Electron Correlation and Charge Transfer in (Ba$_{0.9}$Nd$_{0.1})$CuO$_{2+\delta }$/(CaCuO$_{2})_{2 }$ Superconducting Superlattices

The mechanism of cuprate high-temperature superconductivity is still controversial. It is widely accepted that HTSC occurs primarily within CuO planes of cuprates, yet there is contention concerning the in-plane low-energy physics. Using 2$\times $2 high-Tc superlattices (SL) (Ba$_{0.9}$Nd$_{0.1}$CuO$_{2+x})_{2}$/(CaCuO$_{2})_{2}$, we probe the CuO$_{2}$ planes by applying x-ray emission/absorption spectroscopy. The pulsed-laser deposited SL consist of two separately insulating layers and exhibit a T$c$ of 80K. Superconductivity occurs exclusively within the infinite layer (IL) and not the charge reservoir (CR) of the SL. We demonstrate resonant x-ray emission and absorption to be insightful tools for studying the IL, CR and superlattice structures. We measure the O 1s density of state to be insulating for the component layers and metallic for the superlattice. Using resonant inelastic scattering (RIXS) we make the first direct observation of Zhang-Rice singlets in artificial high-Tc superconducting heteroepitaxial structures. A comparison of the x-ray emission spectra of the SL and its component layers gives evidence of charge transport from the so-called charge reservoir layer to the infinite layer.