SUPERFLUID TRANSITION IN HE-4 FILMS ON HYDROGEN AND ITS EFFECT ON THE FILM-VAPOR COUPLING

We have measured the 2D He-4 superfluid density sigma(s) on hydrogen using a high-Q Al oscillator. We find that only a 1/2 layer of helium remains normal well below the transition on hydrogen as opposed to 4 layers on bare Al. For transition temperatures T(c) below 0.9 K we find that sigma(s)(T(c)) is-proportional-to T(c) as predicted by Kosterlitz-Thouless theory. For transitions above 1 K, where there is significant vapor damping, we see a striking suppression of sigma(s)(T(c)) and find clear evidence for the appearance of excess gas-atom slip in the superfluid phase.