转角铜氧化物中的约瑟夫森效应

Josephson effect in twisted cuprates

当前常压下超导转变温度最高的材料仍然来自铜氧化物家族.然而,铜氧化物超导的微观机理仍未被完全建立起来,成为了凝聚态物理领域最具挑战性的问题之一.测定配对波函数的相位部分是全面理解高温超导机理不可或缺的一环.该实验往往需要将不同晶向的铜氧化物拼接成高质量的约瑟夫森结,十分考验样品的合成制备技术.近年来,利用二维材料中发展起来的范德瓦耳斯堆垛技术,研究者们构建了具有原子级平整界面的转角铜氧化物双晶结,研究了不同掺杂浓度、不同转角下的约瑟夫森隧穿,探索了其中出现s波、d波、以及由于界面耦合演生出的d+id波配对的可能性.本文将回顾转角铜氧化物约瑟夫森结的研究进展,介绍近年来发展起来的转角结制备技术,讨论当前实验测量的结果及其意义,提出尚待解决的关键性问题.

To tunnel,or not to tunnel,that is the question for a Josephson junction constructed by superconductors with unidentified pairing symmetry.Theoretically,Josephson tunneling is forbidden between two d-wave superconductors twisted by 45°.This is in sharp contrast to persistent tunneling between two s-wave superconductors.Experimentally,however,Josephson tunneling is observed in twisted bicrystalline cuprates at around 45°,against the expectation that cuprate superconductors possess d-wave pairing.Due to technical uncertainties,the early studies on twisted bulk cuprates were not widely recognized.The recent advent of van der Waals stacking has allowed a fresh look at this problem.Indeed,twisted thin flakes of cuprates have been realized and the corresponding pairing symmetry has been revisited both experimentally and theoretically.In this work,we overview the recent development on twisted cuprates.After summarizing the theoretical treatment and recent proposals,we introduce the technical progress of making the twisted cuprate junctions in van der Waal stacking,and discuss the recent experimental results of s-,d-,or d+id-wave pairing.In the end,we propose possible directions for future exploration in this field.This paper has three major sections:theories on twisted cuprates in Section 1,techniques of realizing twisted cuprates in Section 2,and experimental results on twisted cuprates in Section 3.Specifically,in Section 1,both the early theory and the latest theoretical proposals are introduced.After discussing the calculated angular dependence of Josephson tunneling between two d-wave or s-wave superconductors,we summarize the predicted features from the emergent d+id-wave pairing.They include unconventional temperature dependence of the critical Josephson current,doubling in frequency of the Fraunhofer pattern or Shapiro steps,and spontaneous Kerr rotation or emergence of Josephson diode effect.In Section 2,the technological progress of van der Waals stacking of cuprate superconductors is presented.Ultrathin twisted Josephson junctions of cuprates can be realized by either dry stacking together with oxygen post-annealing or cryogenic stacking at tens of degrees below 0°C.In Section 3,the recent experimental results on van der Waals stacked twisted cuprates are reviewed.Tunneling in twisted underdoped cuprates realized by post-annealing indicates the existence of s-wave pairing and strong deviation from pure d-wave pairing.This result is contrasted with another study on cryogenically stacked junctions.There,signatures of d+id-wave pairing,such as fractional Shapiro steps,are reported.Still,our recent experiments on 45°-twisted junctions with ultraclean interfaces,which are also realized by cryogenic stacking,show standard Fraunhofer patterns and AC Josephson effect with only integer steps,indicating the absence of d+id-wave pairing.These results have far-reaching influence on understanding the pairing symmetry of twisted cuprates.Future efforts to study the twisted cuprates may include:extending to a wider pool of materials,pushing the thickness to the atomic limit,and adopting other characterization tools.The twisted cuprates may also find applications in high temperature superconducting quantum bit as well as Josephson diodes.