FIRST AND 2ND SOUND IN A UNIFORM BOSE-GAS

We have recently derived two-fluid hydrodynamic equations for a trappe d weakly interacting Bose gas. In this paper we use these equations to discuss first and second sound in a uniform Bose gas. These results a re shown to agree with the predictions of the usual two-fluid equation s of Landau when the thermodynamic functions are evaluated for a weakl y interacting gas. In a uniform gas, second sound mainly corresponds t o an oscillation of the superfluid (the condensate) and is the low-fre quency continuation of the Bogoliubov-Goldstone symmetry-breaking mode . [S1050-2947(97)00812-3].