Superfluid flow dissipation in two dimensions

A model is presented describing the onset of nonlinear dissipation in two-dimensional helium films for allT_KT, whereT_KT is the Kosterlitz-Thouless transition temperature. The superfluid velocity is included in the renormalization equations of Kosterlitz and Thousless, and it is shown that the model predicts a characteristic velocity for a given temperature, film thickness, and frequency where onset occurs. No critical velocity is predicted except atT=0. The model neglects the effects of prinning, and preliminary measurements seem to substantiate this assumption.