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 $2p_{ \sigma}$ 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 $2p_{ \sigma}$, 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.
Categories: cond-mat.str-el cond-mat.supr-con