CHARGE-TRANSFER IN THE HUBBARD-MODEL AND CU-O SUPERCONDUCTIVITY

Charge transfer between cations and anions is discussed in terms of an extended Hubbard model which is appropriate for the Cu-O planes of the high-Tc oxide superconductors. The limitations of the simple Hubbard model for the case that the on-site repulsion is greater than the charge transfer excitation energy is addressed. The role of charge transfer excitation in optical spectra is described with reference to recent experimental results on insulating and metallic samples. Superconductivity and charge-transfer-mediated pairing in the extended Hubbard model is discussed in light of recent numerical and analytical calculations.