Superfluidity of Thin He4 Films

Superfluid persistent currents have been studied in $^{4}\mathrm{He}$ films adsorbed on a porous Vycor glass substrate at temperatures down to 150 mK. Stable superflow was observed in films as thin as 2.4 atomic layers at the lowest temperatures. Critical-velocity and superfluid-mass data were obtained as a function of film thickness. These data indicate that on a Vycor substrate superflow is only possible for films greater than 2 atomic layers in thickness and, further, that only the coverage above the first two layers contributes to the superflow.