ENERGY-GAP, TC, AND PAIR-BREAKING IN THE NESTED-FERMI-LIQUID MODEL FOR HIGH-TC SUPERCONDUCTORS

We employ Eliashberg theory to calculate the ratio R = 2-DELTA/k(B)T(c) for s-wave pairing for a two dimensional tight-binding band. The pairing is mediated in our model by exchange of phonons and higher frequency "excitons" which, by themselves, would yield a T(c) of about 200 K. The additional exchange of an antiferromagnetic spin fluctuation (antiparamagnon) is adjusted to reduce T(c) to the observed 90 K range. We calculate the RPA spin fluctuation propagator self-consistently including the "feedback" effect of the superconductivity and assuming that only wave vectors near "nesting" are important. The addition of the antiparamagnon exchange further enhances R, which depends strongly on the band-filling. The resulting values should, however, be compared with a weak-coupling BCS value that, depending on the band structure and band-filling, can be significantly less than 3.5.