Superfluid density of 4He films adsorbed in porous MCM-41 ceramic

The superfluid transition of He-4 films adsorbed in MCM-41 ceramic with 40-angstrom-diameter, micron-length cylindrical channels is studied with a torsion oscillator technique. For film coverages above 1.7 layers a finite-size Kosterlitz-Thouless transition becomes apparent in the data, with considerable broadening of the KT jump of the superfluid fraction. With decreasing helium coverage the extent of the broadening increases, indicating that the vortex core size is increasing. The data is consistent with the Machta-Guyer theory of the KT transition in a cylindrical channel, but with a coverage-dependent Vortex Core size. At low temperatures a linear decrease of the superfluid fraction with temperature is observed, indicating a zero-dimensional excitation.