Degenerate excitons in strongly spin-orbit coupled heterolayers*

We study the optical response and exciton properties of two-dimensional topological systems with strong spin-orbit coupling, using ab-initio GW and Bethe-Salpeter equation (BSE) calculations. We develop a new method to derive group representations of exciton states directly from ab initio calculations without any assumption on the character of the envelope functions. Our analyses show that, for appropriate systems, it is possible for the two lowest-energy optically active excitons from a single valley to be degenerate. By construction of a smooth gauge, the envelope functions of these degenerate excitons exhibit 1s-like characters. Our finding suggests these material systems are very promising for many applications in optoelectronics.