Colloquium: Modeling the unconventional superconducting properties of expanded A3C60 fullerides

The trivalent alkali fulleride solids of generic composition A3C60, where C60 is the fullerene molecule and A=K, Rb, and Cs, are a well-established family of molecular superconductors. The superconductive electron pairing is of regular s-wave symmetry and is accounted for by conventional coupling of electrons to phonons, in particular by well-understood Jahn-Teller intramolecular C60 vibrations. A source of renewed interest in these systems is the surprising indication of strong electron-electron repulsion phenomena, which has emerged in compounds where the C60-C60 distance is expanded, by either a large cation size or other chemical or physical means. Several examples are now known where this kind of expansion, while leading to a high superconducting temperature at first, gradually or suddenly causes a decline of superconductivity and its eventual disappearance in favor of a Mott insulating state. ... ...