Spectroscopic evidence of the existence of substantialCa 3dderived states at the Fermi level in the Ca-intercalated graphite superconductorCaC6

We have performed soft x-ray photoemission studies of Ca-intercalated graphite superconductor ${\text{CaC}}_{6}({T}_{\text{c}}=11.2\text{ }\text{K})$. The valence-band spectrum shows six main structures, with a peak at the Fermi level $({E}_{\text{F}})$, which correspond to those of calculated DOS. The intensity of the ${E}_{\text{F}}$ peak is resonantly enhanced at the $\text{Ca}\text{ }2p\text{\ensuremath{-}}3d$ threshold, providing spectroscopic evidence for the existence of $\text{Ca}\text{ }3d$ electrons at ${E}_{\text{F}}$, which is confirmed experimentally for the first time. The $\text{Ca}\text{ }2p$ core-level spectrum has a very large asymmetric line shape, which is further possible suggestion of the existence of $\text{Ca}\text{ }3d$ derived conduction electrons at Ca sites. These results strongly support the picture where $\text{Ca}\text{ }3d$ derived interlayer band plays a crucial role for the superconductivity of this material with relatively high ${T}_{\text{c}}$.