Role of oxygen states in the low valence nickelate La4Ni3O8
Authors
Shen, Y.; Sears, J.; Fabbris, G. ; Li, J.; Pelliciari, J. ; Jarrige, I.; HE, Xi; Bozovic, I.; Mitrano, M.; Zhang, Junjie; Mitchell, J.; Norman, M.
Abstract
The discovery of superconductivity in square planar d9 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 d9 nickelate La4Ni3O8 and a prototypical cuprateLa2xSrxCuO4 (x = 0.35), interpreting the results with exact diagonalization calculations. 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 . Nevertheless, the transition-metal-oxygen hopping is larger in La4Ni3O8 than in La2xSrxCuO4, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La4Ni3O8 despite its larger . Our results clarify the essential characteristics of d9 nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.