Interlayer hybridization enables superconductivity in bilayer nickelates

摘要

通过稳定双层镍酸盐（La,Pr）₃Ni₂O₇超导薄膜并采用保护覆盖层，结合X射线吸收和共振非弹性X射线散射光谱，本研究直接探测了绝缘体、超导体和金属态下的电子结构演化。实验与理论分析表明，面内dx²-y²态构成巡游电子骨架，而超导电性仅在面外dz²-p_z-dz²层间杂化形成相干时出现，此时静态自旋序被抑制，自旋激发呈现强阻尼特征。氧化学计量和外延应变共同调控该层间通道，使超导态局限于一个较窄的层间相干性与关联强度窗口内。这些结果揭示了双层镍酸盐中超导所需的微观要素，并提供了多轨道图像来描述其涌现机制。

材料

• (La、Pr)₃Ni₂O₇

方法

• X-ray absorption spectroscopy (XAS)
• resonant inelastic X-ray scattering (RIXS)
• density functional theory (DFT) calculations
• ligand-field multiplet theory
• X-ray absorption spectroscopy
• resonant inelastic X-ray scattering
• theoretical analysis

关键词

• interlayer hybridization
• oxygen stoichiometry
• spin density wave
• orbital selective
• spin density waves
• itinerant states
• multiorbital physics

亮点

• The in-plane dx2-y2 states form an itinerant electron backbone.
• The study provides a multiorbital framework to describe the emergent mechanism of superconductivity in bilayer nickelates.
• Overcome limitation by stabilizing superconducting (La,Pr)₃Ni₂O₇ thin films with a protective capping layer, enabling direct spectroscopic access via X-ray absorption and resonant inelastic X-ray scattering.

结论

• Superconductivity emerges only when coherent d_z2-p_z-d_z2 interlayer hybridization develops, accompanied by suppression of static spin order and appearance of strongly damped spin excitations.
• Oxygen stoichiometry and epitaxial strain jointly regulate the interlayer channel, confining the superconducting phase to a narrow window.
• Superconductivity emerges only when coherent dz2-p_z-dz2 interlayer hybridization develops, accompanied by suppressed static spin order and strongly damped spin excitations.
• Oxygen stoichiometry and epitaxial strain both act on this interlayer channel, placing superconductivity within a narrow window of interlayer coherence and correlation strength.

主要论断

• Superconductivity in bilayer nickelates emerges only when coherent interlayer dz2-pz-dz2 hybridization develops, accompanied by suppression of static spin order
  • 证据: XAS shows out-of-plane spectral weight increases; RIXS shows SDW peak strongly suppressed in superconducting samples; calculated XAS confirms oxygen vacancies disrupt hybridization
• The evolution from insulating to superconducting to metallic states in LaPr₂Ni₂O₇ is dominated by changes in the out-of-plane electronic structure, specifically the activation of coherent dz2-pz-dz2 interlayer hybridization.
  • 证据: O K-edge XAS shows spectral weight transfer in out-of-plane component; Ni L₃-edge RIXS shows broadening of orbital excitations; SDW is suppressed.
• Superconductivity emerges only within a narrow window of oxygen stoichiometry, where static SDW order is suppressed while robust spin exchange interactions persist.
  • 证据: Insulating sample has inner apical oxygen vacancies and strong SDW; SC sample has optimal oxygen; metallic sample has interstitial oxygen and is overdoped.
• Oxygen stoichiometry acts as a dual tuning parameter controlling both carrier density and correlation strength.
  • 证据: Vacancies induce localization and magnetism; interstitials drive overdoped metallic state; optimal oxygen yields coherent interlayer hybridization and superconductivity.

研究流程

• Thin film growth and capping — Capping layer preserves superconductivity for spectroscopy
  • 材料: (La,Pr)₃Ni₂O₇ films on SLAO with amorphous PBCO cap
  • 方法: PLD; E_x situ capping
  • 观察: Transport confirms superconductivity preserved
• X-ray absorption and RIXS measurements — Interlayer dz2-pz-dz2 hybridization is key for superconductivity
  • 材料: XAS at O K-edge and Ni L-edge; RIXS at Ni L-edge
  • 方法: Polarization-dependent XAS; RIXS with 85 meV resolution
  • 观察: In-plane and out-of-plane spectral evolution; SDW peak suppression; Orbital excitation broadening
• Thin Film Fabrication and Capping — Protective capping layer preserves superconductivity during XAS/RIXS measurements.
  • 材料: LaPr₂Ni₂O₇; SrLaAlO₄ substrate; amorphous PrBa₂Cu₃O₇ capping
  • 方法: pulsed laser deposition (PLD); ex situ capping
  • 观察: films with thickness 7-14 nm; insulating, superconducting, and metallic samples
• X-ray Absorption Spectroscopy (XAS) — O K-edge XAS reveals progressive hole doping and reconstruction of dz2-derived states.
  • 材料: LaPr₂Ni₂O₇ films with PBCO cap
  • 方法: O K-edge and Ni L₃-edge XAS in TFY and TEY modes
  • 观察: In-plane XAS shows systematic peak shift to lower energy from insulator to superconductor to metal; out-of-plane XAS shows two components, with spectral weight transfer
• Resonant Inelastic X-ray Scattering (RIXS) — Enhanced itinerancy in SC and metallic samples; magnon damping increases.
  • 材料: LaPr₂Ni₂O₇ films
  • 方法: RIXS at Ni L₃ edge with π polarization; scattering angles 90° and 150°
  • 观察: Orbital excitations at 0.4, 1.0, 1.5 eV broaden in SC and metallic samples; magnetic excitation at ≈70 meV broadens
• Spin Density Wave (SDW) Measurements — SDW order is suppressed in the superconducting state.
  • 材料: LaPr₂Ni₂O₇ films
  • 方法: quasi-elastic RIXS intensity along (H,H) direction
  • 观察: Pronounced SDW peak near (0.25,0.25) in insulating sample; strongly suppressed in SC sample
• DFT Calculations of O K-edge XAS — Inner apical oxygen vacancies in insulating sample disrupt interlayer hybridization.
  • 材料: Pr₃Ni₂O₇, Pr₃Ni₂O₆.75, Pr₃Ni₂O₇.25
  • 方法: VASP with PBE+U (U=4 eV on Ni); core-hole approach
  • 观察: Calculated spectra reproduce overall energy shifts; high-energy out-of-plane feature missing in DFT