Prominent Josephson tunneling between twisted single copper oxide planes of Bi2Sr2-xLaxCuO6+y

Abstract

Josephson tunneling in twisted cuprate junctions provides a litmus test for the pairing symmetry, which is fundamental for understanding the microscopic mechanism of high temperature superconductivity. This issue is rekindled by experimental advances in van der Waals stacking and the proposal of an emergent d+id-wave. So far, all experiments have been carried out on Bi2Sr2CaCu2O8+x (Bi-2212) with double CuO2 planes but show controversial results. Here, we investigate junctions made of Bi2Sr2-xLaxCuO6+y (Bi-2201) with single CuO2 planes. Our on-site cold stacking technique ensures uncompromised crystalline quality and stoichiometry at the interface. Junctions with carefully calibrated twist angles around 45° show strong Josephson tunneling and conventional temperature dependence. Furthermore, we observe standard Fraunhofer diffraction patterns and integer Fiske steps in a junction with a twist angle of 45.0±0.2°. Together, these results pose strong constraints on the d or d+id-wave pairing and suggest an indispensable isotropic pairing component.

Josephson effect in twisted cuprates

“…底下一层真正超导 [27] . 2019年, 复旦大学张远波研 究团队 [28] 报道了在半个原胞厚度的铋锶钙铜氧薄 膜中的测量结果, 发现高温超导的诸多特性都可以 氧氛围进行退火 [9] , 将氧原子重新掺杂到相对旋转 的近邻的两片铜氧层中. 另一方案则是将堆叠过程 在零下数十度的低温下进行 [10][11][12] .…”

“…这说明仅仅观测到约瑟夫森 二极管效应也不能作为d+id波配对存在的证据. 图 3 两个研究团队利用室温堆叠后氧退火(I)和低温堆叠(II)两种方法制备出的样品在一系列转角下的约瑟夫森耦合强度 (a) 清华研究团队利用方法I得到的铋锶钙铜氧欠掺杂区间实验数据 [9] ; (b) 哈佛研究团队利用方法II得到的铋锶钙铜氧最佳掺 杂区间实验数据 [12] ; (c), (d) 清华研究团队利用方法II得到的铋锶钙铜氧最佳掺杂区间、过掺杂区间实验数据 [11] 和铋锶镧铜氧 最佳掺杂区间的实验数据 [10] Fig. 3.…”

“…3. Josephson coupling strength as a function of twist angle from two research groups using two methods of room temperature stacking with oxygen post-annealing (I) and cryogenic stacking (II): (a) Data of underdoped Bi-2212 from the research group in Tsinghua University by using method I [9] ; (b) data of optimally doped Bi-2212 from the research group in Harvard University by using method II [12] ; (c), (d) data of optimally doped Bi-2212, overdoped Bi-2212 [11] , and optimally doped Bi-2201 [10] from the research group in Tsinghua University by using method II. 本征约瑟夫森结的宏观量子相干温度在1 K附近 [30] .…”

“…除了配对波函数振幅部分外, 对于相位部分的 探测也并非全部支持d波变号. 这一挑战主流观点 的实验 [6][7][8][9][10][11] 拼接而成的毫米尺度单晶 [6] 、人工堆叠 [7] 或天然交错 [8] 的微米厚度晶须构建了转角铜氧化物双晶, 实验结 果不符合d波预期: 45°转角时的约瑟夫森电流甚 至可以与0°转角时同样大. 然而, 这些实验中界面 处的结构未得到全面的解析, 大尺寸单晶还存在在 隧穿过程中严重发热的问题.…”

Two-Dimensional van der Waals Superconductor Heterostructures: Josephson Junctions and Beyond

“… The recent progress in vdW heterostructure technology has facilitated this achievement, featuring an atomically flat vdW interface that remains free of oxidation and interdiffusion during junction preparation. The epitaxially precise vdW interfaces contribute to extraordinary crystalline quality, versatile electronic properties, and flexible assembly significantly improve the quality and enrich the variety of 2D vdW JJs. ,− With different combinations, JJs such as SC/vdW gap/SC, SC/topological semimetal/SC, and SC/FM/SC, ,, have been employed in fields ranging from digital electronics to quantum electronics (Figure ).…”