拓扑半金属的红外光谱研究

Optical properties of topological semimetals

拓扑半金属是一类全新的拓扑电子态,展现出丰富而有趣的物理性质.这类材料不仅在未来电子器件方面具有潜在的应用价值,同时也是目前量子材料领域研究的热点和前沿.根据在三维动量空间中能带结构特点的不同,拓扑半金属可以分为Dirac半金属、Weyl金属和Nodal-line半金属等.人们已经利用各种实验技术手段对这些材料的物理性质进行了系统的研究.例如:角分辨光电子能谱可以直接观测到Weyl半金属表面态上连接两个具有相反手性的Weyl费米子的费米弧;软X射线的角分辨光电子能谱还可以直接观测到这些拓扑半金属体态中的Dirac点,Weyl点以及Nodal-line;电输运测量验证了拓扑半金属中由手性反常导致的负磁阻现象;圆偏振光产生光电流的实验证明了Weyl半金属TaAs中存在相反手性的Weyl费米子;此外,人们还发现Weyl半金属具有非常强的非线性效应,主要表现为很强的二次谐波产生和THz发射效应等.红外光谱是一种体敏感的实验手段.该手段不仅覆盖很宽的能量范围(meV到几个eV),而且具有很高的能量分辨率(最高可达几十个μeV量级),因此适合研究拓扑半金属体态的电子结构以及晶格振动行为.研究拓扑半金属材料的红外光谱学性质不仅可以帮助人们更深入理解这类材料的物理性质以及发现新的物理现象,还可以为拓扑半金属在光学领域的研究和应用奠定基础.本文介绍了近年来上面提到的几类拓扑半金属材料的红外光谱研究的进展情况.

Topological semimetal represents a novel quantum phase of matter,which exhibits a variety of fascinating quantum phenomena.This class of materials not only have potential applications in electronic devices,but also represent one of the hottest topics in the field of quantum materials.According to the band structure of these materials in the three-dimensional momentum space,topological semimetals can be classified into Dirac semimetals,Weyl semimetals and nodal-line semimetals.Extensive studies on these materials have been conducted using various techniques.For example,angle-resolved photoemission spectroscopy(ARPES)has directly observed the Fermi arc that connects two Weyl points with opposite chiralities in the surface states of Weyl semimetals;the Dirac points,Weyl points as well as the Dirac nodal line in the bulk states have also been revealed by soft X-ray ARPES;the observation of negative magnetoresistance in transport measurements has been taken as the evidence for the chiral anomaly in Weyl and Dirac semimetals;the chirality of the Weyl fermions have been detected by measuring the photocurrent in response of circularly polarized light;in addition,strong second harmonic generation and THz emission have been observed,indicating strong non-linear effects of Weyl semimetals.Infrared spectroscopy is a bulk-sensitive technique,which not only covers a very broad energy range(meV to several eV),but also has very high energy resolution(dozens of μeV).Investigations into the optical response of these materials not only help understand the physics of the topological phase and explore novel quantum phenomena,but also pave the way for future studies and applications in optics.In this article,we introduce the optical studies on several topological semimetals,including Dirac,Weyl and nodal-line semimetals.