Electronic Character of Charge Order in Square-Planar Low-Valence Nickelates

Abstract

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 also host charge order, but the electronic character of this symmetry breaking is unsettled. Here, using resonant inelastic x-ray scattering at the Ni L_{2} edge, we identify intertwined involvements of Ni 3d_{x^{2}-y^{2}}, 3d_{3z^{2}-r^{2}}, and O 2p_{σ} orbitals in the formation of diagonal charge order in an overdoped low-valence nickelate La_{4}Ni_{3}O_{8}. The Ni 3d_{x^{2}-y^{2}} orbitals, strongly hybridized with planar O 2p_{σ}, largely shape the spatial charge distribution and lead to Ni site-centered charge order. The 3d_{3z^{2}-r^{2}} 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.