Possible superfluidity of molecular hydrogen in a two-dimensional crystal phase of sodium

We theoretically investigate the ground-state properties of a molecular para-hydrogen (p-H2) film in which crystallization is energetically frustrated by embedding sodium (Na) atoms periodically distributed in a triangular lattice. In order to fully deal with the quantum nature of p-H-2 molecules, we employ the diffusion Monte Carlo method and realistic semiempirical pairwise potentials describing the interactions between H-2-H-2 and Na-H-2 species. In particular, we calculate the energetic, structural, and superfluid properties of two-dimensional Na-H-2 systems within a narrow density interval around equilibrium at zero temperature. In contrast to previous computational studies considering other alkali metal species such as rubidium and potassium, we find that the p-H-2 ground state is a liquid with a significantly large superfluid fraction of rho(s)/rho = 0.29(2). The appearance of p-H-2 superfluid response is due to the fact that the interactions between Na atoms and H-2 molecules are less attractive than between H-2 molecules. This induces a considerable reduction of the hydrogen density which favors the stabilization of the liquid phase.