Atomically thin superfluid helium films on solid hydrogen

We have measured both adsorption isotherms and third-sound resonance of atomically thin ⁴He 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±{}0.4 K (layers)³. We have also measured third sound, at 0.18 K, down to a total helium coverage of 0.45±{}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.