Nonadiabatic electron-phonon effects in low carrier density superconductors

Different families of unconventional superconductors present a low charge carrier density as a commontrait, suggesting that the low charge density can be at the basis of a unifying picture for different superconductors.In the past years we have suggested that the electron-phonon interaction can be responsiblefor a high- c T superconducting pairing in a nonadiabatic regime, where nonadiabatic effects are triggeredon by the small electronic Fermi energy associated with the low charge density character. A coherent pictureof such a framework requires however reconciling the low charge density and the small Fermi energywith a finite metallic character (sizable density of states and large Fermi surfaces). In this paper we investigatethe peculiar conditions which are needed to be encountered in order to fulfill these requirements.We discuss the specific case of fullerenes, cuprates and MgB2 alloys by analyzing their specific structuraland electronic properties The comparison between these materials and simple instructive models permitsto underline the different routes to reconcile these characteristics in different compounds. In cuprates andfullerenes the interplay between small Fermi energies and large Fermi surface is strictly connected withstrong electronic correlation effects. A comprehensive understanding of these issues can be useful to thefuture search for new nonadiabatic high- c T materials.