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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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This review systematically summarizes theoretical progress on the interlayer pairing mechanism driven by strong correlation effects in bilayer nickelate superconductors. Starting from key experimental observations, the paper extracts core physical ingredients, including the hybridized electronic structure of Ni-3d({x^2-y^2}) and 3d({z^2}) orbitals, orbital-dependent electronic correlations, Hund’s coupling, and strong interlayer magnetic coupling, and introduces fundamental theoretical frameworks such as the bilayer two-orbital Hubbard model and its (t)-(J) variants. Emphasis is placed on the strong-correlation pairing mechanism rooted in an interlayer valence bond picture in the atomic limit of half-filled d({z^2}) orbitals, with particular stress on the hybridization mechanism: local singlet pairing of d({z^2}) electrons provides condensation energy, while hybridization with itinerant d(_{x^2-y^2}) orbitals promotes superconducting phase coherence. The review further analyzes the pairing symmetry, the dependence of critical temperature on various internal and external parameters, and non-trivial normal-state behaviors including Fermi liquid, non-Fermi liquid, weak insulating, and pseudogap regimes, and discusses the effects of pressure tuning, oxygen stoichiometry, and Kondo scattering induced by oxygen vacancies. The central conclusion points to an unconventional superconductivity picture driven by the synergy between local pairing and itinerant behavior, and briefly mentions weak-coupling theories based on spin fluctuations arising from Fermi surface nesting.

High-temperature superconductivity in pressurized La3Ni2O7 has attracted considerable interest, yet the superconducting phase is rather fragile. Although bulk superconductivity can be achieved by Pr substitution for La, the underlying mechanism is still unclear. A further puzzle is the role of oxygen content: moderate oxygenation enhances superconductivity, whereas high-pressure oxygen annealing suppresses it. Here combining multislice electron ptychography and electron energy-loss spectroscopy, we show that Pr doping mitigates oxygen vacancies and stabilizes a near-stoichiometric La2PrNi2O7 structure. Strikingly, high-pressure oxygen annealing introduces interstitial oxygen atoms that arrange into a stripe-ordered superstructure, which generates excess hole carriers and alters the electronic structure, ultimately suppressing superconductivity under pressure. This contrasts sharply with cuprates, where similar oxygen ordering is known to induce superconductivity. Our findings reveal a competition between interstitial oxygen ordering and superconductivity in bilayer nickelates, providing key insights into the pairing mechanism and guiding principles for engineering more robust superconducting phases.

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Addressing the long-standing puzzle of the pairing mechanism for high-temperature superconductivity in cuprates and nickelates, this paper proposes a picture of itinerant Cooper pairs mediated by oxygen-bridged electron pairs (e⁻-O-e⁻) or metal-bridged hole pairs (h⁺-M-h⁺), based on the dominant role of ionic bonds on the order of eV, the electron affinities of O⁻ and O²⁻ (1.46 eV and -8.08 eV, respectively), and the large double ionization energies of metal atoms (approximately 15–28 eV). Such pairing forms below the pseudogap temperature T*, which is higher than Tc, and follows the relationship of chemical bond → structure → properties, being applicable to cuprates, nickelates, iron-based, and other ionic superconductors. The author verifies the correctness and universality of this mechanism through 32 independent experimental pieces of evidence, especially STM images within the CuO₂ plane and the extremely small pairing size, and points out that any sub-eV or covalent bonding pairing mechanism is unreliable. This theory reveals the missing link between ionic bonds and superconductivity, resolves a four-decade-long puzzle, and demonstrates the feasibility of achieving room-temperature carrier pairing in ionic-bond superconductors. Based on this, the author establishes a new theoretical framework centered on the strongest pairing strength and Bose–Einstein condensation, opening a new path for understanding the mechanism of high-temperature superconductivity and bringing the dream of room-temperature superconductivity closer to reality.

This study employs superconducting density functional theory to perform ab initio calculations on optimally doped infinite-layer nickelates Re₀.₈Sr₀.₂NiO₂ (Re=La, Pr, Nd), treating electron–phonon coupling, screened Coulomb repulsion, and spin fluctuations on an equal footing. The results reveal that these materials are two-band superconductors exhibiting opposite-sign d_(x²−y²)-wave pairing gaps on different Fermi surfaces; when spin fluctuations are switched off, the critical temperature drops to a negligible ~0.01 K, indicating that superconductivity is driven by spin fluctuations. On the large quasi-two-dimensional Fermi surface at the Brillouin zone center, the spin fluctuation strength is an order of magnitude larger than that of electron–phonon coupling and Coulomb repulsion, thereby dominating the pairing mechanism, whereas electron–phonon coupling plays the principal role on the small three-dimensional electron pockets at the zone corners. The nodal d-wave gap structure originates from a pronounced peak of the Lindhard response function at the zone corners. The calculated Fermi surfaces, critical temperature, nodal gaps, and quasiparticle density of states are in agreement with most experimental observations, and the predicted unconventional superconducting properties such as scanning tunneling spectra await direct experimental verification.