INTERACTION BETWEEN O-QUASI-PARTICLES IN CUO2-BASED SUPERCONDUCTORS

We consider the extended Hubbard model including repulsion U(pd) between nearest-neighbor Cu and O holes. In the limit of strong intra-atomic correlations and weak hopping, we derive an effective Hamiltonian for the O holes. This Hamiltonian contains interactions between O holes with a common nearest-neighbor Cu ion. The effective interaction between two holes is repulsive for small U(pd) and attractive otherwise. Bound states between two holes exist only if the effective interaction between them is sufficiently large and negative.