Excitations of a Bose-Einstein Condensate at Non-Zero Temperature: A   Study of Zeroth, First and Second Sound
  Collective excitations of a dilute Bose gas were 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. Hydrodynamic oscillations of the thermal cloud were observed, constituting first sound. An antisymmetric dipolar oscillation of the thermal cloud and the condensate was studied, representing the bulk flow of a superfluid through the normal fluid. The excitations were observed in situ using non-destructive imaging techniques. 
D.M. Stamper-Kurn, H.-J. Miesner, S. Inouye, M.R. Andrews, W. Ketterle