Intramolecular Charge Disproportionation and the Band Structure ofA3C60Superconductors

Realistic electronic model calculations provide convincing evidence for intramolecular charge disproportionation of the LUMO (lowest unoccupied molecular orbitals) band structure in the $\mathrm{Fm}\overline{3}m$ family of $A_3{\mathrm{C}}_{60}$ ( $A=\mathrm{K}$, Rb, Cs) superconductors. The conventional LUMO band splits into three similar subbands showing an overall occupied bandwidth of 1 eV and a steep decrease of the density of states at the Fermi level when the band population deviates from half filling. The disproportionation effect results also in nonzero Jahn-Teller distortions, inhomogeneous charge distribution among carbon atoms, and low frequency reorientation modes of Jahn-Teller distortions on each fullerene site.