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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-H2 molecules, we employ the diffusion Monte Carlo method and realistic semi-empirical pairwise potentials describing the interactions between H2-H2 and Na-H2 species. In particular, we calculate the energetic, structural and superfluid properties of two-dimensional Na-H2 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-H2 ground-state is a liquid with a significantly large superfluid fraction of ~30%. The appearance of p-H2 superfluid response is due to the fact that the interactions between Na atoms and H2 molecules are less attractive than between H2 molecules. This induces a considerable reduction of the hydrogen density which favours the stabilization of the liquid phase.