P. J. Benning

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 KxC60 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 KxC60 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.

Formation of fullerides and fullerene-based heterostructures.

Two potassium fulleride phases, metallic K3C60 and nonmetallic K6C60, are formed when potassium is incorporated into thin C60 films under ultrahigh vacuum conditions. Phase separation is observed for intermediate stoichiometries. Results obtained for the C60-K3C60 heterostructure demonstrate that it is stable against potassium migration from the K3C60 phase. In contrast, the C60-K6C60 interface is not stable and K3C60 is formed.

KxC60 : nonequilibrium thin film growth and spectroscopic results

Abstract This paper examines the electronic structure of the K-C 60 system. High resolution photoemission results show multiple phase formation, even for stoichiometries close to nominal single phase fields. The results are interpreted in terms of the nonequilibrium conditions for growth by vapor phase deposition.