Charge transfer transitions in cuprates

Absorption spectra of cuprates are discussed. Persistent photo-induced conductivity occurs in the visible spectrum (∼2 eV) and is commonly assigned to ligand–metal (LM) charge transfer (CT) transitions. However, LM CT is site local and cannot possibly generate persistent charges. The assignment in this Letter is ‘metal to adjacent metal’ (MM) CT transitions, while the absorption at hν > 3 eV is still assigned to mainly LM CT. Only MM CT, defining the Mott–Hubbard gap, is exclusively polarized in the CuO2 plane, as found experimentally. Since MM CT is strongly affected by the local electric field, doping transfers spectral weight to the IR region.