Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response

Abstract

The spontaneous condensation of excitons into an excitonic insulator phase has been proposed to occur in several materials. However, spontaneous condensation of phonons (i.e., lattice distortions) can produce similar effects. An important open question is whether the exciton or the phonon mechanism is dominant and how phonons affect the ideal excitonic insulator. This work reveals the characteristic features of the collective excitations of coupled exciton-phonon systems and shows how this information can be used to experimentally identify the dominant condensation mechanism.