Defect-Bound Excitons in Topological Materials

Excitons, bound states of electrons and holes, are affected by the properties of the underlying band structure of a material. Defects in lattice systems may trap electronic defect states, to which an electron can be excited to form defect-bound excitons. Here, we examine the effect of band topology on excitons in systems with a single-site defect. We show that in the topological phase, when robust, in-gap, ring-shaped electronic states appear around defects, excitons inherit the properties of these ring states: The excitons' binding energies are lowered as a result of the wide spatial profile of the defect state, and their wave functions have complex shapes and order due to the mixed orbital character of the topological bands. Our study advances the understanding of the role of topology in modifying and controlling defect-bound excitons in quantum materials.