Size-dependent Kosterlitz-Thouless superfluid transition in thin He4 films adsorbed on porous glasses

We report the first quantitative study of the size-dependent suprefluid transition in He4 films adsorbed on porous glasses with pore diameters d from 50 to 10 000. Tc for the submonolayer films decreases as d increases up to d=2900, whereas Tc for the full-pore liquid increases with d. The d dependence of Tc and the dissipation at the transition for the films reveal that the vortex-pair unbinding on length scales smaller than the pore sizes determines the superfluid transition. We obtain the diameter of vortex core a0=25±12 from the fit by letting the 2D coherence length 2+(Tc)=d/2. AE