Electronic states at the Fermi level of dopedC60

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 ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ exhibits a clear metallic edge, although the Fermi level is positioned at low state density [N(${\mathit{E}}_{\mathit{f}}$)=0.8±0.2 states/eV molecule] past the band maximum. The ${\mathrm{K}}_4$ ${\mathrm{C}}_{60}$ phase is insulating. Correlation effects are necessary to explain the data as a function of doping.