Behaviour of the Zhang-Rice singlet in CuGeO3, Bi2CuO4, and CuO

Zhang-Rice singlet states, clearly detected as first ionization state in the photoemission spectra of CuO are not observed in other cuprates such as CuGeO3, and Bi2CuO4. This paper reports a study, based on X-ray photoemission valence band spectra of CuGeO3, Bi2CuO4, and CuO, focused to enquire about the nature of the Zhang-Rice singlets in these compounds. Impurity cluster calculations, that reproduce the correlated satellites and the low lying binding energy states well, in agreement with photoemission experiments, clearly indicate a strong dependence of the Zhang-Rice singlets behaviour on the Cu-O bond ionicity. This finding fully explains the experimental data. The stability of the Z-R singlet can be considered as a peculiar feature of the low-gap cuprates (e.g. CuO) whereas the energy gap increase reduces the singlet-triplet separation affecting the Z-R singlet stability in the more insulating cuprates such as CuGeO3.