Theory of pairing in the Cu-O Plane: Three-Band Hubbard Model and Beyond
  We calculate the effective interaction $W_{eff}$ between two holes added to the ground state of the repulsive three-band Hubbard model. To make contact with Cooper theory and with earlier Hubbard model cluster studies, we first use a perturbative canonical transformation, to generate a two-body Hamiltonian. Then, we extend the results to all orders. The approach is exact in principle, and we obtain a close analytic expression including explicitly the effects of all virtual transitions to 4-body intermediate states. Our scheme naturally lends itself to embody off-site, inter-planar, phonon-mediated and other interactions which are not considered in the Hubbard model but may well be important. The result depends qualitatively on the symmetry of the two-hole state: $^{1}B_{2}$ and $ ^{1}A_{2}$ pairs are special, because the bare holes do not interact by the on-site repulsion (W=0 pairs). The effective interaction in these channels is attractive and leads to a Cooper-like instability of the Fermi liquid; however $W_{eff}$ is repulsive for Triplet pairs. Bound two-hole states of the same nature were reported earlier in small cluster calculations by exact diagonalisation methods; only symmetric clusters are good models of the plane. Once $W_{eff}$ is known, the pair eigenfunction is determined by an integral equation. We present numerical estimates of the binding energy $|\Delta|$ of the pairs, which is in the physically interesting range of tens of meV if unscreened on-site repulsion parameters are used. 
Michele Cini, Gianluca Stefanucci and Adalberto Balzarotti