Strong Superexchange in a d9−δ Nickelate Revealed by Resonant Inelastic X-Ray Scattering

The discovery of superconductivity in a d9−δ nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. We address this question using Ni L-edge and O K-edge spectroscopy of the reduced d9−1/3 trilayer nickelates R4Ni3O8 (where R = La Pr) and associated theoretical modeling. A magnon energy scale of ∼ 80 meV resulting from a nearest-neighbor magnetic exchange of J = 69 (4) meV is observed, proving that d9−δ nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O K -edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates. Layered nickelates thus provide a route to testing the relevance of superexchange to nickelate superconductivity.