Unconventional Superconductivity in La₃Ni₂O₇ from the Perspective of Symmetry

摘要

本研究针对高压下高转变温度的La₃Ni₂O₇块材与常压超导但T_c减半的薄膜之间的差异，发展了一种基于对称性的唯象方法，结合DFT+U计算和实验确定的T_c及结构对称性，分析超导能隙结构。研究发现，两者均呈现s±波配对对称性和两带超导，但主导的微观配对构型不同：加压块材中超导主要由Ni-dz²轨道的面外配对主导，而薄膜中则由Ni-dx²-y²轨道的面内配对主导。T_c的降低归因于薄膜中层间与层内跳跃比率减小，导致主导配对类型从面外转向面内。该结果揭示了对称性在非常规超导中的关键作用，所发展的方法有望推广至其他非常规超导体。

材料

• La₃Ni₂O₇

方法

• DFT
• Bogoliubov–de Gennes theory
• Symmetry analysis
• BCS mean-field theory

关键词

• s± wave pairing
• two gap superconductivity
• interlayer pairing
• orbital selectivity
• fermi surface reconstruction
• pairing symmetry

亮点

• Developed a phenomenological symmetry-based method that can be generalized to other unconventional superconductors.
• Calculated spectra show close agreement with ARPES and STM/STS experimental measurements.
• The dominant pairing type transition explains the approximately halved T_c in thin films relative to pressurized bulk.

结论

• Both pressurized bulk and thin-film La₃Ni₂O₇ exhibit s±-wave pairing symmetry and two-gap superconductivity.
• In pressurized bulk, superconductivity is dominated by out-of-plane pairing of Ni-dz2 orbitals; in thin films, in-plane pairing of Ni-dx2-y2 orbitals dominates.
• The reduction in T_c in thin films is attributed to a decreased ratio of inter-layer to intra-layer hoppings, shifting the dominant pairing type.

主要论断

• Both pressurized bulk and thin-film La₃Ni₂O₇ exhibit s±-wave pairing symmetry and two-gap superconductivity.
  • 证据: Symmetry analysis and free energy minimization identify s± pairing; gap function on Fermi surfaces shows sign change between pockets
• In pressurized bulk, superconductivity is dominated by out-of-plane pairing of Ni-d_z² orbitals; in thin film, in-plane pairing of Ni-dx²-y² orbitals prevails.
  • 证据: Free energy comparison shows different leading pairing types; parameter analysis shows decreased interlayer-to-intralayer hopping ratio in thin film
• The reduced T_c in thin film (half of bulk) is due to transition of dominant pairing type driven by decreased interlayer/intralayer hopping ratio.
  • 证据: Calculations show dominant pairing type shift leads to lower T_c; consistent with experimental dependence on lattice constants

研究流程

• model_construction — Effective tight-binding model for La₃Ni₂O₇.
  • 材料: DFT+U calculations
  • 方法: density functional theory with Hubbard U correction; Wannier downfolding
  • 观察: two-orbital model with d_z² and dx²-y² orbitals
• symmetry_analysis — s±-wave pairing symmetry is preferred.
  • 材料: crystal symmetry information
  • 方法: irreducible representations of point group D₄h; BdG generators
  • 观察: identification of symmetry-allowed pairing types
• numerical_calculation — Dominant pairing configurations differ between bulk and thin film.
  • 材料: tight-binding parameters
  • 方法: self-consistent BCS gap equation; free energy minimization; differential evolution
  • 观察: out-of-plane pairing dominates in bulk; in-plane dominates in thin film
• comparison_with_experiment — Calculated gap structure matches experimental observations.
  • 材料: ARPES, STM/STS data
  • 方法: projected gap function on Fermi surfaces
  • 观察: agreement with RPA calculations and experimental spectra