Superfluidity and structural order in 4He adsorbed on a C20 molecule: Path-integral Monte Carlo calculations

We have studied adsorption of He-4 on the surface of a single C-20 fullerene molecule using the path-integral Monte Carlo method. For a full incorporation of the surface corrugations on the molecular surface the He-4-C-20 interaction is treated with a sum of empirical helium-carbon interatomic pair potentials. Radial density distributions show layer-by-layer growth of He-4, and a detailed analysis of energetics and angular density distributions reveals that the strongly bound first layer, located at a distance of similar to 4.9 angstrom from the center of the C-20 molecule, is in various quantum states as the number of He-4 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=0.31 K. This manifestation of supersolidity on a nanometer scale is understood to be induced by the presence of mobile vacancies.