Atomically thin superfluid helium films on solid hydrogen.

Abstract

We have measured both adsorption isotherms and third-sound resonance of atomically thin $^{4}\mathrm{He}$ films on solid hydrogen. We find helium more weakly bound to a hydrogen surface than to other rare-gas surfaces, with a van der Waals cofficient of 4.6\ifmmode\pm\else\textpm\fi{}0.4 K (layers${)}^{3}$. We have also measured third sound, at 0.18 K, down to a total helium coverage of 0.45\ifmmode\pm\else\textpm\fi{}0.01 atomic layer, indicating an inert coverage at T=0 K of 0.29 atomic layer or less. In common with other weak-binding substrates we also see depressions in third-sound velocity near half-layer intervals.