RQMP (CPM) Seminar

Electronic structure and charge order in square planar low valence nickelates

Yao Shen

Condensed Matter Physics and Materials Science Department
Brookhaven National Laboratory

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 and superexchange akin to the cuprates or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role and superexchange is negligible? Do they host charge order as cuprates and what is the electronic character of this symmetry breaking? Here, we resolve these questions using O K-edge and Ni L-edge RIXS measurements of the low valance nickelate La₄Ni₃O₈ and a prototypical cuprate La_2-xx4 (x=0.35), interpreting the results with exact diagonalization calculations [1,2]. 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 La_2-x4, which leads to a significant superexchange interaction of J~70 meV and an appreciable hole occupation of the ligand O orbitals in La₄Ni₃O₈ despite its larger Δ. Moreover, we identify intertwined involvements of Ni 3d_x²-y², 3d_3z²-r², and O 2_σ orbitals in its formation of diagonal charge order. More specifically, the Ni 3d_x²-y² orbitals, strongly hybridized with planar O 2_σ, largely shape the spatial charge distribution and lead to Ni site-centered charge order. The 3d_3z²-r² orbitals play a small, but non-negligible role in the charge order as they hybridize with the rare-earth 5d orbitals. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.

References
[1] J. Q. Lin et al., Phys. Rev. Lett. 126, 087001 (2021).
[2] Y. Shen et al., Phys. Rev. X 12, 011055 (2022).