Terahertz Studies on Magnetic Weyl Semimetals and Multi-Fold Fermions
Abstract
The birth of topological insulators, which only conduct electricity on the surfaces despite the insulating bulk, has extended importance of the effect of quantum mechanics beyond the traditional strongly correlated materials. In this class of materials, the topological properties are encoded in the quantum mechanical wave functions of the ground state. Even though the correlation is weak, emergent phenomena from Berry curvature, a fictitious magnetic field in the momentum space, have been discovered in topological insulators in transport and optical experiments. The discovery of topological semimetals, a low-carrier-density metallic phase in three dimensional with divergent Berry curvature at certain crossing points in the momentum space, was another breakthrough because usually topological phases need a gap to protect their stabilities. These crossing points are stable even though the systems are gapless. Candidate materials for topological semimetals have been confirmed by band-structure measurement and new transport phenomena have been widely studied. However, how Berry curvature effects of topological semimetals manifest in optical responses and whether they can lead to large effect that could ultimately be used in devices are still widely open questions. In this three-year project, we therefore propose to use and develop ultrafast optics and terahertz tools to identify unprecedented large responses from Berry curvature in two new classes of topological semimetals: magnetic Weyl semimetals and multi-fold fermions/semimetals. In magnetic Weyl semimetals, the crossing point with divergent Berry curvature is two-fold degenerate, while it is multi-fold degenerate in multi-fold fermions/semimetals. The similarity between them is that both of them could exhibit Berry curvature effects in optical responses in the low-energy terahertz regime under certain conditions. Secondly, we aim at controlling the Berry curvature by ultrafast lights to create new phenomena in these two class of semimetals. The proposed studies will be based upon the recent room-temperature studies on large linear magneto-optical effect and giant nonlinear optical response in topological semimetals, which indicated the possibility of unprecedently large effect with a novel mechanism in these topological semimetals. The objective of this project is to investigate new, giant, and robust linear and nonlinear optical responses from Berry curvature in these topological semimetals and identify them as possibly new platforms for faster memory devices and energy-efficient optoelectronics. From a fundamental perspective, the success of this project will represent a major breakthrough in our understanding of novel properties of topological semimetals. The project will build the foundation to tailor new material concepts towards a wide variety of optical applications. From a practical perspective, the developed robust large optical responses in topological semimetals will significantly benefit the fast-growing topological quantum materials with novel properties for spintronic and optoelectronic applications. Moreover, the studies of electrodynamics of these relativistic quasiparticles with and without counterparts in particle physics will have a broad impact in several areas of physics and material science, ranging from quantum field theory, mathematical physics, condensed matter physics and photonics.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 02, 2019
- Source ID
- W911NF1910342
Entities
People
- Liang Wu
Organizations
- Army Contracting Command
- United States Army
- University of Pennsylvania