Nonadiabatic Channels in the Superconducting Pairing of Fullerides

Abstract

We show the intrinsic inconsistency of the conventional phonon mediated theory of superconductivity in relation to the observed properties of Rb$_3$C$_{60}$. The recent, highly accurate measurement of the carbon isotope coefficient $α_{\rm C}=0.21$, together with the high value of $T_c$ (30 K) and the very small Fermi energy $E_{\rm F}$ (0.25 eV), unavoidably implies the opening of nonadiabatic channels in the superconducting pairing. We estimate these effects and show that they are actually the key elements for the high value of $T_c$ in these materials compared to the very low values of graphite intercalation compounds.