Electronic character of charge order in square planar low valence nickelates*

Charge order is a central feature of the physics of cuprate superconductors and is known to arise from a modulation of holes with primarily oxygen character. Low-valence nickelate superconductors of mixed charge-transfer–Mott-Hubbard character [1] also host charge order, but the electronic character of this symmetry breaking is unsettled. Here, using resonant inelastic x-ray scattering at the Ni L₂-edge, we identify intertwined involvements of Ni 3d_x2−y2 , 3d_3z2−r2 , and O 2p_σ orbitals in the formation of diagonal charge order in an overdoped low-valence nickelate La₄Ni₃O₈. The Ni 3d_x2−y2 orbitals, strongly hybridized with planar O 2p_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 reveal that the low-energy physics and ground-state character of these nickelates are more complex than those in cuprates.

[1] Y. Shen et al., Phy. Rev. X 12, 011055 (2022).