Superconductivity in A3C60 compounds is usually assumed to be driven by the electron-phonon interaction. Traditionally, superconductivity is analyzed in terms of the electron-phonon coupling lambda and the Coulomb pseudopotential mu*, where mu* describes the effects of the Coulomb repulsion on superconductivity. For conventional superconductors, mu* is argued to be small due to retardation effects, and the electron-phonon interaction can drive superconductivity. Since retardation effects are expected to be inefficient at reducing mu* for fullerides, it may seem surprising that superconductivity is at all possible, in particular at a large critical temperature.

Due to the potential importance of mu* for alkali-doped fullerides, it is artificial to treat the electron-phonon and the electron-electron interaction separately. The two interactions can be treated on an equal footing in the dynamical mean-field theory (DMFT). We find that a local pairing on the C60 molecules plays a very important role. This results from an interplay between the electron-electron interaction and the interaction between electrons and intramolecular Jahn-Teller phonons. This local pairing is actually favored by the Coulomb repulsion. The result is that the Coulomb repulsion is much less damaging to superconductivity than one might have expected ( Strong Superconductivity with Local Jahn-Teller Phonons in C60 Solids , Phys. Rev. Lett. 90, 167006 (2003)).

Superconductivity in alkali-doped fullerides therefore differs essentially from the picture of conventional superconductors, in the sense that the Coulomb repulsion is mainly overcome by local pairing and screening effects instead of retardation effects. The molecular solid character of C60 and the presence of Jahn-Teller phonons are essential for these results.

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