Viscosity spectra of a dilute Bose fluid

The spectra of longitudinal and transverse current-density fluctuations in a dilute Bose-condensed fluid at finite temperature are evaluated in the co llisionless regime using a microscopic model based on an approximate decoup ling of the equation of motion for the current density. As in earlier work on the degenerate electron gas, the calculation takes account of correlated two-particle excitations and in this approximation the two spectra are sim ply proportional to each other in the case of a contact interaction. We rep roduce some earlier results obtained in the one-loop approximation at zero temperature as well as the Beliaev-Popov values for the width of the phonon excitation. The corresponding contributions to the real part of the dynami cal exchange-correlation potentials at long wavelength are obtained via Hil bert transform. (C) 1998 Elsevier Science B.V.