Possible Enhancement of Superconductivity in Ambient-Pressure La₃Ni₂O₇ Thin Film

摘要

该研究采用涨落交换（FLEX）近似方法，系统分析了常压下La₃Ni₂O₇薄膜超导体在弱关联体系中的两站点两轨道模型，重点研究了空穴掺杂对超导特性的影响。通过对费米面拓扑结构的详细分析，发现当由dz²反键轨道构成的δ口袋在Γ点附近形成时，δ口袋与γ口袋的嵌套以及α与β口袋之间的嵌套，在相应波矢处共同增强了s±波配对。研究进一步提出，这种由费米面嵌套驱动的自旋涨落诱导配对的增强机制，可能为提升超导转变温度提供可行途径。该工作为理解常压镍基薄膜超导体的配对机制及探索更高性能超导材料提供了理论指导。

材料

• La₃Ni₂O₇

方法

• FLEX (fluctuation exchange) approximation
• linearized Eliashberg equation
• density functional theory (DFT)-based tight-binding modeling
• spin susceptibility analysis

关键词

• fermi surface nesting
• s± wave pairing
• spin fluctuation
• hole doping
• γ pocket
• δ pocket
• α and β pockets
• pairing enhancement

亮点

• When a δ pocket composed of the dz2 antibonding orbital emerges near the Γ point, its nesting with the γ pocket, together with the nesting between the α and β pockets, leads to a mutual enhancement of s±-wave pairing.
• If the δ pocket could be restored to the Fermi surface in new material realizations through strain or doping, it could lead to higher superconducting transition temperature.

结论

• We used the FLEX method to study a previously proposed model Hamiltonian for the ambient pressure La₃Ni₂O₇ film superconductor.
• We conclude that the complex spin fluctuations induced by repulsive Coulomb interactions can facilitate s±-wave pairing on the orbitals.
• We found that a non-trivial δ pocket in the studied band structure significantly enhances the tendency toward s±-wave superconductivity.
• B_y artificially tuning the presence or absence of the δ pocket, we confirmed this superconductivity enhancing mechanism.

主要论断

• Fermi surface nesting between the δ pocket (d_z² antibonding orbital) and the γ pocket, together with α-β nesting, mutually enhances s±-wave pairing.
  • 证据: abstract: 'nesting between the δ and γ pockets, together with the nesting between the α and β pockets, leads to a mutual enhancement of s±-wave pairing',full_text: 'spin susceptibility at optimal doping is dominated by peaks at q1 and q2'
• The δ pocket significantly contributes to superconductivity; artificially removing it reduces the pairing eigenvalue λ.
  • 证据: full_text Fig. 4(c,f): λ is consistently higher when δ pocket is present,full_text: 'the enhancement mechanism of spin-fluctuation-induced pairing driven by Fermi surface nesting'
• This nesting-driven enhancement mechanism may provide a viable route to raising T_c in nickelate thin films.
  • 证据: abstract: 'this nesting-driven enhancement of spin-fluctuation-induced pairing may provide a viable mechanism for enhancing superconductivity',full_text: 'presence of the δ pocket and its enhancement mechanism could lead to a higher T_c'

研究流程

• model_construction — The minimal model captures essential physics of ambient-pressure films.
  • 材料: two-site two-orbital Hubbard model for La₃Ni₂O₇ thin film
  • 方法: tight-binding parametrization from DFT; Hubbard-Kanamori interaction
  • 观察: Band structure agrees with DFT
• FLEX_calculations — FLEX captures renormalized Fermi surface and spin fluctuations.
  • 材料: self-consistent Green's functions and susceptibilities
  • 方法: fluctuation exchange (FLEX) approximation; 48×48×48 k-grid; T = 0.01 eV
  • 观察: Fermi surface with α, β, γ, δ pockets; spin susceptibility peaks at nesting vectors
• Eliashberg_analysis — Fermi surface nesting between δ and γ pockets enhances s±-wave pairing.
  • 材料: converged FLEX quantities
  • 方法: linearized Eliashberg equation; power method for largest eigenvalue λ
  • 观察: s±-wave eigenvalue enhanced near optimal doping; d-wave competitive in overdoped region