Probing Chiral Fermion Dynamics in Topological Semimetals

非技术摘要：拓扑半金属化是凝聚态物理学的一个重要前沿领域。在这些材料中，电子能级的交叉点受到拓扑保护。据预测，这些交叉点周围的电子应该会表现出令人着迷的非常规行为，以响应外部扰动，如通过电流或闪光。本项目的目标是从实验上研究拓扑半金属的光学性质。在拓扑半金属中实现新的光学和光电子现象可能会导致有用的技术应用。在拓扑物理和时间分辨光学技术方面都有专长的学生和博士后将被培训为这些领域的下一代科学家。该计划的教育部分的一个关键部分是为提高K-12数学和科学教育的质量做出贡献，特别是在受社会经济影响的地区。技术摘要：Weyl半金属是一种新的物质拓扑相，预计将显示出迷人的性质。尽管用角度分辨光电子能谱、扫描隧道显微镜和电输运等手段系统地研究了这些材料的电子能带结构和输运，但这些材料的光学性质仍在很大程度上没有被探索，尽管人们预测了许多非传统的光学行为。本研究的目标是研究拓扑半金属中手性Weyl费米子的新的光学性质。特别是，有人预测，在平行电场和磁场存在的情况下，具有不同手性的Weyl费米子应该混合在一起，并产生与磁场有关的电导率。仅仅基于输运测量，很难最终观察到这种被称为手征反常的现象。在这个项目中，将使用时间分辨光电流显微镜、瞬变光栅光谱和太赫兹光谱来光学检测Weyl半金属中手性反常的特征。此外，Weyl半金属的光电流响应将被用来绘制其电子波函数的量子几何图形。这将允许检测从Weyl节点产生的浆果曲率单极子和偶极子。在拓扑半金属中实现新的光学和光电现象可能使我们能够通过光学手段控制Weyl费米子及其相关的量子反常，从而导致有用的光学和光电应用。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical abstract:Topological semimetals have recently emerged as an important frontier of condensed matter physics. In these materials, the crossing points of electronic energy levels are topologically protected. It has been predicted that the electrons around these crossing points should display fascinating unconventional behavior in response to external perturbations such as passing an electrical current or shining light. The goal of this project is to experimentally investigate optical properties of topological semimetals. The realization of novel optical and optoelectronic phenomena in topological semimetals may lead to useful technological applications. Students and postdocs with expertise both in topological physics and time resolved optical techniques will be trained as the next generation of scientists in these fields. A crucial part of the educational component of this program is to contribute to improving the quality of K-12 math and science education especially in socio-economically impacted regions.Technical abstract:Weyl semimetal is a new topological phase of matter predicted to display fascinating properties. While the electronic band structure and transport have been systematically studied using angle resolved photoemission spectroscopy, scanning tunneling microscopy and electrical transport, optical properties of these materials remain largely unexplored despite predictions of many unconventional optical behaviors. The goal of this research program is to investigate the novel optical properties of chiral Weyl fermions in topological semimetals. In particular, it has been predicted that, in the presence of parallel electric and magnetic fields, Weyl fermions with different chiralities should intermix and give rise to a magnetic field dependent conductivity. It has been difficult to conclusively observe this phenomenon, known as the chiral anomaly, solely based on transport measurements. In this project, time-resolved photocurrent microscopy, transient grating spectroscopy and terahertz spectroscopy will be used to optically detect signatures of the chiral anomaly in Weyl semimetals. Additionally, photocurrent response of a Weyl semimetal will be used to map out the quantum geometry of its electronic wavefunction. This will allow detection of the Berry curvature monopoles and dipoles that arise from a Weyl node. The realization of the novel optical and optoelectronic phenomena in topological semimetals may allow us to control the Weyl fermions and their associated quantum anomalies by optical means, leading to useful optical and optoelectronic applications.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.