Study of the Superfluid Transition in Two-Dimensional He-4 Films

We have studied the superfluid transition of a thin two-dimensional helium film adsorbed on an oscillating substrate. The superfluid mass and dissipation when analyzed in terms of the dynamic theory of Ambegaokar, Halperin, Nelson, and Siggia support the Kosterlitz-Thouless picture of the phase transition in a two-dimensional superfluid. The value for the jump in the superfluid density at the transition given by Kosterlitz and Thouless, ${\ensuremath{\rho}}_{s}({{T}_{c}}^{\ensuremath{-}})=8\ensuremath{\pi}{k}_{\mathrm{B}}{(\frac{m}{h})}^{2}{T}_{c}$, is in good agreement with estimates from experiment.