CLUSTER APPROACH TO THE 3-BAND HUBBARD-MODEL OF THE CU-O PLANE - SUPERCONDUCTING PAIRS

The fully symmetric Cu-O clusters with a central Cu have two-hole sing let eigenstates which are not affected by the on-site repulsion. These pairs become bound states if the number of holes in the cluster is ap propriate; moreover, the quantization at half-integer values of a magn etic test flux is consistent with a superconducting pattern. To invest igate the mechanism of pairing, we study the symmetric clusters with l ess than or equal to 21 atoms and four holes by exact diagonalization. Further, by the diagrammatic expansion of the ground-state energy and of the scattering amplitude, we obtain the spin-flip diagrams for the effective interaction, which is attractive for singlets and repulsive for triplets. A criterion for pairing in cluster calculations is ther eby obtained.