Role of Oxygen States in the Low Valence Nickelate La4Ni3 O8

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 La4Ni3O8 and a prototypical cuprate La2-xSrxCuO4 (x=0.35). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen (ZSA) 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 Δ. Nevertheless, the transition-metal-oxygen hopping is larger in La4Ni3O8 than in La2-xSrxCuO4, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La4Ni3O8 despite its larger Δ. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.