Superconductivity in bilayer La₃Ni₂O₇: A review focusing on the strong-coupling Hund's rule assisted pairing mechanism

摘要

双层La₃Ni₂O₇中的高温超导源于其独特的双轨道双层电子结构，其中3d_z²轨道近半填充且局域化，通过内顶角氧2p_z轨道产生强层间反铁磁交换；而3dx²-y²轨道约四分之一填充且高度巡游。在强耦合下，Hund规则耦合使同一镍位上两个轨道的自旋对齐，将层间反铁磁交换有效传递至巡游的3dx²-y²轨道，形成有效耦合J⊥。这一机制可简化为针对3dx²-y²带的强耦合双层t-J-J⊥模型，其中J⊥驱动电子形成层间库珀对，实现高临界温度的扩展s波配对超导。同时，强局域的3d_z²电子倾向于形成层间梯子单态，由于缺乏相位相干性，这些单态不直接参与超导凝聚，而是导致赝能隙相的出现。该综述系统阐述了这一强耦合Hund规则辅助配对理论，为理解该体系的高温超导机制提供了统一框架。

材料

• La₃Ni₂O₇

方法

• strong-coupling analysis
• slave-boson mean-field theory
• density matrix renormalization group (DMRG)
• quantum Monte Carlo

关键词

• hund's rule coupling
• interlayer pairing
• extended s wave
• pseudogap
• orbital selectivity
• self doping

亮点

• A strict division of labor between orbitals: d_z2 acts as magnetic glue, dx2-y2 hosts mobile Cooper pairs.
• Electron doping is predicted to enhance T_c, providing a testable route.

结论

• Hund's rule coupling aligns spins of localized d_z2 and itinerant dx2-y2 orbitals, transferring interlayer antiferromagnetic exchange to the dx2-y2 band, resulting in extended s-wave pairing with high T_c.
• The d_z2 orbital forms interlayer rung singlets that yield a pseudogap phase but do not directly contribute to superconductivity.

主要论断

• The 3d_z2 orbital is nearly half-filled and localized, generating strong interlayer antiferromagnetic exchange via the inner apical oxygen 2p_z orbital.
  • 证据: the 3d_z² orbital is nearly half-filled and localized, generating strong interlayer antiferromagnetic exchange via the inner apical oxygen 2p_z orbital
• Hund's rule coupling aligns the spins of the two orbitals on the same nickel site, effectively transferring the interlayer antiferromagnetic exchange to the itinerant 3dx2-y2 orbital, forming an effective coupling J⊥.
  • 证据: Under strong coupling, Hund’s rule coupling aligns the spins of the two orbitals on the same nickel site, effectively transferring the interlayer antiferromagnetic exchange to the itinerant 3dx²-y² orbital, forming an effective coupling J⊥.
• This mechanism can be simplified into a strong-coupling bilayer t-J-J⊥ model for the 3dx2-y2 band, which realizes extended s-wave pairing superconductivity with high T_c.
  • 证据: This mechanism can be simplified into a strong-coupling bilayer t-J-J⊥ model for the 3dx²-y² band, where J⊥ drives electrons to form interlayer Cooper pairs, realizing extended s-wave pairing superconductivity with high critical temperature.

研究流程

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