Superfluid transition inHe4films on hydrogen and its effect on the film-vapor coupling

We have measured the 2D ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ superfluid density ${\mathrm{\sigma }}_{\mathit{s}}$ 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 ${\mathit{T}}_{\mathit{c}}$ below 0.9 K we find that ${\mathrm{\sigma }}_{\mathit{s}}$(${\mathit{T}}_{\mathit{c}}$)∝${\mathit{T}}_{\mathit{c}}$ as predicted by Kosterlitz-Thouless theory. For transitions above 1 K, where there is significant vapor damping, we see a striking suppression of ${\mathrm{\sigma }}_{\mathit{s}}$(${\mathit{T}}_{\mathit{c}}$) and find clear evidence for the appearnace of excess gas-atom slip in the superfluid phase.