COLLECTIVE EXCITATIONS OF BOSE-EINSTEIN-CONDENSED GASES AT FINITE TEMPERATURES

We have applied the Popov version of the Hartree-Fock-Bogoliubov (HFB) approximation to calculate the finite-temperature excitation spectrum of a Bose-Einstein condensate (BEC) of Rb-87 atoms. For lower values of the temperature, we find excellent agreement with recently publishe d experimental data for the JILA time-averaged orbiting potential trap . In contrast to recent comparison of the results of HFB-Popov theory with experimental condensate fractions and specific heats, there is di sagreement of the theoretical and recent experimental results near the BEC phase transition temperature. [S1050-2947(98)50701-9].