Topological Wannier excitons in bismuth chalcogenide nanosheets I: Single-particle properties

TL;DR

This paper investigates the topological properties of bulk Wannier excitons in bismuth chalcogenide nanosheets, revealing their inheritance of electronic band topology, unique exciton states, and the existence of topological edge excitons.

Contribution

It introduces the concept of topological Wannier excitons inheriting band topology and characterizes their dispersion, states, and edge phenomena in bismuth chalcogenide nanosheets.

Findings

Excitons inherit the topology of electronic bands via skyrmion winding numbers.

Identification of a quartet of exciton states with distinct dispersions.

Existence of topological edge states of chiral excitons.

Abstract

We analyze the topology and dispersion of bulk Wannier excitons in nanosheets of topological insulators in the family of bismuth chalcogenides. Our main finding is that excitons also inherit the topology of the electronic bands, quantified by the skyrmion winding numbers of the constituent electron and hole pseudospins as a function of the total exciton momentum. We furthermore show that every $s$-wave exciton state consists of a quartet with a degenerate and quadratically dispersing nonchiral doublet, and a chiral doublet with one linearly dispersing mode as in transition metal dichalcogenides. We also demonstrate the existence of topological edge states of chiral excitons arising from the bulk-boundary correspondence.