Superfluidity and structural order in H4e adsorbed on a C20 molecule: Path-integral Monte Carlo calculations

We have studied adsorption of H4e on the surface of a single C₂₀ fullerene molecule using the path-integral Monte Carlo method. For a full incorporation of the surface corrugations on the molecular surface the H4e-C₂₀ interaction is treated with a sum of empirical helium-carbon interatomic pair potentials. Radial density distributions show layer-by-layer growth of H4e , and a detailed analysis of energetics and angular density distributions reveals that the strongly bound first layer, located at a distance of ∼4.9Å from the center of the C₂₀ molecule, is in various quantum states as the number of H4e atoms changes. This layer, when completed with 32 atoms, is found to be a solid whose structure is commensurate with the underlying molecular surface. Near the completion of the first layer we observe a finite superfluid fraction as well as a solid order at a low temperature of T=0.31K . This manifestation of supersolidity on a nanometer scale is understood to be induced by the presence of mobile vacancies.