Electron correlation and charge transfer in [(Ba0.9Nd0.1)CuO2+delta](2)/[CaCuO2](2) superconducting superlattices

We use x-ray spectroscopy to examine the electronic structure of high-temperature superconducting superlattices [(Ba0.9Nd0.10)CuO2+delta](2)/[CaCuO2](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.