Unconventional pairing in fullerides by nonadiabatic channels

In C60 compounds the frequencies of the phonon involved in the superconductive processes (intra-molecular modes with xph up to 2300 K) are comparable with the typical electronic scale (EF 1/4 2900 K) and thus the two dynamics cannot be treated separately (breakdown of the adiabatic hypothesis). This implies a generalization of the standard theory to include nonadiabatic effects. The strong electronic correlation in the fullerenes causes the contribution of the nonadiabatic processes to be positive with consequent increase of the effective electron-phonon (el-ph) pairing k and of Tc. We show that the inclusion of nonadiabatic terms in the el-ph interaction is a key element for the high values of Tc in fullerenes. In particular Tc as high as 100 K are shown to be compatible with moderate values of k K 1. This implies that it should be possible to increase the value of k, and so Tc, and still have a stable system.