最近量子物質科學協同創新中心、量子材料科學中心博士生蔣慶東在導師謝心澄教授指導下與中心孫慶豐教授、劉海文助理研究員、以及蘇州大學江華教授合作外爾半金屬研究領域取得重要進展，文章在線發表于《物理評論快報》： Topological Imbert-Fedorov Shift in Weyl Semimetals［Physics Review Letters 115,156602］
外爾半金屬由于其理論和實驗方面的重大進展受到凝聚态和材料領域的廣泛關注。外爾半金屬有拓撲非平庸的能帶結構，在基礎物性研究方面具有重要地位。同時，其線性色散關系又使得外爾半金屬成為一種相對論性的電子系統，被稱為是“三維的石墨烯”，在器件應用方面有巨大的潛在價值。外爾半金屬中的低能激發被稱作外爾費米子，它是整個費米子家族（狄拉克費米子，外爾費米子，馬約拉納費米子）中的重要一員。最近，科學家在TaAs族化合物中證實了外爾費米子的存在。外爾費米子靜質量為零且具有特定的手性， 從而可能具有奇特的物理特性。 蔣慶東等人注意到：盡管統計規律不同， 外爾費米子和光子有許多相似特性（例如: 都具有無質量、手性等特點）。因此，外爾半金屬體系中将存在特定的類光學效應。他們的工作主要研究外爾半金屬中的 Goos-Hänchen 位移和Imbert-Fedorov 位移。
光學中Goos-Hänchen位移和Imbert-Fedoro位移是指光在界面反射過程中可能會存在的縱向偏移和橫向偏移。蔣慶東等人的研究表明外爾費米子在界面反射過程中也存在縱向偏移和橫向偏移［圖1］。進一步地，他們發現橫向位移（Imbert-Fedorov 位移）不僅具有手性依賴（谷依賴）的特點，而且起源于體系的拓撲性質， 即外爾半金屬獨特的貝裡曲率。基于Imbert-Fedorov 位移具有手性依賴的特點，多次界面反射後不同手性的外爾費米子能被空間分離［圖2］。這一性質可被利用于:（1）有效表征某個體系是否為外爾半金屬；（2）制備谷電子學器件；（3）通過測定Imbert-Fedorov位移得到體系的貝裡曲率。 正如審稿人評論說：這項理論研究工作解決了該領域兩個較為重要的課題: 如何有效表征外爾半金屬和如何測量貝裡曲率。
本研究得到國家重大科學研究計劃、國家自然科學基金以及量子物質科學協同創新中心等項目經費的資助。
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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.
