二维拓扑绝缘体单层WTe2中的约瑟夫森效应

Topological insulator is a newly discovered phase of matter characterized by gapped bulk states and topologically protected conducting edge states, which is a very important breakthrough in recent condensed matter physics research. One of the most striking proposal is to couple superconductors to a topological insulator. The topological superconductivity resulted from the interaction between the superconductivity and the dissipationless edge states is predicted to be able to support Majorana zero modes, quasi-particles obey non-Abelian exchange statistics whose braiding properties have been proposed as the basic blocks of a fault-tolerant quantum computer. Despite the clear theoretical guide, a conclusive experimental realization of topological superconductivity is still of great challenging. Monolayer Tungsten ditelluride (WTe2) in it’s 1T’ phase has been recently discovered theoretically and experimentally as a two-dimensional topological insulator, whose quantized conducting edge states persist even at 100 K. This project will focus on building superconductor-WTe2-superconductor Josephson junctions, study the supercurrent properties and measure the properties of the edge states by superconducting quantum interference. So far, we have successfully demonstrated the finite supercurrent in few-layer WTe2 Josephson junctions. Our next step is fabricating Josephson junctions based on the monolayer WTe2. Owing to the success in the first period, we believe that the project is highly feasible. If the project can get the support and to be continued, it will provide significant experimental support for understanding topological superconductivity and searching for the Majorana zero modes.

拓扑绝缘体是一种全新的量子物态，由不导电的绝缘体态和受拓扑保护的导电边缘态组成，是近年来凝聚态物理的一个重大突破。该领域最重要的课题之一是将超导体与拓扑绝缘体相耦合。理论预测这两种量子物态相互作用可以产生马约拉纳零能模，这种准粒子服从非阿贝尔统计规律，是构建容错拓扑量子计算机的重要基元。尽管有着清晰的理论指导思路，在实验上实现拓扑超导依然极具挑战。最近的研究表明，单层1T’相WTe2是一种二维拓扑绝缘体，其边缘态在100K时依然稳定存在。本项目拟制备单层WTe2约瑟夫森结，研究其超导电流特性，并利用超导量子干涉效应测量单层WTe2中的边缘态特性。目前，我们已经成功地在多层WTe2约瑟夫森结中观察到了超导电流，接下来将集中力量制备单层WTe2约瑟夫森结。基于充分的前期准备，本项目的可行性高。如果本项目成功开展，将对理解拓扑超导和寻找马约拉纳零能模提供十分重要的实验支持。

拓扑绝缘体由具有带隙的绝缘体态和零带隙的导电表面态组成，因其存在奇异的表面态，近年来在科学界引起了极大的关注。单层1T’相WTe2是最新发现的一种二维拓扑绝缘体材料，实验表明单层WTe2中的量子自旋霍尔边缘态在温度高达100K时依然能够稳定存在。此外，和之前的HgTe量子阱等二维拓扑绝缘体不同，单层WTe2是一种能够稳定、独立存在的二维原子晶体，可以与其他材料构建异质结构，从而实现不同功能的量子器件。本项目基于前人的研究，提出构建超导体-WTe2-超导体平面约瑟夫森结的思路，利用超导量子干涉效应测量单层WTe2中的边缘态特性，并探索WTe2约瑟夫森结中拓扑超导和马约拉纳零能模存在的更多证据。项目组围绕“二维拓扑绝缘体单层WTe2中的约瑟夫森效应”开展研究工作，发展了一种在手套箱中制备空气敏感二维量子材料及其异质结构器件的方法，制备了高质量单层WTe2约瑟夫森结；在临界温度以下观测到了10nA级的超导电流，利用超导量子干涉效应测量了不同层数WTe2中超导电流的实空间分布。实验结果表明，在23nm厚的WTe2中，导电通道全部来自于体态的贡献；随着厚度的较小，边缘态的贡献逐渐增加；到1nm（单层）时，WTe2中的导电通道几乎全部来自于边缘态，而体态的贡献几乎为零。本项目研究结果有力地证明了单层WTe2的二维拓扑绝缘体特性，与理论上预测的结论一致。此外，我们通过构建单层WTe2约瑟夫森结，成功实现了超导和量子自旋霍尔边缘态的相互耦合，在这个体系中有望实现拓扑超导，找到马约拉纳零能模（马约拉纳费米子），为最终发展容错拓扑量子计算奠定物理基础。项目组培养了1名博士研究生和3名硕士研究生，在Light: Science & Applications、Nano Letters等国际期刊发表论文6篇，申请发明专利1项，在国际学术会议做报告3次，项目研究成果获湖南省光学进展奖，研究论文被Nature Nanotechnology、Science Bulletin等国际著名期刊论文引用。