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 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 3dx2-y2, 3d3z2-r2, and O 2pσ orbitals in the formation of diagonal charge order in an overdoped low-valence nickelate La4Ni3O8. The Ni 3dx2-y2 orbitals, strongly hybridized with planar O 2pσ, largely shape the spatial charge distribution and lead to Ni site-centered charge order. The 3d3z2-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.