VORTEX EQUILIBRIUM IN FILM FLOW NEAR A DEFECT

A simple model based on numerical simulations in bulk is used to exami ne the pinning characteristics of vortices in helium films adsorbed in the vicinity of substrate defects. The film profile in the absence of a vortex is first calculated including both the Van der Waals potenti al of a bump defect and surface tension. The displacement and ultimate de-pinning of a vortex line constrained within the profile is then fo llowed as a function of an imposed superfluid flow. The behavior of th e vortices lends insight into recent experimental results sensitive to the presence of pinned vortices and indicates that the relevant pinni ng sites are atomic in dimension.