We have extended the previous fluctuation exchange approximation for the two-dimensional Hubbard model to a Fermi surface geometry approximating that of the YBazCu307_y cuprates. In spite of the poor nesting of this Fermi surface we obtain a broad commensurate peak in the spectral density of the dynamic spin susceptibility, Im Xs(q, to), provided that the coupling is sufficiently large (U/t > 7). The antiparamagnons soften and the quasiparticles exhibit marginal Fermi liquid behavior as the band filling n tends to half filling n = 1. The eigenvalue A d for dx2_y2 pairing increases with U and n, or decreasing T, however, we have not reached the pairing instability where A d = 1 (for example, A d = 0.91 for U/t = 6.8, n = 0.95, and T= 0.0224t). In the second part we use a coupling J(q) which is peaked at Q. Then we obtain a commensurate peak and dx2_y2 wave superconductivity, for example, at U/t = 3.7, n = 0.85, and T c = 0.039t. Below To, a gap and a peak at higher frequency develop rapidly in Im Xs(Q, to), in qualitative agreement with neutron-scattering data.
The density of states N(to) is linear in to which is consistent with the observed low-temperature variation of the penetration depth. In the photoemission intensity along the antinode of the gap the quasiparticle peak shifts to lower frequency and a dip develops as T decreases below To. This behavior is in qualitative agreement with ARPES data obtained on Bi-2212.