Abstract
The discovery of superconductivity in square-planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character akin to the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role? Here, we resolve this question using O -edge resonant inelastic x-ray scattering (RIXS) measurements of the low valence nickelate and a prototypical cuprate (). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen (ZSA) scheme. The nickelate, however, is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer–Mott-Hubbard character with the Coulomb repulsion of similar size to the charge-transfer energy . Nevertheless, the transition-metal-oxygen hopping is larger in than in , leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in despite its larger . Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.
- Received 12 October 2021
- Revised 13 January 2022
- Accepted 26 January 2022
DOI:https://doi.org/10.1103/PhysRevX.12.011055
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The discovery of copper-based high-temperature superconductivity set up a decades-long quest to understand this novel “unconventional” state. One popular approach to doing this is to synthesize superconducting cuprate analog materials and study which elements of their electronic properties are similar or different. A breakthrough in this area occurred recently with the discovery of low-valance nickel-based, or nickelate, superconductors. There is, however, still considerable debate on their basic electronic properties and especially about the importance of their oxygen electronic states. Here, we use x-ray techniques to explore the role of oxygen states in and find that nickel and oxygen act on approximately equal footing in determining the electronic properties of low-valence nickelates.
In this work, we use a state-of-the-art technique called resonant inelastic x-ray scattering. This can directly target electronic states associated with specific atomic species—in this case, nickel and oxygen—and determine the energetic splitting between these states. The oxygen states were found to exist at an energy scale neither very close to nor very far from the nickel states. In this configuration, both nickel and oxygen contribute to the nickelate’s electronic properties and produce magnetic interactions only slightly weaker than the ones found in the cuprates.
These results lay the foundations for understanding the origins of this “nickel age” of superconductivity and establish the need for more complex models than those describing the copper oxide superconductors.