HOLE PAIRING IN HIGH-T_c SUPERCONDUCTORS

摘要

We discuss the peculiar interplay of the C_(4V) symmetry and correlation effects which give rise to singlet hole pairs with zero repulsion energy in the CuO_2 plane of cuprate superconductors. The basic ingredients are: i) the existence of partially occupied nonbonding states belonging to the degenerate E representation and ii) the interaction between equivalent configurations that leads to the spontaneous symmetry breaking in the Hartree-Fock approximation. In cluster calculations further correlation effects produce a net attraction between holes and bound ~1B_2 pairs; substitutional impurities, however, alter the planar symmetry and then destroy pairing very effectively. We study the dependence of the mechanism on the strength of electron-electron interactions, and find binding energies comparable to the experimental superconductivity gap.