COPPER-OXYGEN CHARGE EXCITATIONS AND THE EFFECTIVE-SINGLE-BAND THEORY OF CUPRATE SUPERCONDUCTORS

We propose a formally controlled perturbative method for constructing an effective-single-band (SB) Hubbard theory from a multiband (MB) model for cuprate systems. Our SB theory explicitly incorporates the lowest MB Cu-O charge excitations, and thus possible charge-transfer instabilities, and remains valid, even for the extreme charge-fluctuation limit of the MB model, up to excitation energies of typically approximately 3-4 eV. This is contrasted with recent approximate slave-boson and diagrammatic studies of the MB model.