Exciton-phonon interaction revisited

photo of Fulvio Paleari
Fulvio Paleari
Istituto di Struttura della Materia, Monterotondo (Italy)

The investigation of the coupling of optically excited, bound electron-hole pairs – i.e., excitons – with phonons has become increasingly relevant to both experimental and theoretical research, being related to many spectroscopic features that are nowadays observable in low-dimensional semiconductors. In this case, exciton-phonon coupling is thought to explain fine structures in optical absorption and luminescence spectra, as well as the linewidths, lifetimes, and formation times of excitonic spectral peaks, and also the out-of-equilibrium dynamics of excitations in ultrafast spectroscopy.
In this talk, I will briefly introduce the state-of-the-art Bethe-Salpeter equation used to calculate excitonic properties, and then describe the usual “quasiparticle” model for exciton-phonon interactions, which is based on approximating excitons as weakly interacting bosons.
I will then discuss the strengths and weaknesses of this model using examples from the literature as well as focusing on the case of phonon-assisted luminescence in hexagonal boron nitride.
In the last part I will introduce a more general approach that aims to overcome some issues of the “quasiparticle” model: in this new approach, lattice vibrations are coupling optically generated excitons with “irreducible” excitons undressed of the electron-hole exchange interaction. I will discuss the possible consequences of this change concerning the calculation of exciton-phonon linewidths by showing some preliminary ab initio results for the case of single-layer MoS2.