Narrow bands and electronic structure in unconventional high-Tc superconductors

Electronic structure details play a marginal role in the superconductive properties of conventional superconductors. Electronic energies are by far the most important in these materials and the only relevant electronic parameter, with respect to superconductivity, is essentially the density of states N(0). Cooper oxides and fullerene compounds are characterized by narrow band dispersions and by electronic structures of hundreds meV. This feature is reflected in the high sensitivity of these materials to fine tuning of electron quantities, as shown by the phase diagram of these materials. A similar situation is encountered in recently discovered superconducting MgB2, where the Fermi edge lies only |0.5 eV below the top of the s-bands. We discuss the relevance of these low electronic energy scales in the context of the theory of nonadiabatic electron-phonon coupling, which is achieved when Fermi energies are of the same order as phonon frequencies.