Superconductivity at 52 K in hole-doped C-60

Superconductivity in electron-doped C-60 was first observed almost ten year s ago. The metallic state and superconductivity result from the transfer of electrons from alkaline or alkaline-earth ions to the C-60 molecule, which is known to be a strong electron acceptor. For this reason, it is very dif ficult to remove electrons from C-60-yet one might expect to see supercondu ctivity at higher temperatures in hole-doped than in electron-doped C-60, b ecause of the higher density of electronic states in the valence band than in the conduction band. We have used the technique of gate-induced doping i n a reld-effect transistor conrguration to introduce signircant densities o f holes into C-60. We observe superconductivity over an extended range of h ole density, with a smoothly varying transition temperature T-c that peaks at 52 K. By comparison with the well established dependence of T-c on the l attice parameter in electron-doped C-60, we anticipate that T-c values sign ircantly in excess of 100 K should be achievable in a suitably expanded, ho le-doped C-60 lattice.