Layer-Asymmetry-Induced Topological Superconductivity in High-T_c Bilayer Nickelates

摘要

本研究基于第一性原理构建双轨道模型，探讨基底支撑的双层镍酸盐薄膜（La₃Ni₂O₇）中的拓扑超导机制。在主导层间配对假设下，基底诱导的层不对称势与层交错Rashba自旋轨道耦合使系统进入受对角镜像对称保护的DIII类时间反演不变拓扑超导相。该非平凡拓扑源于自旋分裂键合费米面上配对符号的反转，表现为对角方向非零镜像缠绕数及[11]边缘的Majorana Kramers对。此外，轨道杂化导致偏离对角方向的体Dirac节点及零能平带。该研究确立了双层镍酸盐作为拓扑超导平台的潜力，其可调拓扑转变和边界模式为探测非常规配对提供了新途径。

材料

• La₃Ni₂O₇
• NiO₂ bilayers

方法

• First-principles calculations
• Effective two-orbital model
• Tight-binding Hamiltonian
• Bogoliubov-de Gennes (BdG) Hamiltonian
• Topological analysis
• Fermi-surface formula
• Numerical edge-state calculations
• STM
• Spin-resolved angle-resolved photoemission spectroscopy (ARPES) (proposed probe)

关键词

• topological superconductivity
• time reversal invariant topological superconducting phase
• layer asymmetry
• layer asymmetric potential
• layer staggered rashba spin orbit coupling
• interlayer pairing
• dominant interlayer pairing
• mirror symmetry
• diagonal mirror symmetry
• majorana kramers pair
• mirror winding number
• pairing sign reversal
• bulk gap nodes
• dirac nodes
• chiral charge
• zero energy flat bands
• class diii
• unconventional pairing
• high tc superconductivity
• nodal topological superconductivity
• orbital hybridization
• chiral symmetry
• particle hole symmetry
• time reversal symmetry

亮点

• The substrate naturally provides the layer-asymmetric potential and layer-staggered Rashba spin-orbit coupling needed for the topological phase.
• Realizing the topological superconductivity does not require strong layer asymmetry or SOC, because the topological window is set by comparable energy scales of order meV.
• The Majorana Kramers pair at the [11] edge could survive even when mirror symmetry is broken, provided the time-reversal-protected index stays unchanged.
• The topological regime exhibits a V-shaped bulk tunneling spectrum and zero-energy peaks at the [11] and [21] edges but not at the [10] edge, resembling d-wave signatures but distinguished by layer polarization of the nodal quasiparticles.
• An external electric field that reverses the layer-asymmetric potential can transfer the nodal quasiparticle spectral weight to the opposite layer, changing the top-layer spectrum from V-shaped to nearly fully gapped.
• The layer-uniform Rashba term only weakly affects the topological window and shifts helicity branches without changing their layer composition.
• The authors note a related work on nodal orbital-anti-phase pairing in a mirror-symmetric spinless bilayer nickelate model, but in their case they obtain a nonzero mirror winding number and a Majorana Kramers pair at the [11] edge.

结论

• Substrate-supported bilayer nickelate thin films can host an intrinsic time-reversal-invariant topological superconducting phase protected by diagonal mirror symmetry.
• A modest layer-asymmetric potential and layer-staggered Rashba spin-orbit coupling drive the system into this topological phase over a broad parameter range with a dominant interlayer pairing.
• The nontrivial topology arises from a reversal of the pairing sign on one of the spin-split bonding Fermi surfaces and is characterized by a nonzero mirror winding number along the diagonal direction, as well as a Majorana Kramers pair at the [11] edge.
• The phase also features bulk gap nodes located slightly away from the diagonal directions, featuring chiral charge and zero-energy flat bands.
• The Dirac nodes arise from the combined effects of orbital hybridization, the layer-asymmetric potential, and the staggered Rashba spin-orbit coupling, and they carry topological charges protected by chiral symmetry.
• High-T_c bilayer nickelates are a promising platform for topological superconductivity, where tunable topological transitions and Majorana boundary modes can provide probes of unconventional pairing.

主要论断

• Substrate-supported bilayer nickelate thin films can host an intrinsic time-reversal-invariant topological superconducting phase protected by diagonal mirror symmetry.
  • 证据: Abstract and full text state that the authors demonstrate substrate-supported bilayer nickelate thin films can host an intrinsic time-reversal-invariant topological superconducting phase protected by diagonal mirror symmetry.,Using a two-orbital model derived from first-principles calculations, the authors analyze the system and find the topological phase.
• A modest layer-asymmetric potential and layer-staggered Rashba spin-orbit coupling drive the system into a topological phase over a broad parameter range with dominant interlayer pairing.
  • 证据: Abstract: a modest layer-asymmetric potential and layer-staggered Rashba spin-orbit coupling drive the system into this topological phase over a broad parameter range with a dominant interlayer pairing.,Full text: substrate-induced mirror-symmetry breaking and layer-staggered Rashba SOC are incorporated into the effective model, and the topological phase diagram is calculated.
• The nontrivial topology arises from a pairing-sign reversal on one spin-split bonding Fermi surface and is characterized by a nonzero mirror winding number and a Majorana Kramers pair at the [11] edge.
  • 证据: Abstract: the nontrivial topology arises from a reversal of the pairing sign on one of the spin-split bonding Fermi surfaces and is characterized by a nonzero mirror winding number along the diagonal direction, as well as a Majorana Kramers pair at the [11] edge.,Full text: mirror winding number calculation and edge-state analysis show a Majorana Kramers pair at the projecting point of the diagonal on the [11] edge.
• The topological phase features bulk gap nodes slightly away from the diagonal directions, carrying chiral charge and giving rise to zero-energy flat bands.
  • 证据: Abstract: the phase also features bulk gap nodes located slightly away from the diagonal directions, featuring chiral charge and zero-energy flat bands.,Full text: the Dirac nodes carry topological charge protected by chiral symmetry, and zero-energy flat bands appear on the [21] edge.
• The topological phase can be realized and tuned experimentally, and its tunable topological transitions and Majorana boundary modes can probe unconventional pairing.
  • 证据: Discussion: substrate selection and strain, distance from the interface, and external electric fields can tune the relevant parameters.,Discussion: STM spectra may distinguish U-shaped and V-shaped behavior, and layer polarization of nodal quasiparticles can serve as evidence for dominant interlayer pairing.

研究流程

• system_and_first_principles — Substrate and interface effects define the low-energy electronic structure used for modeling.
  • 材料: La₃Ni₂O₇ bilayer thin films on SrLaAlO₄ substrate; NiO₂ bilayers near the interface; Ni d_z² and d_x²-y² orbitals
  • 方法: First-principles calculations; Band-structure analysis; Identification of spin splitting
  • 观察: Low-energy states are dominated by Ni d_z² and d_x²-y² orbitals; Bonding and antibonding bands show spin splitting along the diagonal
• effective_model_construction — The effective model incorporates the substrate-induced symmetry breaking and interlayer pairing needed to analyze topology.
  • 材料: La₃Ni₂O₇ bilayer film on SrLaAlO₄ substrate; Two-orbital low-energy subspace
  • 方法: Two-orbital tight-binding Hamiltonian; Add layer-asymmetric potential; Add layer-staggered Rashba spin-orbit coupling; Add layer-uniform Rashba term; Fit model parameters to first-principles bands; Construct BdG Hamiltonian in Nambu basis with spin-singlet pairing
  • 观察: Effective model captures band structure and spin splittings; Fitted substrate and Rashba parameters are meV-scale; Layer-staggered Rashba term produces opposite spin textures in the two layers
• topological_analysis — Layer asymmetry and layer-staggered Rashba spin-orbit coupling drive intrinsic topological superconductivity in the effective model.
  • 材料: BdG Hamiltonian; Mirror-symmetry sectors
  • 方法: Symmetry classification in class DIII; Mirror winding number calculation; Fermi-surface formula; Topological phase diagram calculation; Edge-state calculations
  • 观察: Topological superconducting phase occurs over a broad parameter range; Nonzero mirror winding number along the diagonal direction; Majorana Kramers pair at the [11] edge; Bulk Dirac nodes slightly off the diagonal directions; Zero-energy flat bands on the [21] edge
• interpretation_and_experimental_probes — Tunable topological transitions and Majorana boundary modes can provide probes of unconventional pairing in bilayer nickelates.
  • 材料: Topological superconducting state; Bulk nodes; Majorana boundary modes
  • 方法: Discussion of experimental routes; Proposed ARPES, STM, and electric-field tuning probes; Comparison of layer-polarization signatures
  • 观察: Topological phase is experimentally accessible with moderate parameters; Nodal quasiparticles are layer-polarized; STM spectra may be U-shaped or V-shaped depending on layer position; Electric field can transfer spectral weight between layers