Superconductivity in potassium intercalated Ba3 C60

Here we report a new type of superconductor synthesized by intercalation of potassium into Ba3C60. Bulk superconductivity with the onset temperature of 5.6K was observed in K3Ba3C60. This compound forms a body centered cubic lattice which is regarded as a solid solution of isostructural K6C60 and Ba6C60. A naive electron counting indicates that K3Ba3C60 is a half-filled metal in which the Fermi energy exists at the center of the next lowest unoccupied molecular orbital with the t1g symmetry. This electronic state is in a sharp contrast with that of K3C60 which has a half-filled state of the t1u-derived band. The present observation indicates that the half-filled states of the triply degenerate bands yield superconductivity.