Hole pairing in high-T-c superconductors: Symmetry-driven configuration interaction mechanism

A quantum-mechanical effect specific of the symmetry of the Cu-O plane in cuprate superconductors is presented. We show that pairs of holes in degenerate states are such that their mutual on-site Coulomb interaction W vanishes identically. A procedure for obtaining such W = O two-hole states in clusters of any size is outlined. Small clusters also display the effect provided they have the full C-4v symmetry and a Cu atom at the centre. In the many-body problem that arises when degenerate states are at the Fermi level, these holes interact indirectly through their screening clouds. This interaction is analysed by the exact diagonalization of an extended Hubbard Hamiltonian in doped CuO4 clusters and is shown to be attractive with the literature values of all parameters. Bound states of B-1(2) symmetry and binding energies of the order of 10 meV are obtained. The hole-hole interaction readily returns strongly repulsive if impurity potentials lower the symmetry. Off-site interactions fit well into the scheme. If the current estimates of the parameters are correct, this correlation effect alone can explain pairing.