${\mathrm{Sr}}_6$${\mathrm{C}}_{60}$ and ${\mathrm{Ba}}_6$${\mathrm{C}}_{60}$: Semimetallic fullerides

We have studied the electronic structure of the recently discovered superconducting body-centered-cubic alkaline-earth fullerides ${\mathrm{Sr}}_6$ ${\mathrm{C}}_{60}$ and ${\mathrm{Ba}}_6$ ${\mathrm{C}}_{60}$ using the local-density approximation in the density-functional theory. Both fullerides are found to be the novel semimetallic compounds of alkaline-earth atoms and ${\mathrm{C}}_{60}$ clusters in which the hybridization between the alkaline-earth-atom states and the ${\mathrm{C}}_{60}$ π states is essential for their metallic electronic structure, and consequently for the occurrence of the superconductivity.