Disentangling Symmetry-Breaking Charge Order from Oxygen Order in Infinite-Layer Nickelates

As a <i>3d⁹</i> analogue to the cuprates, the recent discovery of superconductivity in infinite-layer nickelates has provided a promising direction to understand high-temperature superconductivity and its competing orders [1,2]. Despite their similarities in crystal structure and valence electron configuration, the cuprates and nickelates are distinguished by significant differences in <i>T</i>_c and competing ground states [3]. In cuprates, charge density waves break symmetry and compete with superconductivity at low temperature, while in nickelates charge order has been relatively elusive. There have been reports of charge order in the infinite layer nickelates [4,5], but it remains unclear whether these signatures arise from an intrinsic correlation-driven density wave or extrinsic modulation of the nickel valence from excess oxygen [6,7]. Due to the rich interplay between superconductivity and charge order found in cuprates and other superconducting systems, it is essential to build an understanding of the nature of charge order in the nickelates to shed light on universality in high temperature superconductivity.<br/><br/>In this work, partially-reduced capped NdNiO_2+x thin films with intentional excess oxygen are characterized using scanning transmission electron microscopy (STEM). Electron ptychography - a phase retrieval algorithm providing highest resolution and 3D information in STEM - directly probes the atomic structure of the thin films. Precise measurement of O, Ni and Nd sites reveals the presence of excess oxygen which order with 3a₀periodicity, a period similar to that of previously reported charge order. Associated oxygen octahedral rotations and accompanying periodic lattice distortions (PLDs) of Nd and Ni sites are present in regions with oxygen ordering. Superlattice peaks which arise from these structural distortions enable mesoscale visualization of the excess oxygen ordering using electron nano-diffraction. We find that the ordering is not uniform across the film but exists in dispersed domains. No superlattice peaks are observed in areas outside of ordered excess oxygen domains, suggesting that the signatures of charge order in NdNiO₂ reflect the presence of excess oxygen rather than intrinsic correlation effects. This work exemplifies the unique capability of a highly localized and sensitive probe, such as electron ptychography, to directly differentiate and disentangle exotic phases arising in finely tuned systems such as nickelates.<br/><br/>[1] Li D. et al. Nature 527, 624-627 (2019).<br/>[2] Osada M. et al. Advanced Materials 33, 45 (2021).<br/>[3] Li D. et al. Physical Review Letters 125, 027001 (2020).<br/>[4] Rossi M. et al. Nature Physics 18, 869-873 (2022).<br/>[5] Tam C. et al. Nature Materials 21, 116-1120 (2022).<br/>[6] Raji A. et al. ArXiv:2306.10507 (2023).<br/>[7] Parzyck C. et al. ArXiv:2307.06486 (2023).<br/> <br/>*STEM characterization was performed at the Cornell Center for Materials Research Facilities supported by National Science Foundation (DMR-1719875). The microscopy work at Cornell was supported by the NSF PARADIM (DMR-2039380), with additional support from Cornell University, the Weill Institute and the Kavli Institute at Cornell. L.B and L.F.K. acknowledge support from Packard foundation.