Interaction-induced crystalline topology of excitons

TL;DR

This paper explores how interactions induce topological properties in exciton bands within centrosymmetric semiconductors, revealing new classes of excitons with quantized shifts and edge states, even when underlying bands are trivial.

Contribution

It introduces the concept of shift excitons, showing their topological nature and experimental accessibility, expanding understanding of exciton topology beyond noninteracting band structures.

Findings

Existence of shift excitons with quantized Wannier state shifts

Topologically nontrivial exciton spectrum with edge states

Experimental detection via optical conductivity measurements

Abstract

We apply the topological theory of symmetry indicators to interaction-induced exciton band structures in centrosymmetric semiconductors. Crucially, we distinguish between the topological invariants inherited from the underlying electron and hole bands, and those that are intrinsic to the exciton wavefunction itself. Focusing on the latter, we show that there exists a class of exciton bands for which the maximally-localised exciton Wannier states are shifted with respect to the electronic Wannier states by a quantised amount; we call these excitons shift excitons. Our analysis explains how the exciton spectrum can be topologically nontrivial and sustain exciton edge states in open boundary conditions even when the underlying noninteracting bands have a trivial atomic limit. We demonstrate the presence of shift excitons as the lowest energy neutral excitations of the Su-Schrieffer-Heeger model in its trivial phase when supplemented by local two-body interactions, and show that they can be accessed experimentally in local optical conductivity measurements.