Electronic states of KxC60 : insulating, metallic, and superconducting character

The recent report of electrical conductivity in the alkali metal fullerides and the discovery of superconductivity at 18 K for KxC(60) has raised fundamental questions about the electronic states on either side of the Fermi level, their occupancy with K intercalation, and the mechanism of superconductivity. Direct photoemission evidence is presented of filling of bands derived from the lowest unoccupied molecular orbital as a function of K incorporation for the metallic and insulating phases. This filling is not rigid band-like, and it reflects disorder in the K sites. Theoretical analysis indicates that KxC(60) is a strong coupling superconductor, and we suggest that the enhanced electron-phonon interaction is related to the unique hybridization of the C sp-derived states.