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This study systematically compares the spin and orbital excitation properties of undoped superconducting infinite-layer nickelate PrNiO₂ and insulating cuprate CaCuO₂ using momentum-resolved and polarization-resolved resonant inelastic X-ray scattering (RIXS) measurements. The results show that the in-plane magnetic exchange integral of PrNiO₂ (approximately 46 meV) is significantly smaller than that of CaCuO₂ (approximately 82 meV), while the out-of-plane exchange integrals are similar (approximately 6–7 meV), indicating that both materials support three-dimensional antiferromagnetic order with comparable three-dimensionality of spin-spin correlations. The orbital excitations (intra-3d transitions) are well described by a single-ion model, but the Ni-dxy peak energy is notably lower than that of Cu-dxy, with opposite dispersion directions—nickelate exhibits orbital excitation propagation driven by nearest-neighbor orbital superexchange coupling, whereas cuprate is dominated by next-nearest-neighbor coupling. Despite a significant difference in charge-transfer energy (larger in the nickelate), the spin and orbital excitation characteristics are generally highly similar, with key distinctions only in the energy and dispersion of the Ni-dxy peak, attributed to differing orbital superexchange coupling mechanisms. This work reveals the core commonalities in magnetism and orbital dynamics between infinite-layer nickelates and cuprates, while also indicating smaller spin fluctuation energies and stronger localization of doped charges on metal sites in the nickelates.

This work employs resonant inelastic X-ray scattering (RIXS) to systematically investigate the electronic and spin excitations in La₃Ni₂O₇ (LNO) thin films under epitaxial strain spanning from approximately –2% to +1.9%. In compressively strained LNO/SrLaAlO₄ films that exhibit ambient-pressure superconductivity with an onset critical temperature above 40 K, dd excitations and spin dynamics resembling those of bulk LNO are observed, yet the spin excitation bandwidth increases by about 10 meV, indicating an enhanced interlayer antiferromagnetic exchange coupling Jz; conversely, tensile-strained LNO/SrTiO₃ films display a pronounced suppression of both spin excitations and Ni 3dz²-related dd excitations. This evolution reflects how strain tunes the Ni 3dz²–O 2pz hybridization and the interlayer distance, thereby modulating the interlayer magnetic coupling strength. The results demonstrate that epitaxial strain effectively controls the interlayer antiferromagnetic superexchange in bilayer nickelates, and the strengthened Jz is closely correlated with the emergence of ambient-pressure superconductivity, lending support to the theoretical picture in which interlayer magnetic exchange facilitates interlayer pairing.

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The spin fluctuations in rare-earth Pr and Nd doped bilayer nickelates La₂LnNi₂O₇₋δ (Ln = La, Pr, Nd) were investigated under ambient pressure using inelastic neutron scattering. In the undoped La₃Ni₂O₇₋δ, a flat spin fluctuation mode at 45 meV was observed; upon doping, this mode splits into two modes at 43 and 48 meV, with an additional weak mode appearing at approximately 60 meV. Notably, the spin fluctuation intensity in La₂NdNi₂O₇₋δ is significantly higher than that in La₃Ni₂O₇₋δ and La₂PrNi₂O₇₋δ. These results are consistent with a description based on the stripe-type antiferromagnetic Heisenberg model, indicating that rare-earth doping enhances the interlayer magnetic coupling, with the interlayer exchange coupling SJ⊥ increasing from about 60 meV to 69–73 meV, while the intralayer coupling remains weak (≤3.5 meV). This enhancement may account for the increase in superconducting transition temperature from 80 K to near 100 K following rare-earth doping. This work reveals the regulatory role of rare-earth doping on spin dynamics and superconducting pairing in bilayer nickelates.

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