| A model for the onset of nonlinear dissipation in AC and DC flows is presented as an extension of the linear theory of Ambegaokar, Halperin, Nelson, and Siggia to include to flow velocity. The renormalization group calculation is carried out on a computer to determine the dissipation as a function of superfluid velocity. We find the onset of nonlinear dissipation is driven by two mechanisms whose relative contributions are temperature dependent. The free vortex contribution is most important at the higher temperatures whereas the bound pair contribution is dominant at low temperatures. The model is fitted to experimental measurements with two weakly coupled adjustable parameters: the vortex diffusivity and a free vortex creation time. Our model does not predict a critical velocity in the usual sense except at T = 0, but we define a characteristic velocity in order to compare the theory with experiment. At T = 0 we find a critical velocity given by the Feynman criterion, where the frequency dependent vortex diffusion length coincides with the zero in the vortex energy. In addition, a novel experimental apparatus using a 3(mu) I.D. capillary to allow us to change the helium film thickness at low temperatures is described. |
