Enhanced pairing interactions in a two-band model

The Kondo picture for the electron-electron attraction due to transfer of pairs of charge carriers between two bands is investigated. This interaction may reduce considerably the Coulomb repulsion, leading to enhancement of the superconducting transition temperature and a substantial weakening of the isotope effect. The effect is large when the empty band is narrow and the energy gap between the two bands is small. The screening of the Coulomb interaction in a two-band model is considered in detail using the random-phase approximation, and the dynamic dielectric functions are derived. It is found that the intraband vertex of the empty band is almost unscreened and that of the occupied one has metallic screening. The screening of the pair-transfer vertex is independent of the intraband Coulomb interactions and may even be magnified. The possible relevance to the superconducting oxides is suggested.