ELECTRONIC-STRUCTURE OF CALCIUM-DOPED C-60

We have studied the electronic structure of solid Ca3C60 and the C60Ca cluster. In Ca3C60, two Ca atoms are placed at the tetrahedral interstitial sites of the face-centered cubic C60 lattice and the third Ca atom is placed either at the on-center or at the off-center position of the octahedral interstitial site. In all cases including C60Ca cluster where Ca atom is placed at the center of the C60 cage, Ca 4s electrons transfer to the C60 t1u state. In addtion, the strong hybridization between the t1g band (the second lowest-unoccupied state of the C60 cluster) and the 4s band of the Ca atom at the off-center position of the octahedral interstitial site has been found. Hence, the t1u band of the recently discovered superconductor Ca5C60 is expected to be completely occupied unlike alkali-doped C60 superconductors, and thus carriers should be in the next hybridized band of C60 t1g and the 4s of the octahedral-site Ca atoms at the off-center positions.