相变，比如水变成冰，正常态变成超导态，通常对应着对称性的改变。1980年整数量子霍尔效应的发现（诺贝尔物理奖）使得拓扑的概念应用到物理相变中。量子霍尔效应所对应的相变并不伴随着对称性的改变，改变了人们对于相变和对称性破缺的看法。近些年，随着拓扑绝缘体的发现和进一步研究，人们对基本物质态的划分有了新的认识：物质材料可以划分为拓扑非平庸和拓扑平庸这两类。最近，三维狄拉克半金属作为一种新的拓扑非平庸材料，由于其能带的导带和价带在狄拉克点接触，并且在三维动量空间中表现出线性色散关系，已成为凝聚态物理领域的重要研究方向之一。三维狄拉克半金属可以看成是石墨烯的三维形式或是无能隙的拓扑半金属态。有趣的是，三维狄拉克半金属作为一种量子材料和物相，处于各种拓扑材料的相临界点。理论上通过改变或调制三维狄拉克半金属的参数，可以使其转变成拓扑绝缘体、外尔半金属、乃至拓扑超导体等其它拓扑物质态。对于拓扑超导体，其体内是有能隙的超导体，表面产生一种被称为Majorana费米子的无能隙态。Majorana费米子是一种理论预测的新奇准粒子。其中，Majorana零能准粒子态满足非阿贝尔统计，可以用于拓扑量子计算，即可容错的量子计算。因此拓扑超导的研究不仅有着重要的基本物理意义，而且有极其重要的应用前景。然而当前在拓扑超导方面的实验研究仍存在不少争议和困难，拓扑超导体和Majorana费米子的发现还没有被实验完全证实。
最近量子物质科学协同创新中心、古天乐代言太阳集团网址量子材料科学中心王健研究员等人在前期三维狄拉克半金属 Cd3As2 单晶电输运研究的基础上（Physical Review X 5, 031037(2015)），与中心危健研究员、贾爽研究员等人合作，在Cd3As2单晶表面用钨针尖做硬点接触实验发现：接触区域变成超导体，同时点接触谱表明其超导特性是非常规的。更重要的是，在超导谱中发现了可能由Majorana费米子引起的零压电导峰。这项工作的理论负责人刘雄军研究员、刘海文博士、谢心澄教授通过理论分析进一步揭示出拓扑超导的可能性。不同于国际上通常采用超导近邻效应探寻拓扑超导或Majorana费米子的方法，硬点接触的实验是通过针尖在三维狄拉克半金属表面微米量级的接触区域产生压力、掺杂等效果，使得处于拓扑材料相临界点的三维狄拉克半金属在接触区域被调制成可能的拓扑超导体。这就为探索拓扑超导提供了新的思路和实验手段，为最终实验证实拓扑超导体和Majorana费米子开辟了新的途径，科学意义非常重大。相关文章于2015年11月2日在线发表于国际顶级学术刊物《自然•材料》上 (Nature Materials (2015) doi:10.1038/nmat4456)：http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4456.html 。北大王健研究员、危健研究员与刘雄军研究员为文章并列通讯作者。北大博士生王贺、王慧超与助理研究员刘海文为文章并列第一作者。
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| 图：a. 在三维狄拉克半金属表面硬点接触区发现超导；b. 点接触谱给出的超导能隙特征，表现出非常规超导的特性；c. 实验观测到的可能由Majorana费米子引起的零压电导峰。 |
上述研究得到国家重大科学研究计划、国家自然科学基金、中组部“青年千人”计划、高等学校博士学科点专项科研基金以及量子物质科学协同创新中心等项目经费的资助。
Nature Materials reports the observation of potential topological superconductivity in 3D Dirac semimetal Cd3As2 crystals
The discoveries of quantum Hall effect and topological insulators broaden the understanding of fundamental states of quantum matter. In topological view, the quantum matter can be divided into topological trivial and non-trivial materials. The three-dimensional (3D) topological Dirac semimetal is a new type of topologically non-trivial materials, in which the conduction and valence band touch only at discrete points and disperse linearly along all (three) momentum directions—a natural 3D counterpart of graphene, as well as a gapless topological semimetal. More interestingly, the 3D Dirac semimetal is on the boundary of various topological materials. It means that by modulation, the 3D Dirac semimetal can be driven into other topological states, such as Weyl semimetal, topological insulator and even topological superconductors. In particular, topological superconductors are superconducting in bulk state but support gapless Majorana fermions in the boundary. In solid state physics, Majorana fermions are new quasiparticles from the theoretical point of view, and it is shown that Majorana zero modes can be applied for topological quantum computation. Thus, topological superconductivity is of great importance in both fundamental science and potential applications. However, so far the experimental demonstrations are still under debate.
Recently, Prof. Jian Wang etc, based on previous transport studies on 3D Dirac semimetal Cd3As2(Physical Review X 5, 031037(2015)), in collaboration with Prof. Jian Wei, Prof. Xiong-Jun Liu, Prof. X. C. Xie and Prof. Shuang Jia at Peking University, discovered superconductivity induced by hard point contact on 3D Dirac semimetal Cd3As2 crystals. The point contact spectroscopy measurement reveals the characteristics of unconventional superconductivity. Furthermore, the zero bias conductance peak is observed, which might originate from Majorana fermions. This work indicates that the 3D Dirac semimetal could be modulated to potential topological superconductor in the contact region by hard point tip or probe. More importantly, the results reveal a new way to detect and study topological superconductivity by using hard tip/point contact on topological non-trivial materials, which is different with the prevailing proximity effect method for creating topological superconductivity or Majorana fermions.
The paper was online published in Nature Materials on November 2, 2015 (Nature Materials (2015) doi:10.1038/nmat4456): http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4456.html. Prof. Jian Wang, Prof. Jian Wei and Prof. Xiong-Jun Liu are corresponding authors of this paper. He Wang, Huichao Wang and Dr. Haiwen Liu contributed equally to this work.
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| Figure: a. The discovery of superconductivity in hard point contact region on the surface of 3D Dirac semimetal Cd3As2 crystal. b. The point contact spectroscopy shows the characteristics of unconventional superconductivity. c. The observed zero bias conductance peak, which might be from Majorana fermions. |
The work was supported by National Basic Research Programs of China, National Natural Science Foundation of China, 1000 Talents Program for Young Scientists of China, the Research Fund for the Doctoral Program of Higher Education (RFDP) of China, and Collaborative Innovation Center of Quantum Matter, China.
