Superfluid transition in He4 films on hydrogen and its effect on the film-vapor coupling
We have measured the 2D He4 superfluid density s on hydrogen using a high-Q Al oscillator. We find that only 1/2 layer of helium remains normal well below the transition on hydrogen as opposed to 4 layers on bare Al. For transition temperatures Tc below 0.9 K we find that s(Tc) Tc as predicted by Kosterlitz-Thouless theory. For transitions above 1 K, where there is significant vapor damping, we see a striking suppression of s(Tc) and find clear evidence for the appearnace of excess gas-atom slip in the superfluid phase. © 1992 The American Physical Society.