RUI: Probing the interplay between magnetism and relativistic fermions in the Weyl semimetals PrAlGe1-xSix with infrared spectroscopy and magneto-spectroscopy

摘要：创造更高效的电子设备，可再生和可持续的能源，或提高计算速度和信息存储能力依赖于发现和利用材料的新特性。近年来，出现了一类新的半导体，其特点是“无质量”载流子，即狄拉克费米子，具有大的电子迁移率，因此可以以低功率损耗传输电力。此外，拓扑的特殊性使得控制电荷的磁性（自旋）成为可能。本项目旨在通过低温和宽带磁光光谱的方法，探测和表征最近发现的一类半导体中具有拓扑保护自旋的狄拉克费米子的性质。将样品暴露在频率横跨三个数量级（从太赫兹到紫外线）的电磁辐射中，将它们冷却到负450华氏度，并在外部磁场的存在下，探索狄拉克费米子的内在电子和磁性能，以及它们可能的技术应用潜力。除了它的科学相关性，这个项目提供暑期研究机会和动手实验技能，从一个主要的本科院校（PUI）的大学生显著数量。它还允许首席研究员继续参与外展活动，并在暑期项目中指导来自代表性不足群体的高中生。技术摘要：在Weyl半金属中，反转和/或时间反转对称性的破坏产生了具有保护自旋手性的体无质量（Dirac）电子（Weyl费米子），使它们成为新兴技术应用的有希望的候选物，如量子计算或基于自旋的器件（自旋电子学）。该项目涉及低温红外磁谱测量，结合密度泛函计算，以探测和表征一个特别相关系统中的Weyl费米子，REAlX半金属（其中RE是稀土元素Pr， La或Ce中的一种，X是Si或Ge）。Dirac（线性）电子带的接触产生点（i型Weyl）和线节点（ii型Weyl），并且根据稀土元素的不同，REAlX可以实现非磁性基态和磁性基态。这些所谓的Weyl锥的特征可以在电子分支之间的带内和带间跃迁谱中找到，也可以在应用磁场的量子化朗道能级的能量标度中找到，从而使红外磁光谱学成为一个特别相关的工具。这项研究的主要动机是研究无质量载流子和磁相互作用之间的耦合，因为它可以产生新的电子状态，并为技术应用带来希望。具体来说，目的是探索REAlX中Weyl锥的存在性，并表征其在动量空间中的范围、形状和位置。此外，通过研究磁有序对Weyl锥的影响，目的是回答该领域的一些悬而未决的问题，例如稀土离子之间磁耦合的起源，或者在该系统中观察到的Berry曲率对反常霍尔效应的实现的作用。这些都是重要的基础问题，同时也为不需要外加磁场的自旋操纵提供了新的实践场所。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical Abstract:Creating more efficient electronic devices, renewable and sustainable energy sources, or increasing computational speed and information storage capabilities rely on discovering and exploiting novel properties of materials. In recent years, a new class of semiconductors have emerged, featuring “massless” charge carriers, i.e. Dirac fermions, with large electronic mobility, and hence allowing transport of electricity with low power losses. In addition, topological particularities make it possible to also control the magnetic property (spin) of electrical charges. This project aims to probe and characterize the properties of Dirac fermions with topologically protected spin in a recently discovered class of semiconductors, by means of low-temperature and broad-band magneto-optical spectroscopy. Exposing the samples to electromagnetic radiation with frequency spanning three orders of magnitude (from terahertz to ultraviolet), cooling them to negative 450 F, and in the presence of external magnetic field, the intrinsic electronic and magnetic properties of Dirac fermions, and possibly their potential for technological applications are explored. In addition to its scientific relevance, this project provides summer research opportunities and hands-on experimental skills to a significant number of college students from a predominantly undergraduate institution (PUI). It also allows the principal investigator to continue involvement in outreach and mentor high school students from underrepresented groups during summer programs.Technical Abstract:In Weyl semimetals, breaking of inversion and/or time-reversal symmetries give rise to bulk massless (Dirac) electrons with protected spin chirality (Weyl fermions), making them promising candidates for emerging technological applications, such as quantum computing or spin-based devices (spintronics). This project involves low temperature infrared magneto-spectroscopy measurements, combined with density functional calculations, in order to probe and characterize Weyl fermions in a particularly relevant system, the REAlX semimetals (where RE is one of the rare earth elements Pr, La or Ce and X is Si or Ge). Touching of Dirac (linear) electronic bands gives rise to both point (type-I Weyl) and line nodes (type-II Weyl), and depending on the rare earth element, both non-magnetic and magnetic ground states can be realized in REAlX. Hallmarks of these so-called Weyl cones can be found in the intra and inter-band transition spectrum between electronic branches, as well as in the energy scaling of quantized Landau levels with applied magnetic field, thus making infrared magneto- spectroscopy a particularly relevant tool. The primary motivation of this research is to investigate the coupling between massless carriers and magnetic interactions, as it can generate novel electronic states and hold promises for technological applications. Specifically, the aim is to probe the existence of Weyl cones in REAlX, and to characterize their extent, shape and position in momentum space. In addition, by studying the effects of magnetic ordering on Weyl cones, the goal is to answer some open questions in the field, such as the origins of magnetic coupling between the rare earth ions, or the role of Berry curvature on the realization of the anomalous Hall effect, observed in this system. These are important fundamental problems and at the same time, may provide new practical venues for spin manipulation without application of external magnetic field.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.