Charge-transfer and magnetic-pairing mechanisms in the extended Hubbard model of high-Tcoxides

Charge-transfer and spin-interaction-mediated pairing is derived within a strong-coupling expansion for an extended multiband Hubbard model describing Cu-O–based superconductors. We include both intrasite (${\mathit{U}}_{\mathit{d}}$,${\mathit{U}}_{\mathit{p}}$) and intersite (${\mathit{U}}_{\mathit{p}\mathit{d}}$) Coulomb repulsions. The basic carriers are oxygen-hole quasiparticles in ${\mathit{p}}_{\mathrm{\sigma }}$ orbitals. They interact via a charge-transfer mechanism driven by ${\mathit{U}}_{\mathit{p}\mathit{d}}$ and via a magnetic mechanism, which is due to a constructive interplay between the enhanced (by ${\mathit{U}}_{\mathit{p}\mathit{d}}$) Kondo (Cu-O) exchange and the superexchange of the Cu spins. Within the BCS limit we find that ${\mathit{U}}_{\mathit{p}\mathit{d}}$ enhances the relative weight of s versus d-wave pairing, with the dominance of s-wave pairing for large transfer energies Δ=${\mathit{E}}_{\mathit{p}}$-${\mathit{E}}_{\mathit{d}}$ and at low-to-intermediate doping. The charge-transfer-induced coupling is negligible in this large-Δ limit. In the small-Δ limit it induces a 10–20 % enhancement of the BCS ${\mathit{T}}_{\mathit{c}}$.