamic oscillations of the thermal cloud, was observed. An antisymmetric dipolar oscillation of the thermal cloud and the condensate was also studied. This excitation represents the bulk flow of a superfluid through the normal fluid and has similarities to second sound. The detailed theoretical description of these results is currently a challenge for many-body theorists.