相變，比如水變成冰，正常态變成超導态，通常對應着對稱性的改變。1980年整數量子霍爾效應的發現（諾貝爾物理獎）使得拓撲的概念應用到物理相變中。量子霍爾效應所對應的相變并不伴随着對稱性的改變，改變了人們對于相變和對稱性破缺的看法。近些年，随着拓撲絕緣體的發現和進一步研究，人們對基本物質态的劃分有了新的認識：物質材料可以劃分為拓撲非平庸和拓撲平庸這兩類。最近，三維狄拉克半金屬作為一種新的拓撲非平庸材料，由于其能帶的導帶和價帶在狄拉克點接觸，并且在三維動量空間中表現出線性色散關系，已成為凝聚态物理領域的重要研究方向之一。三維狄拉克半金屬可以看成是石墨烯的三維形式或是無能隙的拓撲半金屬态。有趣的是，三維狄拉克半金屬作為一種量子材料和物相，處于各種拓撲材料的相臨界點。理論上通過改變或調制三維狄拉克半金屬的參數，可以使其轉變成拓撲絕緣體、外爾半金屬、乃至拓撲超導體等其它拓撲物質态。對于拓撲超導體，其體内是有能隙的超導體，表面産生一種被稱為Majorana費米子的無能隙态。Majorana費米子是一種理論預測的新奇準粒子。其中，Majorana零能準粒子态滿足非阿貝爾統計，可以用于拓撲量子計算，即可容錯的量子計算。因此拓撲超導的研究不僅有着重要的基本物理意義，而且有極其重要的應用前景。然而當前在拓撲超導方面的實驗研究仍存在不少争議和困難，拓撲超導體和Majorana費米子的發現還沒有被實驗完全證實。

最近量子物質科學協同創新中心、beat365官方网站量子材料科學中心王健研究員等人在前期三維狄拉克半金屬 Cd3As2 單晶電輸運研究的基礎上（Physical Review X 5, 031037(2015)），與中心危健研究員、賈爽研究員等人合作，在Cd3As2單晶表面用鎢針尖做硬點接觸實驗發現：接觸區域變成超導體，同時點接觸譜表明其超導特性是非常規的。更重要的是，在超導譜中發現了可能由Majorana費米子引起的零壓電導峰。這項工作的理論負責人劉雄軍研究員、劉海文博士、謝心澄教授通過理論分析進一步揭示出拓撲超導的可能性。不同于國際上通常采用超導近鄰效應探尋拓撲超導或Majorana費米子的方法，硬點接觸的實驗是通過針尖在三維狄拉克半金屬表面微米量級的接觸區域産生壓力、摻雜等效果，使得處于拓撲材料相臨界點的三維狄拉克半金屬在接觸區域被調制成可能的拓撲超導體。這就為探索拓撲超導提供了新的思路和實驗手段，為最終實驗證實拓撲超導體和Majorana費米子開辟了新的途徑，科學意義非常重大。相關文章于2015年11月2日在線發表于國際頂級學術刊物《自然•材料》上 (Nature Materials (2015) doi:10.1038/nmat4456)：http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4456.html 。北大王健研究員、危健研究員與劉雄軍研究員為文章并列通訊作者。北大博士生王賀、王慧超與助理研究員劉海文為文章并列第一作者。

上述研究得到國家重大科學研究計劃、國家自然科學基金、中組部“青年千人”計劃、高等學校博士學科點專項科研基金以及量子物質科學協同創新中心等項目經費的資助。

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.

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.