Anderson’s model for localized impurity states hybridized with the conduction band is extended to include transfers of pairs of electrons between the band states and the localized levels. The interplay between the hybridization and the pair-transfer coupling is analyzed. It is found that when the effect of the hybridization is small and the localized states are not resonant, the temperature range over which the system is superconducting is increased by the pair-transfer mechanism. In the opposite case, the pair-transfer coupling combines with the hybridization to reduce that range. Thus, pair-transfer interaction can be detrimental to superconductivity.

• Received 9 December 1993

DOI:https://doi.org/10.1103/PhysRevB.49.15377