Similarity and dissimilarity of doped fullerenes and oxide superconductors from the viewpoint of electronic structure

Photoemission spectroscopy shows that a very narrow peak and a broad distribution of density-of-states coexist near the Fermi level (E_F) in K₃C₆₀, while the narrow peak at E_F is not present in nonmetallic K₄C₆₀. This shows that K₄C₆₀, is a semiconductor with an energy gap at E_F, contrary to the prediction from band structure calculations, and suggests that electron correlation is important in understanding the electronic structure of doped fullerenes, both metallic K₃C₆₀ and nonmetallic K₄C₆₀. The combination of photoemission, inverse photoemission and polarized oxygen-K-absorption in high T_c oxide superconductors shows that the hole-doping results in a similar electronic structure to that of doped fullerenes with the coexistence of a very narrow peak and a broad distribution of density-of-states near E_F. We discuss the similarities and differences between doped fullerenes and high-T_c oxide superconductors based upon a comparison of their electronic structures.