Digital modulation of the nickel valence state in a cuprate-nickelate heterostructure

Layer-by-layer oxide molecular beam epitaxy has been used to synthesize cuprate-nickelate multilayer structures of composition (La$_2$CuO$_4$)$_m$/LaO/(LaNiO$_3$)$_n$. In a combined experimental and theoretical study, we show that these structures allow a clean separation of dopant and doped layers. Specifically, the LaO layer separating cuprate and nickelate blocks provides an additional charge that, according to density functional theory calculations, is predominantly accommodated in the interfacial nickelate layers. This is reflected in an elongation of bond distances and changes in valence state, as observed by scanning transmission electron microscopy and x-ray absorption spectroscopy. Moreover, the predicted charge disproportionation in the nickelate interface layers leads to a thickness-dependent metal-to-insulator transition for $n=2$, as observed in electrical transport measurements. The results exemplify the perspectives of charge transfer in metal-oxide multilayers to induce doping without introducing chemical and structural disorder.