Weak coupling theory of nickel-based 327 superconductors

摘要

本文综述了加压双层镍酸盐 La₃Ni₂O₇ 超导电性的弱耦合理论研究，主要采用随机相近似（RPA）、涨落交换近似（FLEX）和泛函重整化群（FRG）三种方法，基于包含 Ni dx²-y² 和 d3z²-r² 轨道的双层双轨道 Hubbard 模型。研究表明，压力下费米面由电子型 α 口袋和空穴型 β、γ 口袋构成，其中主要由 d3z²-r² 轨道构成的 γ 口袋与其他口袋存在强嵌套，显著增强反铁磁自旋涨落并介导吸引配对。不同方法普遍预言主导配对对称性为 s± 波：γ 与 α 口袋超导能隙同号，β 口袋反号，且与 d3z²-r² 主导的层间配对通道密切相关。RPA 高效描述自旋和电荷磁化率，FLEX 通过自洽修正准粒子与自旋涨落，FRG 可无偏处理超导、自旋密度波和电荷密度波等竞争序。综述认为弱耦合理论能有效关联费米面几何与自旋涨落配对机制，并指出未来需与强耦合图像及更精确的第一性原理参数相结合。

材料

• La₃Ni₂O₇

方法

• Random phase approximation (RPA)
• Fluctuation-exchange approximation (FLEX)
• Functional renormalization group (FRG)
• DFT
• Cluster dynamical mean-field theory
• Constrained random phase approximation

关键词

• high temperature superconductivity
• spin fluctuations
• fermi surface nesting
• s± wave pairing
• unconventional superconductivity
• interlayer pairing
• gamma pocket

亮点

• The review systematically compares RPA, FLEX, and FRG, emphasizing their complementary strengths and their convergence on spin-fluctuation-mediated s±-wave pairing.
• The weak-coupling understanding is extended to ambient-pressure La₃Ni₂O₇ thin films, including substrate strain effects and the debated presence of the γ pocket.
• Electron-phonon coupling alone is insufficient to generate superconductivity, but interlayer electron-phonon coupling can cooperate with electronic correlations to promote interlayer d3z2-r2 pairing.

结论

• Weak-coupling theories, primarily RPA, FLEX, and FRG, generally predict s±-wave pairing symmetry in the pressurized bilayer nickelate La₃Ni₂O₇.
• The superconducting pairing is mediated by spin fluctuations enhanced by Fermi surface nesting, with the γ pocket dominated by the Ni d3z2-r2 orbital playing a key role.
• Interlayer coupling between Ni 3d_z2 orbitals is dominant in the superconducting pairing, and the pairing state is sensitive to crystal field splitting and interlayer interactions.
• FRG studies of the pressure dependence of T_c in La₃Ni₂O₇ are consistent with experimental observations, supporting an itinerant weak-coupling picture.

主要论断

• Weak-coupling approaches (RPA, FLEX, and FRG) broadly predict s±-wave pairing symmetry in pressurized La₃Ni₂O₇, with the superconducting gap maintaining the same sign on the gamma and alpha pockets and the opposite sign on the beta pocket.
  • 证据: The abstract states: 'a dominant s±-wave pairing symmetry is widely predicted across different methodologies. In this state, the superconducting gap function maintains the same sign on the γ and α pockets but reverses sign on the β pocket.',The full text conclusion states: 'Techniques such as the RPA…, fluctuation-exchange approximation (FLEX)…, and FRG… mainly agree on s±-wave symmetry.'
• The gamma pocket, predominantly derived from the Ni d3z2-r2 orbital, is central to the weak-coupling pairing mechanism because its strong nesting with other Fermi surface pockets enhances antiferromagnetic spin fluctuations that mediate attractive pairing.
  • 证据: The abstract states: 'The γ pocket, predominantly derived from the d3z2−r2 orbital, exhibits strong nesting with the other pockets. This nesting significantly enhances antiferromagnetic spin fluctuations, which in turn mediate attractive pairing interactions.',The full text states: 'These complementary techniques collectively reinforce the conclusion that spin-fluctuation-mediated pairing, driven by the interlayer nesting between the d3z2−r2-dominated γ pocket and other Fermi surface sheets, is the primary mechanism underlying superconductivity in the bilayer nickelate system.'
• Weak-coupling theory effectively links Fermi surface geometry to spin-fluctuation-mediated superconductivity in La₃Ni₂O₇, but future work needs integration with strong-coupling pictures, more precise ab initio-derived parameters, and definitive experimental tests.
  • 证据: The abstract states: 'Finally, we outline outstanding challenges and future directions, emphasizing the need for closer integration with strong-coupling pictures, more precise ab initio-derived model parameters, and definitive experimental tests to distinguish between competing theoretical scenarios.',The full text conclusion states: 'Theoretical research on nickel-based high-temperature superconductivity is still in its early stages and requires more precise experimental guidance. Further development of both weak-coupling and strong-coupling theories is necessary.'

研究流程

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