PAIR-TRANSFER SUPERCONDUCTIVITY IN THE ANDERSON MODEL

Anderson's model for localized impurity states hybridized with the con duction band is extended to include transfers of pairs of electrons be tween the band states and the localized levels. The interplay between the hybridization and the pair-transfer coupling is analyzed. It is fo und that when the effect of the hybridization is small and the localiz ed states are not resonant, the temperature range over which the syste m is superconducting is increased by the pair-transfer mechanism. In t he opposite case, the pair-transfer coupling combines with the hybridi zation to reduce that range. Thus, pair-transfer interaction can be de trimental to superconductivity.