Pairing symmetry and superconductivity in La₃Ni₂O₇ thin films

摘要

该研究采用重整化平均场理论，基于包含 (dz2) 和 (dx2-y2) 轨道的双层 (t-J) 模型，系统探究了 La₃Ni₂O₇ 薄膜的超导配对对称性。通过自洽求解，揭示出由 (dz2) 轨道强层间超交换耦合驱动的 (s_±) 波配对，与加压块体情形一致，并成功重现了角分辨光电子能谱观测到的 (beta) 费米面口袋上无节点超导能隙结构，计算得到的超导转变温度约 60 K 与实验吻合。轨道分辨分析表明，(beta) 口袋的无节点特征源于 (dz2) 和 (dx2-y2) 轨道层间配对的协同作用，同时面内 (dz2) 与 (dx2-y2) 轨道间形成 (d) 波配对分量，可进一步增强主导的 (s_±) 波配对。该工作揭示了 La₃Ni₂O₇ 薄膜复杂费米面上不同配对通道间的多样协作与竞争关系，并讨论了衬底应变、氧空位等因素对配对对称性的潜在调制，为理解镍酸盐超导机理提供了重要理论依据。

材料

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方法

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关键词

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亮点

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结论

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主要论断

• Superconductivity in La₃Ni₂O₇ thin films has s±-wave pairing symmetry.
  • 证据: RMFT self-consistent solution yields s±-wave form factors,Figure 2(a) shows nodeless β pocket and opposite-sign pockets
• The pairing is driven by strong interlayer superexchange coupling of the dz2 orbital.
  • 证据: Interlayer pairing bond Δ_perp^z-z is dominant,Large gap on α pocket reflects strong J_perp
• The nodeless gap on the β pocket results from interlayer pairing within both dz2 and dx2-y2 orbitals.
  • 证据: Orbital-resolved gaps show joint contributions with no node on β pocket (Fig. 3(b,d)),Both orbitals exhibit same sign phase on that pocket
• An inplane inter-orbital d-wave pairing component further enhances the dominant s±-wave.
  • 证据: Δx2-y2 inplane channel develops d-wave form factor,Projection onto Fermi surface shows gap amplification (Appendix A)
• The calculated T_c of ≈60 K is in agreement with experimental values for thin films.
  • 证据: Gap magnitudes drop to zero at T_c≈60 K in RMFT,Experimental reports give 40–60 K T_c under ambient pressure

研究流程

• Model construction — The model captures essential electronic structure for studying superconductivity in La₃Ni₂O₇ thin films.
  • 材料: bilayer two-orbital t-J model; DFT-derived tight-binding Hamiltonian; Heisenberg exchange parameters (J_perp, J_parallel)
  • 方法: exact diagonalization for superexchange coupling estimation; Gutzwiller renormalized mean-field theory framework
  • 观察: effective low-energy Hamiltonian suitable for RMFT
• RMFT self-consistent calculation — Self-consistent RMFT yields an s±-wave superconducting ground state.
  • 材料: mean-field order parameters; pairing bond amplitudes
  • 方法: self-consistent solution of RMFT equations; application of Gutzwiller renormalization factors
  • 观察: s±-wave pairing symmetry; dominant interlayer s± driven by dz2 orbital; finite pairing bond magnitudes
• Gap structure analysis — The nodeless β pocket gap arises from cooperative interlayer pairing of both orbitals, and an additional d-wave component enhances the order.
  • 材料: superconducting gap matrices projected on Fermi surface; orbital-resolved gap components
  • 方法: gap projection onto Fermi pockets (α, β, γ); orbital decomposition of pairing bonds
  • 观察: nodeless gap on β pocket, sign flip on γ pocket; nodeless character originates from interlayer dz2 and dx2-y2 pairs; inplane inter-orbital d-wave component enhances s±
• Interpretation and experimental comparison — The calculated T_c and gap structure support the s±-wave scenario and the role of interlayer superexchange, consistent with thin-film experiments.
  • 材料: temperature dependence of gap values; experimental ARPES gap and T_c data
  • 方法: RMFT at varying temperatures; comparison with experimental T_c and gap anisotropy
  • 观察: gaps vanish sharply at T_c ≈ 60 K, comparable to experimental 40–60K; gap anisotropy on β pocket consistent with ARPES nodeless shape