Abstract
We have developed a model for the onset of nonlinear dissipation in thin superfluid helium films below the static transition temperature ({ital T}{sub KT}) by extending the linear response theory of Ambegaokar, Halperin, Nelson, and Siggia to include flow velocity. Contributions from both vortex pairs and free vortices are included, the relative contributions of which are controlled by two weakly coupled adjustable parameters: the vortex diffusivity {ital D} and a free vortex creation time {tau}{sub {ital f}}. The model does not predict a critical velocity in the usual sense except at {ital T}=0; however, we define a characteristic velocity for the onset of nonlinear dissipation to compare with experiments. At {ital T}=0 we find a critical velocity given by the Feynman criterion, where the frequency-dependent diffusion length coincides with the zero in the vortex pair energy. Applications of this model to experiments with ac and dc flows are discussed.
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