Band filling and width control in fullerides

Highly doped states of fullerene C₆₀ solids have been investigated. A single phase of binary Ba₄C₆₀ was first synthesized and confirmed to be a noncubic ambient pressure superconductor. By cointercalation of alkali metals and Ba, an isostructural series of alkali-Ba-C₆₀ ternaries have been synthesized. This new class of compounds allows us to vary the band-filling and band-filling with keeping the body-centered-cubic (bcc) structure unchanged. Systematic synthesis study of these Ba-including fullerides uncovered various aspects of the highly doped fullerides. Of particular interest is that the criteria for the occurrence of superconductivity are considerably different between the low and high reduction states. Superconductivity at the low reduction state is strictly limited at the half-filling of the t_1u band and the fcc-related structures, while that at the high reduction state appears at several valence states irrespective of crystal structures.