Spectral-weight transfer in effective single-band models for the high-Tccuprates

The low-energy physics of the Emery model, representing the high-${\mathit{T}}_{\mathit{c}}$ cuprates, can be understood by separating intracell interference within ${\mathrm{CuO}}_4$ cells from intercell correlations described by an effective single-band Hubbard model. This decomposition explains the anomalous transfer of spectral intensity from the high- to the low-energy scale, the asymmetry between electron and hole doping, and the breakdown of low-energy sum rules. These features are related to the existence of an antibound charge transfer exciton band, are are very sensitive to the strength of the Coulomb repulsion ${\mathit{U}}_{\mathit{d}}$.