PUB - Publikationen an der Universität Bielefeld

We have extended the previous fluctuation exchange approximation for the two-dimensional Hubbard model to a Fermi surface geometry approximating that of the YBa2Cu3O7-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 chi(S)(q, omega), provided that the coupling is sufficiently large (U/t >/similar to 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 lambda(d) for d(x2-y2) pairing increases with U and n, or decreasing T; however, we have not reached the pairing instability where lambda(d) = 1 (for example, lambda(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 d(x2-y2) wave superconductivity, for example, at U/t = 3.7, n = 0.85, and T-c = 0.039t. Below T-c, a gap and a peak at higher frequency develop rapidly in Im chi(s)(Q, omega), in qualitative agreement with neutron-scattering data. The density of stares N(omega) is linear in omega 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 T-c. This behavior is in qualitative agreement with ARPES data obtained on Bi-2212.