Spin fluctuations and the superconducting state in doped insulators

We propose a model of electron pairing via spin fluctuations in doped insul ators. The bare states for the superconducting condensate correspond to imp urity bands in the original band gap of the undoped material. We obtain a c omplete set of equations for the superconducting state. We show that fermio n pairing in impurity bands of extended states is possible, and thus so is superconductivity, if localized spin-0 bosons are produced. The latter are necessarily accompanied by localized spin-1 bosons, which are responsible f or the relationship between singlet and triplet pairing channels of quasipa rticles. (C) 1998 American Institute of Physics. [S1063-7761(98)01511-X].