Role of Oxygen States in the Low Valence Nickelate La${}_4$Ni${}_3$O${}_8$

Abstract

The discovery of superconductivity in square-planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character akin to the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role? Here, we resolve this question using O $K$-edge resonant inelastic x-ray scattering (RIXS) measurements of the low valence nickelate La${}_4$Ni${}_3$O${}_8$ and a prototypical cuprate La${}_{2-x}$Sr${}_x$CuO${}_4$ ($x=0.35$). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen scheme. The nickelate, however, is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer/Mott-Hubbard character with the Coulomb repulsion $U$ of similar size to the charge-transfer energy $\hat{I}”$. Nevertheless, the transition-metal-oxygen hopping is larger in La${}_4$Ni${}_3$O${}_8$ than in La${}_{2-x}$Sr${}_x$CuO${}_4$, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La${}_4$Ni${}_3$O${}_8$ despite its larger $\hat{I}”$. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.

13 pages; to appear in PRX