Prof. Barry Bradlyn spoke about the discovery of topological materials as one of the most transformative recent breakthroughs in condensed matter physics, revealing new conceptual surprises in established topics such as the phases of matter and the behavior of electrons in insulators. Most work on topological insulators has focused on their metallic surface states that are present regardless of how dirty the system is. In this talk, I will emphasize that topological materials feature anomalous bulk properties that can be probed by external fields even when the surfaces are gapped. Using the tools of topological quantum chemistry, I will show how magnetic and nonmagnetic topological insulators protected by crystal symmetries can be characterized in the bulk. Focusing on “higher-order” topological insulators (HOTIs) protected by inversion symmetry, I will review how magnetic HOTIs have a nontrivial bulk magnetoelectric response and anomalous gapped surfaces. Then, by introducing a new notion of spin-resolved band topology, I will show how to extend this understanding to nonmagnetic helical HOTIs.
Seminar Link: Picoelectrodynamics Theory Network - YouTube