Exciton-phonon interaction calls for a revision of the "exciton" concept

Abstract

The concept of optical exciton -a photo-excited bound electron-hole pair within a crystal -is routinely used to interpret and model a wealth of excited-state phenomena in semiconductors. Beside originating sub-band gap signatures in optical spectra, optical excitons have also been predicted to condensate, diffuse, recombine, relax. However, all these phenomena are rooted on a theoretical definition of the excitonic state based on the following simple picture: "excitons" are actual particles that both appear as peaks in the linear absorption spectrum and also behave as well-defined quasiparticles. In this paper we show, instead, that the electron-phonon interaction decomposes the initial optical (i.e., "reducible") excitons into elemental (i.e., "irreducible") excitons, the latter being a different kind of bound electron-hole pairs lacking the effect caused by the induced, classical, electric field. This is demonstrated within a real-time, many-body perturbation theory approach starting from the interacting electronic Hamiltonian including both electron-phonon and electronhole interactions. We then apply the results on two realistic and paradigmatic systems, monolayer MoS2 (where the lowest-bound optical exciton is optically inactive) and monolayer MoSe2 (where it is optically active), using first-principles methods to compute the exciton-phonon coupling matrix elements. Among the consequences of optical-elemental decomposition, we point to a homogeneous broadening of absorption peaks occurring even for the lowest-bound optical exciton , and we provide a lower bound for the exciton linewidths. More generally, our findings suggest that the optical excitons gradually lose their initial structure and evolve as elemental excitons . These states can be regarded as the real intrinsic excitations of the interacting system, the ones that survive when the external perturbation and the induced electric fields have vanished.