Comunicazione
Excitons at room temperature in a two-dimensional quantum spin Hall insulator.
Consiglio A., Syperek M., Stühler R., Holewa P., Wyborski P., Dusanowski L., Reis F., Höfling S., Thomale R., Hanke W., Claessen R., Di Sante D., Schneider C.
Since the prediction in graphene of quantum spin Hall (QSH) effect, the research field on topological states of matter saw a remarkable increase in interest. A boost came from the verification, in 2007, of this state. Future applications are then made possible by a vanishing bulk conductivity, combined with symmetry-protected helical edge states. To date, however, the use of QSH materials is limited to very low temperatures because of small band gaps. A progress in this direction is made by bismuthene (honeycomb layer of bismuth atoms) grown on a silicon carbide substrate. This material exhibits a direct gap of 1.3 eV at the $K-point$, proposing itself as an excellent QSH insulator for studying excitons in the visible and near-infrared range. During the presentation the theoretical results on the optical resonances in Bi:SiC, obtained via many-body perturbation theory and Bethe-Salpeter equation, will be discussed. Together with spectroscopic and ARPES data, it will be shown that the strong electron-hole interaction has indeed a relevant effect on the optical excitations in Bi:SiC. These results provide the first evidence of excitons in a 2D QSH insulator.