Source capture
Authors Xinxin Wang, Yaqi Chen, Cui Ding, Lizhi Xu, Jian-Jian Miao, Guangdi Zhou, Zhuoyu Chen, Yu-Jie Sun, Jin-Feng Jia, Qi-Kun Xue
arXiv / Publisher https://arxiv.org/abs/2605.14806
PDF https://arxiv.org/pdf/2605.14806
Relevance score 5.843
Primary category cond-mat.supr-con
Published 2026-05-15
Research paradigm Experimental
Sample form Thin Film

Summary

This study employs atomic-resolution scanning tunneling microscopy and spectroscopy to characterize 1.5 unit-cell-thick (La,Pr)3Ni2O7 ultrathin films grown on SrLaAlO4. Through low-temperature ultrahigh vacuum sample transfer, an ordered √2×√2 surface reconstruction is preserved, and a U-shaped spectrum with two gap scales (approximately 14 and 20 meV) and a flat zero-bias conductance is observed in the tunneling spectra, indicating a nodeless superconducting gap. In contrast, if the sample is exposed to ultrahigh vacuum for a longer time during transfer without cooling, although the surface reconstruction and a transport superconducting onset temperature above 40 K are maintained, the tunneling spectrum becomes V-shaped, and the wide-energy spectrum shows that oxygen deficiency mixes spectral weight related to density waves. By comparing samples with different transfer times, it is determined that controlling the oxygen content is necessary to obtain an intrinsic superconducting gap, providing atomic-scale observational evidence for the intrinsic nodeless superconducting gap in bilayer nickelate ultrathin films.

Materials

Methods

Keywords

Highlights

  • 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.

Conclusions

  • 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 Tc remain, mixing density-wave-related spectral weight.
  • Oxygen-controlled transfer is a prerequisite for accessing the intrinsic superconducting gap in bilayer nickelate films.

Main claims

  • 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.
    • Evidence: 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.
    • Evidence: 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.
    • Evidence: V-shaped spectra show broad dip at density-wave energy scales; non-superconducting bulk spectrum shows similar gap-like suppression

Workflow

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