Superconductivity in alkali-doped fullerides

Alkali-doped fullerides differ in important respects from conventional superconductors. In the latter omega(ph) << W, where omega(ph) and W are the phononic and electronic energy scales, respectively. Retardation effects are therefore believed to reduce drastically the effects of the Coulomb repulsion, allowing the phonom induced attraction to drive superconductivity. For the alkali-doped fullerides, on the other hand, omega(ph) similar to W. We discuss retardation effects extensively, and argue that these effects should not drastically reduce the Coulomb repulsion, raising questions about how superconductivity is possible. It is then important to treat the Coulomb repulsion and the electron-phonon interaction on an equal footing, rather than treating the Coulomb repulsion as a small empirical parameter mu*, as is done for conventional superconductors. For this reason we use the dynamical mean-field theory. We find that the interplay between the electron-electron and electron-phonon interactions is crucial, leading to a so-called local pairing, where the electrons tend to pair on the molecules. This results from the important phonons being intramolecular Jahn-Teller phonons. This local pairing helps forming a coherent superconducting state and it makes the superconductivity quite resistant to the Coulomb repulsion. It also explains the strong doping dependence in these systems.