三维拓扑半金属的红外光谱研究

The discovery of Weyl femions in the Weyl semimetal TaAs has triggered enormous interest in studying three dimensional (3D) topological semimetals, which not only harbor new quantum states of matter but also provide a good platform for many intriguing physical phenomena. In the meantime, 3D topological semimetals also have broad application prospect. For example, their unique quantum properties are expected to play a very important role in the breakthrough of topological electronics and quantum computer techniques. However, the optical spectroscopic investigations on the 3D bulk properties of these materials are still lacking. There are many theoretical predictions needed to be verified by experiments. Particularly, it is proposed by theoretical calculations that 3D topological semimetals will act quite different from traditional metals and insulators, which requires further explorations.. Based on the above background, we are planning to investigate topological semimetals by utilizing Fourier transform infrared spectroscopy in combination with applied magnetic field: detecting the charge dynamics of various 3D topological semimetals; verifying and exploring their exotic behaviors under magnetic field; trying to manipulate the topological phase by magnetic field. Through the research of this project, we hope to enhance people’s knowledge on the bulk properties of topological semimetal, inspect some theoretical predictions, explore the yet hidden novel physical properties, and provide experimental fundations for their application prospect.

近两年来，随着外尔费米子在外尔半金属TaAs中的发现，凝聚态物理领域掀起了三维拓扑半金属的研究热潮。这类材料不仅蕴含着全新的量子物态，为很多新奇物理现象的实现提供了平台，同时也具有广阔的应用前景，比如它们所特有的一些量子属性对拓扑电子学和量子计算机技术的突破具有非常重要的意义。然而，针对这类材料內禀的三维体性质方面的光学研究还非常缺乏，尚有大量的理论预言有待验证；特别是在磁场下预期会表现出与传统金属和绝缘体截然不同的行为，有待进一步的探索。. 根据上述背景，本项目主要采用傅里叶变换红外光谱与外加磁场的结合对新奇的拓扑半金属开展研究：探测各种三维拓扑材料的电荷动力学响应；验证和探索它们在磁场下的奇异行为；并尝试用磁场对各种拓扑相进行调控。通过本项目的研究，希望能够加强人们对拓扑半金属体态性质的认识，验证理论上的预言，探索该类材料中蕴藏的新奇物性，并为其应用前景提供实验基础。

三维拓扑半金属由于具有独特的输运性质从而在低功耗器件方面表现出巨大的应用前景，而其奇异的拓扑非平庸量子态也吸引了大量科研人员的关注。本项目利用红外光谱的实验手段对几种不同的三维拓扑半金属材料的电荷动力学行为进行了研究，一方面对这些体材料中三维狄拉克、外尔费米子的存在提供了谱学上的证据，可以作为表面敏感实验手段的补充和验证，另一方面还发现了一些新奇的电荷动力学行为。主要成果如下：1.发现线节点半金属材料PbTaSe2在100meV附近出现了一个随温度降低而发生略微红移的带间跃迁峰，同时在730meV和1.3eV附近的两个高能带间跃迁峰则随温度降低发生蓝移，这些罕见现象的起源机制还在分析过程中；2.发现完美补偿的半金属材料PtBi2的反射率谱在40meV之下出现了非常尖锐的等离子体边，且随温度降低明显向低能移动，反映了其极低的载流子浓度以及费米能级随温度的变化；3.发现重费米子体材料CeAgSb2的光诱导反射率幅值在45K左右出现了一个弯折，并推断该弯折起源于近藤效应中参与杂化轨道的crossover效应。