Exchange-driven pairing of delocalized carriers in high-temperature superconductors

Abstract

A semiquantitative pairing model is derived from the properties of a realistic Hamiltonian for ${\mathrm{CuO}}_2$ sheets. Due to large interactions between oxygen ions, to first-order delocalized carriers pair via localized spin-system holes on the Cu sublattice. The largest carrier-Cu couplings are dynamic exchange processes involving the Cu($d^{10}$) and Cu($d^8$) configurations. Up to carrier concentrations of ∼30–40 %, exchange-mediated attraction overcomes the large carrier Coulomb repulsion, present because of dielectric local screening. The same mechanism is shown to be capable of explaining high-temperature superconductivity in ${\mathrm{BiO}}_3$. The pairing interaction is attractive for triplet and repulsive for singlet s-wave pairs.