Interlayer pairing mechanism for bilayer nickelate superconductors

摘要

该综述系统梳理了双层镍酸盐超导体中基于强关联效应的层间配对机制理论进展。文章从关键实验观测出发，提炼出核心物理要素，包括杂化的Ni-3d_(x²-y²)与3d_(z²)轨道电子结构、轨道依赖的电子关联、洪德耦合以及强层间磁耦合，并引入了双层两轨道哈伯德模型及其t-J变体等基本理论框架。重点阐述了基于半填充d_(z²)轨道原子极限下层间价键图像的强关联配对机理，尤其强调杂化机制：即d_(z²)局域单重态配对提供凝聚能，其与巡游d_(x²-y²)轨道的杂化促进超导相位相干。文章进一步分析了配对对称性、临界温度对各种内外部参数的依赖关系，以及正常态的费米液体、非费米液体、弱绝缘和赝能隙等非平庸行为，并讨论了压力调控、氧含量及氧空位诱导的近藤散射的影响。全文核心结论指向一种由局域配对与巡游性协同作用驱动的非传统超导图像，并简要提及了基于费米面嵌套自旋涨落的弱关联理论。

材料

• La₃Ni₂O₇
• La₄Ni₃O₁₀
• La₂PrNi₂O₇
• La_3-xSr_xNi₂O₇

方法

• 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

关键词

• 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

亮点

• 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.

结论

• 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.

主要论断

• 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.'

研究流程

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