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 pairs in single-particle degenerate states which, because of the configuration interaction, form singlet states with zero on-site repulsion (W = 0 pairs). In the many-body problem the effective interaction due to the spin exchange with the holes of b(1) symmetry is attractive. The exact diagonalization of the extended Hubbard Hamiltonian with 4 holes using well-established parameters for several clusters shows that the ground state has B-1(2)(xy) symmetry and that binding energies are of 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 rely on off-site inter actions; when the symmetry is broken the repulsion wins. Besides the correct symmetry of the pair, we obtain good agreement with experiment for the binding energies and the singlet-triplet energy separation of the quasiparticles.