Electronic structure ofCaxC60fullerides

Photoemission and inverse photoemission studies of ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ films show insulating character for x<3±0.3 as states derived primarily from the LUMO orbitals of ${\mathrm{C}}_{60}$ are occupied. Metallic character is observed at higher Ca content as a second band of states is partially occupied. This band has mixed character since it is derived from the LUMO+1 orbitals of ${\mathrm{C}}_{60}$ and the Ca s states. These states are responsible for superconductivity in ${\mathrm{Ca}}_5$ ${\mathrm{C}}_{60}$.