Copper-oxygen charge excitations and the effective-single-band theory of cuprate superconductors

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Abstract
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 ∼3–4 eV. This is contrasted with recent approximate slave-boson and diagrammatic studies of the MB model.