On the T 4-law for the density of the normal component in Bose liquids

Abstract

The density of the normal component of an interacting Bose liquid is evaluated in the limit of low temperatures using the representation of the thermodynamic grand potential as a functional of the temperature-dependent Green's functions. It is shown that the density of the normal component vanishes at absolute zero. For low temperatures the T 4-law of Landau's semiphenomenological theory is derived without any corrections. The elementary excitations with imposed drift velocity are calculated for small energies and momenta as the poles of the Green's two-point functions at absolute zero. These poles yield a phonon spectrum modified according to the imposed drift velocity.