FLEX-THEORY FOR HIGH-T_C SUPERCONDUCTIVITY DUE TO SPIN FLUCTUATIONS

Using the Hubbard model we develop a microscopic theory for high-temperature superconductivity due to the exchange of antiferromagnetic spin fluctuations. We treat the corresponding pairing mechanism self-consistently within the framework of the FLuctuation EXchange (FLEX) approximation and study some extensions. Solving the generalized Eliashberg equations for hole- and electron-doped superconductors we obtain both phase diagrams, respectively, and always a d-wave gap function. Furthermore, for hole-doped cuprates we find three characteristic temperature scales which are in qualitative agreement with the experimental situation: a pseudogap temperature T*, below which a gap opens in the density of states, a mean-field transition temperature for superconductivity below which we obtain Cooper-pairs without long-range phase coherence ("pre-formed Cooper-pairs"), and a critical temperature T_c, where these pairs become phase coherent.