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

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.