Exchange-Driven Intermixing of Bulk and Topological Surface State by Chiral Excitons in Bi₂Se₃

Topological surface states (TSS) in the topological insulator Bi₂Se₃ are often characterized using light-based probes, such as the circular photoelectric effect. However, microscopic theory considering electron-hole interactions and their effect on the optical response of TSS and surface localization has not yet been explored. Here, we study excitonic effects in the bulk and surface of Bi₂Se₃ using the ab initio GW and Bethe-Salpeter equation (GW-BSE) approach with a fully-relativistic spinor formalism. We identify bright excitons with p-like symmetry, as well as multiple series of excitons with chiral optical dipole selection rules spread over a wide energy range of 0-2.82 eV. The electron-hole pair forming the chiral excitons exhibit the characters of both bulk states and topologically protected surface states, which are intermixed by the Coulomb exchange interaction. Our results address fundamental questions about the degree to which electron-hole interactions can relax the topological protection of surface states in topological insulators by elucidating the complex intermixture of bulk and surface states excited in optical measurements and their coupling to light.