Mimicry of d-wave Andreev Signatures by Interlayer Pairing

摘要

该文针对双层镍酸盐La₃Ni₂O₇中体态无节点能隙与点接触输运呈d波样特征之间的矛盾，在理论上假定由强层间反铁磁耦合驱动的层间配对为主导配对通道，并在两轨道紧束缚模型中引入层间与层内配对振幅以复现ARPES/STM的多能隙结构。作者指出，当点接触对两层耦合不均匀时，一个在层交换镜面对称下为奇的接触势可充当作用于层赝自旋的有效Zeeman场，从而在势垒区束缚随势垒宽度和强度移动的隙内Andreev态；在镜面本征基中，该势垒混合配对符号相反的镜面扇区并打开符号反转的Andreev通道。对La₃Ni₂O₇顶部层势垒的模拟表明，其亚隙电导可重现通常被当作d波指纹的取向依赖零偏压电导峰及侧峰，说明无节点的层间配对态可模拟d波Andreev信号，为调和相互冲突的实验证据提供了自然图像。

材料

• La₃Ni₂O₇
• bilayer nickelates

方法

• Bogoliubov-de Gennes (BdG) Hamiltonian calculations
• two-orbital tight-binding model
• Blonder-Tinkham-Klapwijk (BTK) formalism
• multichannel BTK conductance calculations
• open-slab calculations
• wave-function matching

关键词

• interlayer pairing
• d wave pairing
• andreev bound states
• zero bias conductance peak (zbcp)
• mirror odd barrier
• layer pseudospin
• effective zeeman field
• mirror selective andreev response
• sign reversal pairing
• nodeless gap
• point contact andreev reflection spectroscopy
• orientation dependent conductance

亮点

• The layer degree of freedom acts as a pseudospin, allowing a nonmagnetic mirror-odd potential to behave as an effective pseudospin Zeeman field.
• Interlayer pairing produces symmetry-enforced opposite-sign gaps in the two mirror sectors residing on closely spaced Fermi surfaces, unlike canonical s± pairing in iron-based superconductors.
• The d-wave-like ZBCP should be tunable through the layer asymmetry of the contact and disappear in the mirror-even limit of equal coupling to the two layers.
• A mirror-even barrier yields a two-gap s-wave conductance spectrum, while a mirror-odd barrier produces a ZBCP along one direction and side dips.
• Uncontrolled variations in barrier parity and junction orientation may account for differences among existing point-contact experiments.

结论

• A nodeless interlayer pairing can naturally mimic d-wave Andreev signatures when the point contact is not evenly coupled to the two layers.
• A contact potential odd under layer-exchange mirror symmetry acts like an effective Zeeman field on the layer pseudospin, binding in-gap Andreev states that sweep through the gap with barrier width and strength.
• In a realistic model for La₃Ni₂O₇, the simulated subgap conductance for a top-layer barrier reproduces the orientation-dependent ZBCP usually interpreted as a fingerprint of d-wave pairing.
• Interlayer pairing can reconcile a nodeless bulk gap with strongly contact-dependent and orientation-dependent conductance spectra in high-T_c bilayer nickelates.
• A ZBCP in point-contact spectroscopy is not by itself definitive evidence for d-wave pairing.

主要论断

• A nodeless interlayer pairing state can mimic d-wave Andreev signatures when the point contact couples unevenly to the two layers.
  • 证据: abstract statement that nodeless interlayer pairing mimics d-wave Andreev signatures for uneven point-contact coupling,minimal-model derivation showing mirror-odd potential acts as effective Zeeman field,conductance calculations showing ZBCP for mirror-odd but not mirror-even barriers
• A contact potential odd under layer-exchange mirror symmetry acts as an effective Zeeman field on the layer pseudospin and binds in-gap Andreev states.
  • 证据: block-diagonal partner-frame Hamiltonian where mirror-odd potential enters with the same sign for electron and hole,round-trip phase condition giving bound-state energy dependent on barrier width and strength,numerical bound-state energies reaching zero in several parameter regimes
• In a realistic La₃Ni₂O₇ model, a top-layer barrier reproduces the orientation-dependent ZBCP and side dips usually taken as a d-wave fingerprint.
  • 证据: open-slab calculations showing sub-gap Andreev branch approaching zero inside the coexistence window,multichannel BTK conductance showing broad ZBCP along x and zero-bias dip along y,side dips below normal-state conductance along x
• A ZBCP in point-contact spectroscopy is not by itself definitive evidence for d-wave pairing in bilayer nickelates.
  • 证据: comparison of simulated mirror-odd and mirror-even barrier conductance with conflicting reported point-contact spectra,consistency of nodeless ARPES/STM gaps with interlayer pairing
• The d-wave-like ZBCP should be tunable through layer asymmetry of the contact and disappear in the mirror-even limit of equal coupling to the two layers.
  • 证据: calculated contrast between mirror-odd (ZBCP) and mirror-even (gapped) barrier conductance

研究流程

• model_construction — A mirror-odd contact potential behaves as an effective Zeeman field on the layer pseudospin, binding in-gap Andreev states.
  • 材料: minimal bilayer model with interlayer coupling and interlayer pairing; two-orbital tight-binding model for La₃Ni₂O₇
  • 方法: BdG Hamiltonian in Nambu basis with layer and particle-hole Pauli matrices; separation of contact potential into mirror-odd and mirror-even components; transformation to mirror (bonding/antibonding) eigenbasis
  • 观察: mirror-odd potential acts as an effective Zeeman field on the layer pseudospin; pairing sign reversal moves into the choice of Andreev partners across layers
• bound_state_calculation — Interlayer pairing binds in-gap Andreev states whose energy depends on barrier width and strength.
  • 材料: bilayer model with potential barrier on last sites
  • 方法: analytic round-trip phase condition for Andreev bound states; SU(2) rotation treatment including interlayer coupling; numerical computation of bound-state energies versus barrier width and strength
  • 观察: in-gap Andreev states sweep through the gap with barrier width and strength; bound-state energy can reach zero in several parameter regimes
• conductance_calculation — A mirror-odd barrier produces resonant Andreev reflection and a ZBCP, while a mirror-even barrier yields a gapped conductance.
  • 材料: normal-metal–barrier–superconductor junction in the minimal model
  • 方法: Blonder-Tinkham-Klapwijk formalism; multichannel BTK conductance formula; Lorentzian resonance analysis
  • 观察: mirror-odd barrier produces a sharp zero-bias conductance peak at resonance; mirror-even barrier leaves conductance fully gapped; finite detuning shifts or splits resonances
• application_to_La3Ni2O7 — Simulated subgap conductance for a top-layer barrier reproduces the orientation-dependent ZBCP and side dips usually interpreted as a d-wave fingerprint.
  • 材料: realistic two-orbital tight-binding model of La₃Ni₂O₇; normal lead with same bands; barrier covering interface sites
  • 方法: open-slab calculation of sub-gap Andreev states; multichannel BTK conductance evaluation; momentum-resolved coexistence-channel analysis
  • 观察: broad ZBCP along one direction and zero-bias dip along the other; side dips below normal-state conductance; mirror-even barrier recovers conventional s-wave BTK line shape; ZBCP survives across a wide parameter regime
• interpretation — Interlayer pairing with a mirror-odd barrier reconciles nodeless bulk gaps with d-wave-like transport signatures in bilayer nickelates.
  • 材料: existing point-contact, ARPES, and STM experimental reports
  • 方法: comparison of simulated conductance with reported spectra
  • 观察: nodeless bulk gaps from ARPES/STM are consistent with interlayer pairing; ZBCP and orientation dependence can arise without d-wave pairing