Spectroscopy of Topological Materials and their Technological Applications

Abstract

Topological materials have been at the forefront of condensed matter physics and material science research in the past decade. The concept of topology has greatly shaped our modern understanding of matter including how band topology can tailor the exotic electronic and structural properties of a material. The prediction and subsequent experimental observation of topological semimetals stimulated an exploration of a substantial number of new materials. These range from verifying new exciting theoretical predictions to examining compilations of vast databases obtained through high-throughput computational and machine-learning approaches. In this thesis, we explore these new research directions by using new spectroscopic techniques to probe topology and other phonon-related phenomena in materials and by investigating potential applications of topological materials in modern-day technologies.

We begin the dissertation by illustrating a chronicle of research into topology in condensed matter physics over the decades, the techniques of x-ray and neutron spectroscopies, and an overview of current progress in modern technologies. The first part of the dissertation describes using x-ray and neutron spectroscopies as a novel probe of topology. In particular, we describe how the nesting of Weyl nodes may lead to exotic phenomena such as anomalies in phonon dispersions and the stabilization of exotic magnetic orders, both of which are measurable using x-ray and neutron probes. We then focus on probing phonon transport in materials using x-ray scattering along two interesting directions. We demonstrate a method that overcomes the difficult task of measuring thermal properties of a thin film on a substrate. By measuring the gradual diffusive lattice relaxation subsequent to laser-induced heat using ultrafast x-ray diffraction, we are able to extract thermal conductivity of a thin film as well as the thermal boundary conductance between the film-substrate interface with lateral sensitivity and with information about local defects. Second, we probe a nonequilibrium state of matter termed many-body localization by demonstrating a departure from thermalization of phonons in disordered semiconductor superlattices.

The second part of the dissertation focuses on technological applications of topological materials following their experimental discovery and recent theoretical developments. Topological materials may possess unique material properties resulting in applicability and possible large-scale implementation into certain technological niches based on current necessities. We investigate how topological materials may be part of future thermometric material candidates as they can acquire large entropy through unique Landau level quantization and large Berry curvature-induced anomalous Nernst effect. We describe measurements of nonlinear Hall effects in these materials which highlight their possible use in energy harvesting and terahertz applications. We also describe ideas of using topological materials as future post-copper back-end-of-the-line interconnects in integrated circuits due to favorable resistivity scaling with decreasing size. Finally, we showcase some research into spintronic-related logic and memory applications due to the generation of large spin-orbit-torques in these materials resulting in efficient current-driven switching capabilities.

We conclude by presenting an outlook of future research directions in the field of topological materials and the prospects of their integration into modern-day electronic and thermal technologies.