High Tc Superconductivity in MgB2 by Nonadiabatic Pairing

The evidence for the key role of the s bands in the electronic properties of MgB2 points to the possibility of nonadiabatic effects in the superconductivity of these materials. These are governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the s bands. We show that the nonadiabatic theory leads to a coherent interpretation of Tc ? 39 K and the boron isotope coefficient aB ? 0.30 without invoking very large couplings and it naturally explains the role of the disorder on Tc. It also leads to various specific predictions for the properties of MgB2 and for the material optimization of these types of compounds.