外尔半金属材料探索

Topological quantum state can be classified as topological semimetal and topological insulator. The former is the classifcation of the band structure of metalic system, and the later is for insulators. Distinguished from normal metal, topological semimetal has its Fermi surface consisting of and only of band-crossing points. These crossing points can be looked as the magnetic monopoles in the crystal momentum space of solids. Therefore, the topological invariant to identify the topologicla semimetal is the Fermi Chern number. According to the degeneracy and the distribution of magnetic monopoles, topological semimetal can have three members, namely Dirac semimetal, Weyl semimetal and Node-Line semimetal. Among them the Weyl semimetal is the most topologically robust and has characteristic Fermi arcs and specific magneto-transport properties due to chiral anomaly. The applicant and his group has predicted the TaAs family to be the Weyl semimetal conserving time-reversal invariant symmetry and this prediction has been experimentally confirmed. This proposal is aiming to searching for more advanced Weyl semimetal materials, which will give the experimental studies both in theoretical guidness and material platform. Thus, this will advance the field of topological semimetal greatly.

拓扑量子态可以大致分为拓扑半金属和拓扑绝缘体。前者是对金属体系的能带进行的拓扑分类，而后者是针对绝缘体。不同于普通金属，拓扑半金属的费米面有且只有能带交叉点构成，这些能带交叉点其实是晶格动量空间的磁单极子。因此表征拓扑半金属的拓扑不变量,费米陈数,就是费米面包裹的净磁荷个数。根据磁单极子在晶格动量空间的分布和简并，拓扑半金属可以分为Dirac半金属，外尔(Weyl)半金属和Node-Line半金属这三类，其中以外尔(Weyl)半金属最为拓扑稳定，且具有特征性的表面费米弧和手性反常导致的特殊磁疏运性质。申请人及其团队理论预言TaAs家族是保持时间反演而破缺中心对称的Weyl半金属，并获得实验证实。本项目拟进一步探索更多、更好的Weyl半金属材料,为实验提供理论指导和物质基础,推动拓扑半金属领域的进展。

本项目主要进行外尔半金属以及含有新型费米子激发的拓扑半金属材料探索，预定的研究任务都顺利完成。取得的主要研究成果有：.1、进一步完善和增强自主开发的拓扑物性计算方法，基于对称性和能带表示理论对拓扑物态进行分类和拓扑不变量计算，大大加快了拓扑材料的搜索效率，建立了世界上首个非磁性拓扑材料数据库；.2、理论预言碳化钨家族材料中可能存在三重简并费米子态，及其与外尔费米子态的共存与调控；.3、与实验小组密切合作，合成、制备了高质量MoP，WC和ZrTe等单晶样品，对它们的电子能带结构，输运性质等进行了系统研究；.4、发展了强关联体系计算方法，应用LDA+Gutzwiller方法，理论预言了首个关联拓扑外尔半金属材料CeRu4Sn6；.5、发现了首个铁磁性外尔半金属Co3Sn2S2，并研究了其红外光谱，自旋波谱随温度的演化。.在该项目的支持下，总共发表了SCI论文20余篇，包括Nature 3篇，Nature子刊 3篇，Science子刊 3篇，Phys. Rev. Lett.或Phys. Rev. X 6篇等高端论文。

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