Electron Correlation and Charge Transfer in (Ba$_{0.9}$Nd$_{0.1})$CuO$_{2+\delta }$/(CaCuO$_{2})_{2 }$Superconducting Superlattices Observed with Resonant Inelastic X-ray Scattering

In-plane CuO$_{2}$ physics of the 2$\times $2 high-T$c$ superlattice (Ba$_{0.9}$Nd$_{0.1}$CuO$_{2+x})_{2}$/(CaCuO$_{2})_{2}$ was investigated by applying x-ray emission/absorption spectroscopy. The superlattices are fabricated by pulsed-laser molecular beam epitaxy (MBE) in a layer-by-layer fashion.\footnote{G. Balestrino, S. Lavanga, P. G. Medaglia, P. Origiani, A. Paoletti, G. Pasquini, A. Tebano, and A. Tucciarone, Appl. Phys. Lett. \textbf{79}, 99 (2001).} The$_{ }$superlattices consist of two layers; an infinite layer (IL) and the charge reservoir (CR). Each insulating layer is alternately deposited to produce superlattices exhibiting a T$c$ of 80K.\footnote{G. Balestrino, P. G. Medaglia, P. Origiana, A. Tebano, C. Aruta, S. Lavanga, and A. A. Varlamov, Phys. Rev. Lett. \textbf{89,} 156402 (2002).} We measure the O 1$s$ density of states 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-temperature superconducting heteroepitaxial structures. Zhang-Rice singlet polarization dependent studies are performed, and the absorption and emission results are compared to local-density approximation theory. X-ray emission spectra of the superlattice and its component layers gives evidence of charge transport from the charge reservoir to the infinite layer. Cu-edge resonant x-ray emission is performed to probe \textit{dd} excitations in the component layers and superlattice.