Topological Wannier Excitons in Bismuth Chalcogenide Nanosheets

Abstract

Current optoelectronic technologies depend crucially on insulators and semiconductors, whose response to photoexcitation at room temperature is largely dominated by bound electron-hole pairs known as excitons. While the physics of excitons in regular materials are very well understood, knowledge of their behavior in topological insulators is still severely limited. These novel phases of matter are receiving much attention due to their excellent prospects for energy-efficient electronics, (pseudo)spintronics devices and quantum-information processing. Therefore, a full understanding of light-matter interactions in these materials requires the investigation of physical and topological properties of bulk excitons.

In this work, we analyze the topology and dispersion of bulk Wannier excitons in thin nanosheets of bismuth selenide (Bi₂Se₃), a prototypical three-dimensional topological insulator. We find that excitons inherit the topology of the underlying electronic band structure, quantified by the winding numbers of the constituent electron and hole pseudospins as a function of the total exciton momentum. We also show that every s-wave exciton state consists of a nonchiral doublet of degenerate states with quadratic dispersion for low momenta, as well as a chiral doublet with one linear mode and one quadratic mode. We derive an effective model for the chiral excitons and consider their topological properties. Furthermore, we analyze the many-body screening due to the coherent surface states on the effective bulk electron-hole interaction, and consider the effect of surface plasmons. Our study is backed up by self-consistent numerical calculations and paints a complete picture of bulk excitons in quasi-two-dimensional topological materials with a band inversion at the Γ point. This picture can now in principle be used for the investigation of interactions between excitons themselves and other excitations such as plasmons and photons.