最近量子物质科学协同创新中心、量子材料科学中心博士生蒋庆东在导师谢心澄教授指导下与中心孙庆丰教授、刘海文助理研究员、以及苏州大学江华教授合作外尔半金属研究领域取得重要进展，文章在线发表于《物理评论快报》： Topological Imbert-Fedorov Shift in Weyl Semimetals［Physics Review Letters 115,156602］
外尔半金属由于其理论和实验方面的重大进展受到凝聚态和材料领域的广泛关注。外尔半金属有拓扑非平庸的能带结构，在基础物性研究方面具有重要地位。同时，其线性色散关系又使得外尔半金属成为一种相对论性的电子系统，被称为是“三维的石墨烯”，在器件应用方面有巨大的潜在价值。外尔半金属中的低能激发被称作外尔费米子，它是整个费米子家族（狄拉克费米子，外尔费米子，马约拉纳费米子）中的重要一员。最近，科学家在TaAs族化合物中证实了外尔费米子的存在。外尔费米子静质量为零且具有特定的手性， 从而可能具有奇特的物理特性。 蒋庆东等人注意到：尽管统计规律不同， 外尔费米子和光子有许多相似特性（例如: 都具有无质量、手性等特点）。因此，外尔半金属体系中将存在特定的类光学效应。他们的工作主要研究外尔半金属中的 Goos-Hänchen 位移和Imbert-Fedorov 位移。
光学中Goos-Hänchen位移和Imbert-Fedoro位移是指光在界面反射过程中可能会存在的纵向偏移和横向偏移。蒋庆东等人的研究表明外尔费米子在界面反射过程中也存在纵向偏移和横向偏移［图1］。进一步地，他们发现横向位移（Imbert-Fedorov 位移）不仅具有手性依赖（谷依赖）的特点，而且起源于体系的拓扑性质， 即外尔半金属独特的贝里曲率。基于Imbert-Fedorov 位移具有手性依赖的特点，多次界面反射后不同手性的外尔费米子能被空间分离［图2］。这一性质可被利用于:（1）有效表征某个体系是否为外尔半金属；（2）制备谷电子学器件；（3）通过测定Imbert-Fedorov位移得到体系的贝里曲率。 正如审稿人评论说：这项理论研究工作解决了该领域两个较为重要的课题: 如何有效表征外尔半金属和如何测量贝里曲率。
本研究得到国家重大科学研究计划、国家自然科学基金以及量子物质科学协同创新中心等项目经费的资助。
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ICQM members publish an article in Physical Review Letters reporting "The Topological Imbert-Fedorov Shift in Weyl Semimetals
A PhD student Qing-Dong Jiang and his advisor Prof. X.C. Xie of The International Center for Quantum Materials (ICQM), School of Physics, Peking University, published an article in “Physical Review Letters” with the title: The Topological Imbert-Fedorov Shift in Weyl Semimetals.(PRL,115,156602) This work was done in collaboration with research scientist Haiwen Liu, Prof. Qing-Feng Sun from ICQM, and Prof. Hua Jiang from Soochow University.
As a system hosting Weyl fermions, Weyl semimetals have attracted wide attention. The Weyl semimetals have the unique energy dispersion, which promisingly leads to new functionalities and applications. Recently, the Weyl semimetal has been identified by the angle-resolved photoemission spectroscopy (ARPES) experiments. Analogous to photons, the Weyl fermions are massless with a certain chirality. This analogy indicates that many photonic phenomena can find their counterparts in Weyl systems. This work investigates the Goos-Hänchen (GH) and Imbert- Fedorov (IF) shift in Weyl semimetals.
The Goos-Hänchen shift and the Imbert-Fedorov shift are the optical phenomena which describe the longitudinal and transverse lateral shifts at the reflection interface, respectively. Q.-D. Jiang, et al. studied the wave-packet dynamics in Weyl semimetal, and found that the longitudinal and transverse shift can happen at the interface of Weyl semimetal. Furthermore, they demonstrate that the IF shift depends on the chirality, which originates from the topological property, i.e. the Berry curvature of the system. Based on these unique properties, the topological IF shift can be used to:
(1) Identifying a Weyl semimetal; (2) designing valleytronic devices; (3) detecting the Berry curvature of the system.
As commented by one of the referees: this work solved two important problems, i.e., to characterize Weyl semimetal and to detect the Berry curvature.
The work was supported by National Basic Research Programs of China, National Natural Science Foundation of China, and Collaborative Innovation Center of Quantum Matter, China.
