Paper

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.

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 paper develops a unified itinerant electron description for the nature of spin-density wave (SDW) order and magnetic excitations in Ruddlesden-Popper nickelates. The central element is the mirror symmetry of the NiO₂ multilayer blocks, which organizes the low-energy electronic states into mirror-even and mirror-odd sectors. It is shown that the dominant interband nesting between mirror-opposite sectors drives a mirror-selective itinerant SDW instability, whose collective modes naturally reproduce the spin-wave-like spectra observed experimentally. In La₄Ni₃O₁₀, the SDW further induces a secondary mirror-even charge density wave, giving rise to intertwined spin and charge textures. These results demonstrate that magnetism in multilayer nickelates is intrinsically itinerant rather than of local-moment origin, and establish mirror-selective interband SDW order as a unifying organizational principle for magnetic correlations in these systems.

This study systematically analyzed the electronic structure of strained La₃Ni₂O₇ thin films using density functional theory calculations, revealing that biaxial compressive strain primarily elongates the outer apical Ni–O bonds while leaving the inner apical Ni–O bonds nearly unchanged, thereby significantly enhancing the Jahn–Teller splitting energy Δ_JT, yet the interlayer d_z² orbital hopping parameter t_⊥^z exhibits only a weak variation. Given that superconductivity emerges only when the in-plane lattice constant falls below a critical value, these results identify strain-enhanced Δ_JT as a key microscopic tuning parameter. The calculated Fermi surface topology and Hall response agree well with angle-resolved photoemission spectroscopy (ARPES) and Hall measurements on LaAlO₃ and SrLaAlO₄ substrates, confirming that Jahn–Teller distortion plays a central role in optimizing superconductivity in bilayer nickelates.

Lattice-Charge Coupling in a Trilayer Nickelate with Intertwined Density Wave Order

This study employs density functional theory combined with dynamical mean-field theory (DFT+DMFT) to systematically analyze the layer-resolved electronic structure and correlation effects in low-n tetragonal nickelates. The results reveal that the electronic correlation strength of Ni-d orbitals in undoped systems increases with layer number, and that the inner NiO₂ planes consistently exhibit stronger correlations than the outer ones, a discrepancy originating from the inhomogeneous spatial charge distribution. For the n=2 and n=3 compounds, which are non-superconducting due to excessive hole doping, an electronic compensation strategy via Cl substitution at spacer-layer oxygen sites is proposed, and virtual crystal approximation simulations tune the nominal Ni valence to match that of the optimally superconducting n=6 system. Calculations demonstrate that Cl doping significantly enhances the Ni-d mass enhancement factor in the low-layer-number systems, driving them into the strongly correlated metallic regime while preserving the low-energy electronic structure. This work highlights the critical role of layer-resolved electronic correlations in the superconductivity mechanism and predicts that spacer-layer Cl doping is a viable pathway to convert low-n tetragonal nickelates into potential superconducting candidates.

This study employs a multi-orbital itinerant electron framework, combined with Hartree-Fock and random phase approximation methods, to systematically analyze the magnetic ground states and transverse spin excitations of bilayer and trilayer nickelates. For the bilayer system, although the double-stripe order has slightly lower energy, the excitation spectrum of the single-stripe state exhibits anisotropic low-energy cone-shaped dispersion at the wavevector Q_BL and isotropic high-energy excitations near the Γ point, showing qualitative consistency with resonant inelastic X-ray scattering and neutron scattering experiments; it is also found that the energy of the mirror-even interlayer optical mode at Q_BL coincides with that of the mirror-odd mode at Γ. In the trilayer system, both mirror-odd and mirror-even spin-density wave orders can be stabilized, with the mirror-odd state having lower energy and hosting a near-zero-gap mode predominantly from the middle layer, whereas the mirror-even state supports only one acoustic mode and two gapped optical modes; comparison with experimental data supports the mirror-odd order picture. The results demonstrate that magnetic excitations can serve as a sensitive probe to distinguish magnetic order configurations and reinforce the conclusion that the magnetism in multilayer nickelates shares a common itinerant origin.

Based on experimental evidence of orbital-selective electronic correlations, this work proposes a new magnetic description framework for bilayer nickelate La₃Ni₂O₇. In the bad-metal regime of the normal state, the system lies close to an orbital-selective Mott phase, with the electronic spectrum decomposing into coherent d_(x²-y²) quasiparticles and incoherent d_(z²) local moments. An effective spin model incorporating superexchange between local moments and RKKY interactions mediated by coherent electrons is constructed, where the RKKY contribution dominates third-neighbor coupling and introduces magnetic frustration. The model naturally stabilizes a noncoplanar antiferromagnetic ground state with a wavevector near (π/2, π/2) and antiferromagnetic interlayer stacking, consistent with neutron scattering experiments. The calculated spin-wave spectrum contains low-frequency acoustic and high-frequency optical branches, with the acoustic branch softening at the ordering wavevector and an overall bandwidth of about 80 meV, in agreement with resonant inelastic X-ray scattering data. These results reveal that orbital-selective correlations are the essential ingredient determining magnetism in bilayer nickelates and indicate that low-energy magnetic fluctuations and short-range exchange interactions may provide the pairing glue for unconventional superconductivity.

Using angle-resolved photoemission spectroscopy (ARPES) with an optimized surface treatment, we probe the low-energy electronic structure of the infinite-layer nickelate superconductor La0.8Ca0.2NiO2 and its parent LaNiO2 thin films, resolving clear quasiparticle peaks for the first time in this system. In La0.8Ca0.2NiO2, the imaginary part of the electron self-energy exhibits a roughly linear dependence on energy with a slope that progressively increases from the (π/2, π/2) to the (π, 0) direction, revealing momentum-dependent marginal-Fermi-liquid-like scattering. Both films display low-energy spectral weight that is gradually suppressed from the diagonal toward the antinodal region, with stronger suppression in the parent compound; however, finite Fermi-level spectral weight persists over the entire Fermi surface, and no leading-edge shift or back-bending is observed, indicating the absence of the pseudogap typical of cuprates. These findings demonstrate that momentum-selective correlations and marginal-Fermi-liquid-like scattering can emerge without a detectable pseudogap, providing a crucial benchmark for identifying the essential electronic ingredients of the high-Tc normal state and suggesting that the pseudogap is not a requirement, while charge-transfer energy and the orbital character of doped carriers may play decisive roles.