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

非技术摘要：创造更高效的电子设备、可再生和可持续能源，或提高计算速度和信息存储能力，都有赖于发现和开发材料的新特性。近年来，出现了一类新的半导体，其特征是具有大的电子迁移率的无质量的电荷载流子，即狄拉克费米子，因此可以以低功率损耗传输电能。此外，拓扑的特殊性也使得控制电荷的磁性(自旋)成为可能。该项目旨在通过低温和宽带磁光光谱学的方法，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。将样品暴露在频率为三个数量级(从太赫兹到紫外线)的电磁辐射下，冷却到负450F，并在外磁场存在的情况下，探索了狄拉克费米子的本征电子和磁性，以及它们可能的技术应用潜力。除了它的科学相关性，这个项目还为来自以本科生为主的机构(PUI)的相当数量的大学生提供暑期研究机会和动手实验技能。技术摘要：在Weyl半金属中，反转和/或时间反转对称的破坏会产生具有受保护的自旋手性的无质量(Dirac)电子(Weyl Fermions)，使它们成为新兴技术应用的有前途的候选者，例如量子计算或基于自旋的设备(自旋电子学)。这个项目涉及低温红外磁谱测量，结合密度泛函计算，以便探测和表征特别相关的REAlX半金属系统中的Weyl费米子(其中RE是稀土元素Pr、La或Ce之一，X是Si或Ge)。Dirac(线性)电子带的接触同时产生点节点(I型Weyl)和线节点(II型Weyl)，并且根据稀土元素的不同，REAlX中既可以实现非磁性基态，也可以实现磁性基态。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光谱中找到，也可以在量子化朗道能级与外加磁场的能量标度中找到，因此使红外磁谱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与磁相互作用之间的耦合，因为它可以产生新的电子态，并有望在技术上应用。具体地说，目的是探索REAlX中Weyl锥的存在，并表征它们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，目的是回答该领域中一些悬而未决的问题，如稀土离子之间磁耦合的起源，或者Berry曲率在该系统中观察到的反常霍尔效应实现中的作用。这些都是重要的基本问题，同时可能为在不施加外部磁场的情况下操纵自旋提供新的实用场所。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。

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.