Commensurate-incommensurate transition of 4He adsorbed on a single C60 molecule.

Path-integral Monte Carlo calculations have been performed to study (4)He adsorption on a single C(60) molecule. Helium corrugations on the fullerene molecular surface are incorporated with the (4)He-C(60) interaction described by the sum of all (4)He-C interatomic pair potentials. Radial density distributions show a layer-by-layer growth of (4)He with the first adlayer being located at a distance of ~6.3 Å from the center of the C(60) molecule. The monolayer shows different quantum states as the number of (4)He adatoms N varies. For N = 32, we find a commensurate solid, with each of the 32 adsorption sites on the molecular surface being occupied by a single (4)He atom. Various domain-wall structures are observed as more (4)He atoms are added and the first layer crystallizes into an incommensurate solid when it is completely filled. This commensurate-incommensurate transition of the helium monolayer is found to be accompanied by re-entrant superfluid response at a low temperature of 0.31 K with the superfluidity being totally quenched at N = 32, 44, and 48. Finally, the different quantum states observed in the helium monolayer around C(60) are compared with phase diagrams proposed for the corresponding layer on a graphite surface.