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

TL;DR

This study investigates the electronic structure of low valence nickelate La$_4$Ni$_3$O$_8$ using RIXS, revealing a mixed charge-transfer/Mott-Hubbard character and significant oxygen involvement, which informs understanding of their superconductivity.

Contribution

The paper provides the first detailed experimental analysis of oxygen states in La$_4$Ni$_3$O$_8$, clarifying its electronic regime and contrasting it with cuprates.

Findings

La$_4$Ni$_3$O$_8$ exhibits mixed charge-transfer/Mott-Hubbard character.

Oxygen orbitals have significant hole occupation despite large charge-transfer energy.

Transition-metal-oxygen hopping is larger in La$_4$Ni$_3$O$_8$ than in cuprates.

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 $\Delta$. 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 $\Delta$. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.