Theory of Excitons in 2D Materials -- Valley-Spin Physics and Topological Effects
The research on atomically thin 2D materials have opened up opportunities in exploring new phenomena and properties as well as related applications absent in conventional bulk materials. In this talk, we will present recent theoretical and computational studies of excitons in 2D semiconductors and their coupling to light. In the first part, by ab initio GW-BSE calculations, we will demonstrate the presence of a series of dark excitons in monolayer transition metal dichalcogenides, and how they could be brightened by applying an external magnetic field or coupling to lattice vibrations. We then discuss a novel valley-spin character of bright excitons, which leads to interesting ultrafast phenomena in monolayer transition metal dichalcogenides. In the second part, we demonstrate the unusual optical selection rules resulting from a valley-dependent topological band effect. We further connect our theoretical discoveries to experimental results and explore their potential applications.