An extended multiband Hubbard model of the two-dimensional CuO2 planes of high-Tc superconductors is examined by exact diagonalization of the Hamiltonian for small clusters with periodic boundary conditions. Holes are shown to bind in the limit of large Cu-site and nearest-neighbor Cu-O Coulomb repulsion. A simple explanation of the source of the binding energy is given in this strong-coupling limit. The more realistic case of relatively weak nearest-neighbor repulsion is also examined, and suggestive evidence is presented that the effective attraction will also exist in the thermodynamic limit for this case.
Stephan, W. H., von der Linden, W., & Horsch, P. (1989). Attractive interactions from repulsive forces in a multiband Hubbard model. Physical Review / B, 39(4), 2924-2927. https://doi.org/10.1103/PhysRevB.39.2924