The isolated neutral ${\mathrm{C}}_{60}$ molecule is characterized by a triply degenerate lowest unoccupied level of ${\mathit{t}}_{1\mathit{u}}$ symmetry. In the alkali-metal-intercalated ${\mathrm{C}}_{60}$ crystals, this state is partially filled and is thus subject to a Jahn-Teller distortion. To examine the relation between this effect and the superconducitvity of these crystals, we first study the stable atomic distortions as a function of the occupancy of the ${\mathit{t}}_{1\mathit{u}}$ state, stressing the similarity between this problem and the single vacancy in silicon. We then show that the on-ball electron-lattice coupling is not strong enough for ${\mathrm{C}}_{60}$ to behave as a negative-U system. We finally relate the dimensionless electron-phonon coupling constant λ to the magnitude of the Jahn-Teller energies.

• Received 3 September 1991

DOI:https://doi.org/10.1103/PhysRevB.44.12106