High-T sub c Superconductivity Mediated by Charge Transfer Excitations

The high-T sub c copper oxide superconductors have low electron density and small Cu-O separations, so that charge-transfer excitations between nearest neighbor cations and anions are unscreened. Then it is possible to have a low-lying (0.5 eV) excitonic resonance in the particle-hole spectrum. We discuss this using an extended Hubbard model and indicate how the electron-electron interaction mediated by the exchange of such a resonance can give rise to a high-T sub c superconducting transition.