Superconductivity at 52 K in hole-doped C-60 (Retracted article. See vol 422 pg 93 2003)

Superconductivity in electron-doped C-60 was first observed almost ten years 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 difficult to remove electrons from C-60-yet one might expect to see superconductivity at higher temperatures in hole-doped than in electron-doped C-60, because 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 in a reld-effect transistor conrguration to introduce signircant densities of holes into C-60. We observe superconductivity over an extended range of hole 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 lattice parameter in electron-doped C-60, we anticipate that T-c values signircantly in excess of 100 K should be achievable in a suitably expanded, hole-doped C-60 lattice.