Atomically resolved intrinsic superconducting gap in (La,Pr)₃Ni₂O₇ films

摘要

本研究利用原子分辨率扫描隧道显微镜和谱学技术，对生长在SrLaAlO₄上的1.5个单胞(La,Pr)₃Ni₂O₇超薄膜进行了表征。通过低温超高真空样品转移，保持了有序的√2×√2表面重构，并在隧道谱中观测到具有两个能隙尺度（约14和20 meV）且零偏压电导平坦的U形谱，表明超导能隙无节点。相比之下，若转移过程中样品在超高真空中暴露时间较长且未冷却，虽仍保持表面重构和40 K以上的输运超导转变起始温度，但隧道谱呈现V形，且宽能谱显示氧缺失会使密度波相关的谱权重混合。通过对比不同转移时间的样品，确定了获得本征超导能隙需控制氧含量，提供了双层镍酸盐超薄膜本征无节点超导能隙的原子尺度观测证据。

材料

• (La、Pr)₃Ni₂O₇

方法

• Scanning tunneling microscopy/spectroscopy (STM/STS)
• Gigantic-oxidative atomic-layer-by-layer epitaxy (GAE)
• Transport measurements

关键词

• u shaped gap
• nodeless superconducting gap
• double gap structure
• oxygen stoichiometry
• surface reconstruction
• √2×√2

亮点

• Provides atomic-scale visualization of the intrinsic nodeless superconducting gap in bilayer nickelate ultrathin films.
• Transfer-time comparison serves as a built-in control experiment, identifying oxygen content as critical for the local gap line shape.

结论

• Atomically resolved STM/STS on oxygen-sufficient films reveals homogeneous U-shaped double-gap spectra with ≈14 and ≈20 meV gaps and flat zero-conductance bottoms, indicating a nodeless superconducting gap.
• Oxygen loss during transfer (longer UHV exposure) leads to V-shaped spectra even when surface reconstruction and transport T_c remain, mixing density-wave-related spectral weight.
• Oxygen-controlled transfer is a prerequisite for accessing the intrinsic superconducting gap in bilayer nickelate films.

主要论断

• Rapid cryogenic UHV transfer preserves an ordered √2×√2 surface and yields reproducible U-shaped spectra with two gap scales (≈14 and ≈20 meV) and extended flat zero-conductance bottoms, indicating a nodeless superconducting gap.
  • 证据: STM shows ordered reconstruction; STS shows U-shaped double-gap with strongly suppressed zero-bias conductance
• Longer UHV transfer times (without cooling) produce V-shaped spectra despite preserved surface reconstruction and transport T_c > 40 K, indicating oxygen loss degrades the local gap.
  • 证据: Comparison of transfer times: <5 min gives U-shaped; >10 min gives V-shaped; wide-energy spectra show density-wave-related dip in slow-transfer samples
• Oxygen loss can mix density-wave-related spectral weight, so the local superconducting gap line shape is more sensitive to oxygen content than transport or surface reconstruction.
  • 证据: V-shaped spectra show broad dip at density-wave energy scales; non-superconducting bulk spectrum shows similar gap-like suppression

研究流程

• thin_film_growth — High-quality ultrathin films with ordered surface.
  • 材料: (La,Pr)₃Ni₂O₇; SrLaAlO₄ substrate
  • 方法: gigantic-oxidative atomic-layer-by-layer epitaxy
  • 观察: 1.5 unit-cell thick films with √2×√2 surface reconstruction
• surface_characterization — Atomically resolved surface with ordered reconstruction.
  • 材料: samples transferred under UHV cryogenic conditions
  • 方法: STM imaging
  • 观察: atomically flat terraces; √2×√2 reconstruction confirmed
• sts_spectroscopy — Intrinsic nodeless superconducting gap with two gap scales (≈14 and ≈20 meV).
  • 材料: fresh and oxygen-exposed surfaces
  • 方法: scanning tunneling spectroscopy with lock-in technique
  • 观察: U-shaped double-gap spectra with zero residual conductance for fast transfer; V-shaped spectra for slow transfer