Failure of the Migdal-Eliashberg theory of superconductivity in Rb3C60

We discuss the compatibility of the most accurate experimental data with the ordinary Migdal-Eliashberg theory of superconductivity in the fullerene compound Rb3C60 BY using different model phonon spectra we conclude that the experimental data can be fitted only by invoking an electron-phonon coupling of order lambda similar or equal to 3. This exceedingly high value is unphysical and it is not consistent with the basic assumptions of the Migdal-Eliashberg theory. On the contrary, by relaxing the adiabatic hypothesis on which the Migdal-Eliashberg theory rests, the experimental data can be fitted by much more realistic values of lambda. This generalized theory predicts also characteristic features absent in the Migdal-Eliashberg framework which can be experimentally tested.