ANTIFERROMAGNETIC, CHARGE-TRANSFER, AND PAIRING CORRELATIONS IN THE 3-BAND HUBBARD-MODEL

The CuO2 sheets common to the superconducting cuprates are believed to be characterized by a charge-transfer gap in their insulating antiferromagnetic state. The three-band Hubbard model with an on-site Cu Coulomb interaction U(d), which is large compared to the difference in energy-epsilon between the O and Cu sites, provides a basic model for such a system. We have carried out Lanczos and Monte Carlo studies of a CUO2 lattice described by a three-band Hubbard model. For U(d) large compared with epsilon, and epsilon comparable to or larger than the bandwidth of the lower hole band, we find strong antiferromagnetic correlations and evidence for a charge-transfer gap at a filling of one hole per Cu. The antiferromagnetic correlations decrease with either hole or electron doping, and we see that the additional holes go primarily on the O sites, while additional electrons go onto the Cu sites. For large values of the intersite Cu-O Coulomb interaction V, the hole-doped system exhibits a charge-transfer instability. As V is reduced, this is reflected as a peak in the charge-transfer susceptibility near epsilon + 2V almost-equal-to U(d), which we find is washed out by the strong Cu-0 hybridization at realistic values of V. Attractive pairing interactions are found in both the d-wave and extended s*-wave channels near the antiferromagnetic boundary.