Spectroscopic Evidence Of The Existence Of Substantial Ca 3d Derived States At The Fermi Level In The Ca-Intercalated Graphite Superconductor Cac6

We have performed soft x-ray photoemission studies of Ca-intercalated graphite superconductor CaC6 (T-c=11.2 K). The valence-band spectrum shows six main structures, with a peak at the Fermi level (E-F), which correspond to those of calculated DOS. The intensity of the E-F peak is resonantly enhanced at the Ca 2p-3d threshold, providing spectroscopic evidence for the existence of Ca 3d electrons at E-F, which is confirmed experimentally for the first time. The Ca 2p core-level spectrum has a very large asymmetric line shape, which is further possible suggestion of the existence of Ca 3d derived conduction electrons at Ca sites. These results strongly support the picture where Ca 3d derived interlayer band plays a crucial role for the superconductivity of this material with relatively high T-c.