ELECTRONIC STATES AT THE FERMI-LEVEL OF DOPED C-60

The density of states of the lowest unoccupied molecular orbital band as a function of K doping was investigated by yield spectroscopy excited with visible and near-uv photons. The Fermi level position was determined at each composition with high precision. The superconducting phase K3Co60 exhibits a clear metallic edge, although the Fermi level is positioned at low state density [N(E(f))=0.8 +/- 0.2 states/eV molecule] past the band maximum. The K4C60 phase is insulating. Correlation effects are necessary to explain the data as a function of doping.