The evidence for the key role of the σ bands in the electronic properties of MgB2 points to the possibility of non-adiabatic effects in the superconductivity of these materials. These were governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the σ bands. It was shown that the non-adiabatic theory led to a coherent interpretation of TC = 39K and an B isotope coefficient of 0.30 without invoking very large couplings and it naturally explains the effect of disorder on TC. It also led to various specific predictions for the properties of MgB2 and for the material optimization of these types of compounds.

High TC Superconductivity in MgB2 by Nonadiabatic Pairing. E.Cappelluti, S.Ciuchi, C.Grimaldi, L.Pietronero, S.Strassler: Physical Review Letters, 2002, 88[11], 117003 (3pp)