Superfluidity and anomalous correlations in a two-dimensional Bose gas

We show that the onset of superfluidity in a finite homogeneous 2D Bose gas is associated with the appearance of an anomalous thermal density induced by the presence of a finite-size condensate. We investigate the temporal frequency signatures of the Bose field, and show that thermally excited vortices in the field are weakly coupled to the longitudinal excitations. We apply the concept of locking of the superfluid velocity field by the condensate to estimate the superfluid density in a metastable circulating flow of a superfluid Bose gas in a 2D toroidal geometry.