Nonadiabatic channels in fullerene compounds

Abstract

The recent discovery of superconductivity with Tc=52 K in hole doped C60 classifies the fullerene compounds among the highest-Tc materials. Although there are strong evidences of a phonon based pairing in fullerenes, these anomalously high values of the critical temperature appear well beyond the electron-phonon upper limit of generally assumed to be 25–30 K. In addition, C60 compounds are characterized by a small Fermi energy and a sensible degree of electronic correlation, all elements that usually disfavour the superconductivity onset. These considerations point towards a more exotic phonon-based superconductive pairing than the conventional BCS or Migdal-Eliashberg theory. We discuss this framework in the context of the nonadiabatic metal picture, where electron energies are comparable with phonon frequencies. In fullerene compounds high phonon frequencies range up to ≈2000 K whereas the Fermi energy is as small as 0.25 eV. As a consequence, we show that the electron dynamics strongly interferes with ...