Band filling and width control in fullerides

Highly doped states of fullerene C-60 solids have been investigated. A sing le phase of binary Ba4C60 was first synthesized and confirmed to be a noncu bic ambient pressure superconductor. By cointercalation of alkali metals an d Ba, an isostructural series of alkali-Ba-C, ternaries have been synthesiz ed. This new class of compounds allows us to vary the band-filling and band -filling with keeping the body-centered-cubic (bcc) structure unchanged. Sy stematic 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 differen t between the low and high reduction states. Superconductivity at the low r eduction state is strictly limited at the half-filling of the t(1u) band an d the fee-related structures, while that at the high reduction state appear s at several valence states irrespective of crystal structures.