Failure of the Migdal-Eliashberg theory of superconductivity in Rb3C60.

We discuss the compatibility of the most accurate experimental data with th e ordinary Migdal-Eliashberg theory of superconductivity in the fullerene c ompound Rb3C60 BY using different model phonon spectra we conclude that the experimental data can be fitted only by invoking an electron-phonon coupli ng of order lambda similar or equal to 3. This exceedingly high value is un physical 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 fit ted by much more realistic values of lambda. This generalized theory predic ts also characteristic features absent in the Migdal-Eliashberg framework w hich can be experimentally tested.