Since the discovery of high-temperature superconductivity in cuprates, Josephson junction based phase-sensitive experiments are believed and used to provide the most convincing evidence for determining the pairing symmetry. Regardless of different junction materials and geometries used, quantum tunneling involved in these experiments is essentially a nanoscale process, and thus, actual experimental results are extremely sensitive to atomic details of the junction structures. The situation has led to controversial results as to the nature of the pairing symmetry of cuprates: while in-plane junction experiments generally support $d$-wave pairing symmetry, those based on out-of-plane ($c$-axis) Josephson junctions between two rotated cuprate blocks favor $s$-wave pairing. In this work, we revisit the $c$-axis experiment by fabricating Josephson junctions with atomic-level control in their interface structure. We fabricate over 90 junctions of ultrathin ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ (BSCCO) flakes by state-of-the-art exfoliation technique and obtain atomically flat junction interfaces in the whole junction regions as characterized by high-resolution transmission electron microscopy. Notably, the resultant uniform junctions at various twist angles all exhibit a single tunneling branch behavior, suggesting that only the first half of a unit cell on both sides of the twisted flakes is involved in the Josephson tunneling process. With such well-defined geometry and structure and the characteristic single tunneling branch, we repeatedly observe Josephson tunneling at a nominal twist angle of 45°, which is against the expectation from a purely $d$-wave pairing scenario. Our results strongly favor the scenario of a persistent $s$-wave order parameter in the junction.

中文翻译：

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在由扭曲的超薄 Bi2Sr2CaCu2O8+xFlakes 制成的约瑟夫森结中存在单波配对

自从发现铜酸盐的高温超导性以来，基于约瑟夫森结的相敏实验被认为并用于为确定配对对称性提供最令人信服的证据。无论使用不同的结材料和几何形状，这些实验中涉及的量子隧穿本质上是一个纳米级过程，因此，实际实验结果对结结构的原子细节极为敏感。这种情况导致了关于铜酸盐配对对称性质的有争议的结果：虽然面内结实验通常支持$d$-波配对对称性，那些基于平面外（$C$-axis) 两个旋转的铜酸盐块之间的约瑟夫森结有利于 $秒$-波配对。在这项工作中，我们重新审视$C$-轴实验，通过在界面结构中制造具有原子级控制的约瑟夫森结。我们制造了 90 多个超薄接头${双}_2{锶}_2{\mathrm{钙铜}}_2{\mathrm{哦}}_{8+X}$(BSCCO) 薄片通过最先进的剥离技术在整个结区获得原子平坦的结界面，如高分辨率透射电子显微镜所表征。值得注意的是，在各种扭曲角下得到的均匀结都表现出单一的隧道分支行为，这表明只有扭曲薄片两侧的晶胞的前半部分参与了约瑟夫森隧道过程。凭借如此明确的几何形状和结构以及特征性的单隧道分支，我们反复观察约瑟夫森隧道以 45° 的名义扭曲角，这与纯粹的预期相反。$d$-wave 配对场景。我们的结果强烈支持持续存在的情况$秒$- 交界处的波阶参数。