We use x-ray spectroscopy to examine the electronic structure of high-temperature superconducting superlattices $[({\mathrm{Ba}}_{0.9}{\mathrm{Nd}}_{0.10}){\mathrm{CuO}}_{2+\delta }{]}_2/[{\mathrm{CaCuO}}_2{]}_2$. The O $2p$ density of states reveals the insulating character of the individual component layers and the metallic character of the superlattices. We report the first direct observation of Zhang-Rice singlets in artificial high-temperature superconducting heteroepitaxial structures. The experimental findings in the superlattices and its component layers offer evidence of charge transport from the so-called charge reservoir layer to the superconducting infinite layer. This suggests a strong link between superconductivity and both electron correlation and charge transfer within the superlattices.

• Received 6 July 2004

DOI:https://doi.org/10.1103/PhysRevLett.96.017003