Vortices in Very Thin 4He Films

The structure of vortex lines is considered for the case of very thin 4He films, where the active super fluid thickness is less than one atomic layer. The vortex core size and core energy can be extracted from experimental studies of the finite-size Kosterlitz-Thouless transition. For pure 4He films the core size should scale as the mean separation between atoms in the active layer. This would lead to an increase in the core size as the film is thinned, and hence a progressive broadening of the finite-size KT transition as the transition temperature is reduced below 1K. The addition of 3He to the film will also increase the core size, further broadening the transition and giving rise to an initial upward shift of the apparent Tc.