Synthesis and electronic properties of Nd\(_{n+1}\)Ni\(_{n}\)O\(_{3n+1}\) Ruddlesden-Popper nickelate thin films

The rare-earth nickelates possess a diverse set of collective phenomena including metal-to-insulator transitions, magnetic phase transitions, and, upon chemical reduction, superconductivity. Here, we demonstrate epitaxial stabilization of layered nickelates in the Ruddlesden-Popper form, Nd\(_{n+1}\)Ni\(_n\)O\(_{3n+1}\), using molecular beam epitaxy. By optimizing the stoichiometry of the parent perovskite NdNiO\(_3\), we can reproducibly synthesize the \(n = 1 - 5\) member compounds. X-ray absorption spectroscopy at the O \(K\) and Ni \(L\) edges indicate systematic changes in both the nickel-oxygen hybridization level and nominal nickel filling from 3\(d^8\) to 3\(d^7\) as we move across the series from \(n = 1\) to \(n = \infty\). The \(n = 3 - 5\) compounds exhibit weakly hysteretic metal-to-insulator transitions with transition temperatures that depress with increasing order toward NdNiO\(_3\) (\(n = \infty)\).