Room temperature excitons in the atomically thin topological insulator Bi:SiC

The research on topological states of matter saw a remarkable increase in interest since the prediction in graphene of the quantum spin Hall (QSH) effect. Another boost came from the verification, in 2007, of this state of matter. Interesting applications are then made possible by combining symmetry-protected helical edge states and a vanishing bulk conductivity. However, to date, the use quantum Hall materials is limited to very low temperatures because of small band gaps. A progress in this direction is made possible by bismuthene (honeycomb layer of bismuth atoms) grown on a silicon carbide substrate. This material exhibits indeed a direct gap of 1.3 eV at the K-point, proposing itself as an excellent quantum spin Hall insulator for studying excitons in the visible range. In this presentation will be discussed the theoretical results, obtained via many-body perturbation theory and Bethe-Salpeter equation, on the optical resonances in Bi:SiC. By comparing these data with experimental spectroscopic and pump-probe ARPES ones, it will shown that the strong electron-hole interaction have indeed a relevant effect on the optical excitations in Bi:SiC. The results provide the first evidence of excitons in a 2D QSH insulator at room temperature.