CONFIGURATION-INTERACTION AND PAIRING IN SUPERCONDUCTING CUPRATES

We propose a quantum-mechanical effect specific of the symmetry of the Cu-O plane of cuprate superconductors. The mechanism considers hole p airs in single-particle degenerate states which, because of the config uration interaction, form singlet states with zero on-site repulsion ( W = 0 pairs). In the many-body problem the effective interaction due t o the spin exchange with the holes of b(1) symmetry is attractive. The exact diagonalization of the extended Hubbard Hamiltonian with 4 hole s using well-established parameters for several clusters shows that th e ground state has B-1(2)(xy) symmetry and that binding energies are o f the order of 10 meV. Unlike other electronic mechanisms proposed so far, this effect is due to the symmetry of the system and does not rel y on off-site inter actions; when the symmetry is broken the repulsion wins. Besides the correct symmetry of the pair, we obtain good agreem ent with experiment for the binding energies and the singlet-triplet e nergy separation of the quasiparticles.