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

The discovery of superconductivity in a ${d}^{9\ensuremath{-}\ensuremath{\delta}}$ 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 ${d}^{9\ensuremath{-}1/3}$ trilayer nickelates ${R}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{8}$ (where $R=\mathrm{La}$, Pr) and associated theoretical modeling. A magnon energy scale of $\ensuremath{\sim}80\text{ }\text{ }\mathrm{meV}$ resulting from a nearest-neighbor magnetic exchange of $J=69(4)\text{ }\text{ }\mathrm{meV}$ is observed, proving that ${d}^{9\ensuremath{-}\ensuremath{\delta}}$ 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.