Unveiling orbital landscapes in strongly correlated bulk nickelates with $s$-NIXS

Abstract

We leverage $s$-orbital non-resonant inelastic X-ray scattering ($s$-NIXS) to perform orbital imaging on three bulk rare-earth nickelates spanning a range of formal nickel valence (3$d$ electron filling) from Ni${}^{3+}$ (3$d^7$) to Ni${}^{1+}$ (3$d^9$). Our results directly reveal the ground states of these compounds all with minimal theoretical input. In particular, we demonstrate the low-spin orbital configuration of trivalent LaNiO${}_3$, the $d_{x^2-y^2}$ configuration of monovalent LaNiO${}_2$, and resolve the effective $e_g$ crystal field splitting in the distorted octahedral environment of divalent La${}_2$NiO${}_4$. This work illustrates the potential of $s$-NIXS to study the ground state and excited states of strongly correlated materials without needing complex theoretical analysis of spectroscopic data.

3 figures, 10 supplemental figures, 1 table