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The recent discovery of superconductivity in the bilayer Ruddlesden-Popper nickelate La 3 Ni 2 O 7 under high pressure has generated much interest in the superconducting pairing mechanism of nickelates. Despite extensive work, the superconducting pairing symmetry in La 3 Ni 2 O 7 remains unresolved, with conflicting results even for identical methods. We argue that different superconducting states in La 3 Ni 2 O 7 are in close competition and highly sensitive to the choice of interaction parameters as well as pressure-induced changes in the electronic structure. Our study uses a multiorbital Hubbard model, incorporating all Ni 3 d and O 2 p states. We analyze the superconducting pairing mechanism of La 3 Ni 2 O 7 within the random phase approximation and find a transition between d -wave and sign-changing s -wave pairing states as a function of pressure and interaction parameters, which is driven by spin fluctuations with different wave vectors. These spin fluctuations with incommensurate wave vectors cooperatively stabilize a superconducting order parameter with d x 2 − y 2 symmetry for realistic model parameters. Simultaneously, their competition may be responsible for the absence of magnetic order in La 3 Ni 2 O 7 , demonstrating that magnetic frustration and superconducting pairing can arise from the same set of incommensurate spin fluctuations.

The effects of Co doping in a series of La3Ni2−xCoxO7−δ samples before and after high oxygen pressure annealing have been investigated. The structural refinemen

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This study employed the six-terminal method to perform high-precision transport measurements on Ruddlesden–Popper nickelate bilayer and trilayer single crystals, self-consistently extracting the in-plane and out-of-plane resistivities. The results reveal strong intrinsic electronic anisotropy, with out-of-plane resistivity exhibiting a non-monotonic temperature dependence that signals a universal interlayer coherent-to-incoherent crossover. Under pressure, the maximum superconducting transition temperature is inversely proportional to the resistivity anisotropy at ambient pressure, indicating that stronger interlayer electronic coherence favors superconductivity. Moreover, out-of-plane resistivity serves as a sensitive probe of magnetic and density-wave orders, whereas the in-plane resistivity shows a weaker response. These findings highlight interlayer coherence as a key tuning parameter that both tracks magnetic correlations and is closely linked to superconductivity, providing stringent constraints for microscopic theories of nickelate high-temperature superconductivity.

Using resonant X-ray scattering and spectroscopy, we investigate the relationship between spin-density wave (SDW) order and superconductivity in bilayer nickelate La₂PrNi₂O₇ thin films. Superconductivity is found to emerge only in regions without SDW and with complete oxygen stoichiometry, whereas oxygen deficiency promotes SDW order, indicating phase separation between the two. Further Ni-L₃ and O-K edge spectroscopy reveal that the superconducting phase exhibits a metallic ground state dominated by Ni d⁸ and oxygen ligand-hole character; oxygen deficiency induces electron localization and the appearance of low-energy excitations. Combined with theoretical analysis, we propose that ligand holes primarily reside on the interlayer apical oxygen sites, forming stable interlayer five-spin polaron states as the ground state of superconducting bilayer nickelates. This study demonstrates that oxygen stoichiometry is a key parameter controlling interlayer coupling and electronic structure, and that SDW order is not the intrinsic parent state of superconductivity.

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By stabilizing bilayer nickelate (La,Pr)₃Ni₂O₇ superconducting thin films with a protective capping layer and employing X-ray absorption and resonant inelastic X-ray scattering spectroscopy, this study directly probes the evolution of electronic structures across insulating, superconducting, and metallic states. Experimental and theoretical analyses reveal that the in-plane d_{x²-y²} states constitute an itinerant electron backbone, whereas superconductivity emerges only when the out-of-plane d_{z²}-p_z-d_{z²} interlayer hybridization becomes coherent, accompanied by suppression of static spin order and the appearance of strongly damped spin excitations. Oxygen stoichiometry and epitaxial strain jointly regulate this interlayer channel, confining the superconducting phase to a narrow window of interlayer coherence and correlation strength. These findings elucidate the microscopic prerequisites for superconductivity in bilayer nickelates and provide a multi-orbital framework to describe its emergent mechanism.

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