Observation of room temperature excitons in an atomically thin topological insulator

TL;DR

This paper reports the first observation of room temperature excitons in a 2D topological insulator, bismuthene, revealing strong Coulomb interactions and excitonic effects in a material with non-trivial topology.

Contribution

It provides experimental evidence of excitons in a 2D quantum spin Hall insulator at room temperature, supported by ab-initio calculations, linking excitonic and topological physics.

Findings

Observation of strong optical resonances at the direct gap

Confirmation of excitonic states via ab-initio calculations

First evidence of excitons in a 2D topological insulator at room temperature

Abstract

Optical spectroscopy of ultimately thin materials has significantly enhanced our understanding of collective excitations in low-dimensional semiconductors. This is particularly reflected by the rich physics of excitons in atomically thin crystals which uniquely arises from the interplay of strong Coulomb correlation, spin-orbit coupling (SOC), and lattice geometry. Here we extend the field by reporting the observation of room temperature excitons in a material of non-trivial global topology. We study the fundamental optical excitation spectrum of a single layer of bismuth atoms epitaxially grown on a SiC substrate (hereafter bismuthene or Bi/SiC) which has been established as a large-gap, two-dimensional (2D) quantum spin Hall (QSH) insulator. Strongly developed optical resonances are observed to emerge around the direct gap at the K and K' points of the Brillouin zone, indicating the formation of bound excitons with considerable oscillator strength. These experimental findings are corroborated, concerning both the character of the excitonic resonances as well as their energy scale, by ab-initio \emph{GW} and Bethe-Salpeter equation calculations, confirming strong Coulomb interaction effects in these optical excitations. Our observations provide the first evidence of excitons in a 2D QSH insulator at room temperature, with excitonic and topological physics deriving from the very same electronic structure.