Coulomb interactions and pairing in a two-band picture

The screening and renormalization of the Coulomb repulsion in the conduction band of a two-band model are considered. It is shown that when the second band lies above the conduction band and it is empty at low temperatures, the transfer of pairs of charge carriers between the two bands is the relevant interband process for reducing the Coulomb repulsion in the conduction band. By considering a two-band situation arising from the hybridization of the conduction band with shallow impurity states having a large intrasite repulsion, it is shown that the pair-transfer process is effective in reducing the Coulomb repulsion even in situations where the second band is very narrow and possesses by itself a very large repulsion. The implications for enhanced pairing in systems with the appropriate band structure or in those near the metal-insulator transition caused by doping, are discussed.