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

Path-integral Monte Carlo calculations have been performed to study He-4 adsorption on a single C-60 molecule. Helium corrugations on the fullerene molecular surface are incorporated with the He-4-C-60 interaction described by the sum of all He-4-C interatomic pair potentials. Radial density distributions show a layer-by-layer growth of He-4 with the first adlayer being located at a distance of similar to 6.3 angstrom from the center of the C-60 molecule. The monolayer shows different quantum states as the number of He-4 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 He-4 atom. Various domain-wall structures are observed as more He-4 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3685848]