What Does the Single-Particle Spectrum Imply on the Pairing Nature and Pairing Mechanism in La₃Ni₂O₇?

摘要

针对双层镍酸盐La₃Ni₂O₇中配对机制的争议，本文利用角分辨光电子能谱（ARPES）和扫描隧道显微镜（STM）揭示的低各向异性无节点全间隙作为约束，提出以布里渊区对角线上的配对能隙为关键探针。对称性分析表明，沿该对角线上dx2-y2与dz2轨道间的杂化消失，使得γ口袋和α/β口袋的能隙分别反映两个轨道的本征配对强度。基于dz2轨道主导的杂化驱动配对机制会导致α/β口袋在对角线方向出现能隙节点，与实验观测的U形dI/dV谱相矛盾；而dx2-y2轨道主导的洪特规则驱动配对机制则产生整个费米面上的均匀全间隙，与ARPES和STM结果一致。弱耦合随机相近似计算也因dz2轨道的态密度优势在对角线附近给出节点或近节点行为，与实验冲突。因此，该工作澄清了dx2-y2轨道在配对中的主导地位，并确立洪特规则驱动的配对机制为La₃Ni₂O₇中最相关的超导配对图像。

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

• The pairing gap along the BZ diagonal serves as a decisive probe of the pairing mechanism in La₃Ni₂O₇.
  • 证据: Symmetry analysis shows orbital hybridization vanishes along the BZ diagonal, causing the pairing gaps on the γ and α/β pockets to reflect the intrinsic pairing strengths of dz2 and dx2-y2 orbitals respectively.
• The Hund’s rule driven pairing mechanism with dominant dx2-y2 orbital pairing yields a full gap consistent with ARPES and STM experiments.
  • 证据: Mean-field model of Hund’s rule mechanism produces a U-shaped STM spectrum and nodeless gap distribution, matching the experimental observations of a nearly isotropic full gap.
• Weak-coupling (RPA) and orbital-hybridization driven strong-coupling theories produce gap nodes or near-nodes conflicting with experiments because they overestimate the role of the dz2 orbital.
  • 证据: RPA calculation shows gap nodes near the BZ diagonal on the α/β pockets due to dz2-dominated pairing from its large density of states.,Orbital-hybridization mechanism inevitably forces gap nodes on the α/β pockets along the diagonal (V-shaped STM).

研究流程

• symmetry_analysis — The pairing gap along the BZ diagonal directly reflects the intrinsic pairing strengths of dz2 and dx2-y2 orbitals.
  • 材料: two-orbital tight-binding model (from Ref. Hu et al.)
  • 方法: mirror reflection symmetry about the Brillouin zone diagonal to deduce vanishing orbital hybridization
  • 观察: The tight-binding Hamiltonian decouples into pure dz2 and dx2-y2 components along the BZ diagonal; the γ pocket is purely dz2, and α/β pockets are purely dx2-y2.
• model_comparison — Only the Hund’s rule driven mechanism with dominant dx2-y2 pairing can reproduce the ARPES/STM observations.
  • 材料: ARPES and STM data from literature (full gap, U-shaped dI/dV); tight-binding model; mean-field Hamiltonians for three pairing mechanisms
  • 方法: projecting mean-field pairing gaps onto the Fermi surface; simulating STM dI/dV curves using BCS density of states convolved with thermal kernel (T=4.2 K)
  • 观察: Orbital-hybridization mechanism: V-shaped STM, gap nodes on α/β pockets along diagonal (inconsistent with data).; Hund’s rule mechanism: U-shaped STM, full gap on all pockets (consistent).; Adding interlayer NN pairing allows perfect fit to ARPES and STM.
• weak_coupling_evaluation — Weak-coupling theories inherently overestimate dz2 pairing due to large DOS, leading to conflict with experimental full gap.
  • 材料: Hubbard-Kanamori model; RPA code; standard interaction parameters (U, J, U')
  • 方法: random-phase-approximation (RPA) to compute spin-fluctuation mediated pairing; Fourier transform to obtain real-space pairing components
  • 观察: RPA yields gap nodes/near-nodes on α/β pockets along BZ diagonal; dz2 orbital pairing dominates over dx2-y2
• interpretation — Hund’s rule driven pairing mechanism with dominant dx2-y2 orbital pairing is the most relevant pairing picture for La₃Ni₂O₇.
  • 材料: all above results
  • 方法: synthesis of symmetry analysis, model comparison, and RPA
  • 观察: Hund’s rule mechanism is the only one that satisfies the experimental constraint; dx2-y2 orbital is the dominant pairing channel; weak-coupling and hybridization-driven theories fail due to nodes