Interlayer pairing mechanism for bilayer nickelate superconductors

Summary

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(x2-y2) and 3d(z2) 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(z2) orbitals, with particular stress on the hybridization mechanism: local singlet pairing of d(z2) electrons provides condensation energy, while hybridization with itinerant d(x2-y2) 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.

Materials

• La₃Ni₂O₇
• La₄Ni₃O₁₀
• La₂PrNi₂O₇
• La_3-xSr_xNi₂O₇

Methods

• DFT
• DMFT
• random phase approximation (RPA)
• functional renormalization group (FRG)
• density-matrix renormalization group (DMRG)
• variational Monte Carlo (VMC)
• cluster dynamical mean-field theory (CDMFT)
• Schwinger boson approach
• auxiliary-field Monte Carlo
• slave-boson mean field theory

Keywords

• interlayer pairing
• hybridization mechanism
• two component theory
• hund's coupling
• superconducting dome
• non fermi liquid
• pseudogap
• kondo effect
• maximal tc/j ratio
• spin density wave

Highlights

• The hybridization mechanism: local singlet pairs in the dz2 orbital provide pairing energy, and their hybridization with itinerant dx2-y2 promotes superconducting phase coherence.
• The two-component theory predicts a universal maximal T_c/J ratio for unconventional superconductors, correctly estimating T_c for bilayer and trilayer nickelates.
• The interlayer valence bond state is identified as the parent state of bilayer nickelates.
• Pressure-induced metallization and hybridization are crucial for superconductivity, while oxygen vacancies induce Kondo scattering and suppress T_c.

Conclusions

• Anisotropic s±-wave pairing with gap minima along the zone diagonal on the dx2-y2 and dz2 pockets and an isotropic gap on the dz2 pocket.
• Crucial role of dz2-orbital metallization for superconductivity, predicting enhanced T_c by small hole doping away from half filling.
• A maximal T_c/J ratio under optimal conditions gives correct magnitudes of T_c for bulk and thin film bilayer nickelates.
• Non-Fermi liquid behavior induced by strong interorbital scattering, turning into FL weakly coupled to VBS around half-filling, or a weakly insulating state for larger hole doping.
• Kondo scattering by local moments and suppression of interlayer Cooper pairing caused by inner apical oxygen vacancies.

Main claims

• Superconductivity in bilayer nickelates arises from an interlayer pairing mechanism where localized d_z2 singlet pairs provide condensation energy and hybridization with itinerant dx2-y2 orbitals establishes phase coherence.
  • Evidence: Abstract: 'hybridization mechanism, where the d_z2 local singlet pairs provide the pairing energy and their hybridization with itinerant dx2-y2 promotes superconducting phase coherence.',Full text: 'the electrons form local interlayer singlet pairs, and their hybridization with the metallic electrons induces global superconducting phase coherence.'
• Strong interlayer magnetic coupling, confirmed by RIXS and INS, is a key ingredient for both superconductivity and density waves.
  • Evidence: Full text: 'This strong interlayer coupling was confirmed by resonant inelastic X‑ray scattering (RIXS) … and inelastic neutron scattering (INS) measurements … which is essential for both the superconductivity and the density waves observed at ambient pressure.'
• The hybridization scenario predicts an anisotropic s‐wave pairing symmetry with possible gap minima/nodes on the dx2-y2 Fermi sheets, qualitatively consistent with STM and ARPES experiments on thin films.
  • Evidence: Full text: 'it predicts an isotropic s‐wave pairing on the … Fermi surface, and anisotropic extended s‐wave pairing with possible nodes or gap minima along the zone diagonal on the … Fermi surfaces.',Full text: 'qualitatively consistent with current experimental observations on thin films HHWen2025; ZYChen2025.'
• The Hund’s coupling mechanism yields a fully isotropic s‐wave gap and a Fermi liquid normal state, with T_c decreasing monotonically with hole doping.
  • Evidence: Full text: 'Ref. Wang2026 found that the Hund mechanism gives several different predictions: 1) The pairing symmetry is a fully isotropic s‐wave … 2) The normal state is generally a Fermi liquid. … 3) T_c decreases monotonically as the hole doping increases.'
• Pseudogap features observed experimentally above T_c are explained in the two‐component theory by preformed local interlayer singlet pairs that lack long‑range phase coherence.
  • Evidence: Full text: 'several experiments reported observation of pseudogap‑like features above the superconducting phase LYang2024; ZYChen2025; ZYChen2026Three.',Full text: 'the local spin singlet pairs … play the role of preformed Cooper pairs, causing the pseudogap feature in the DOS within the temperature range T_c < T < T_pair^MF.'
• Oxygen stoichiometry critically influences superconductivity: moderate oxygen content enhances orbital delocalization and T_c, while excess or deficiency suppress it.
  • Evidence: Full text: 'oxygenation enhances the orbital delocalization and drives superconductivity … overdoped oxygen atoms are harmful … they induce too many holes that strongly suppress the interlayer magnetic correlation',Full text: 'tuning oxygen content leads to a dome‑like structure in bilayer nickelates as reported in Refs. Hwang2026halfdome; Nie2025PRL_crossover; Nie2025Sr.'

Workflow

Not available in this batch.