Superfluidity in H₂films

Path integral Monte Carlo calculations were used to study a 2D system of H₂ molecules. The ground state on a flat substrate is a triangular lattice, a localized solid. We find that the introduction of some weakly attractive impurities (specifically alkali metal atoms) in the right concentration (∼0.01 Å^-2) stabilizes a liquid hydrogen phase of concentration 0.04 Å^-2 which undergoes a Kosterlitz-Thouless superfluid transition below approximately 1.2 K.