Theoretical Spectroscopy of 2D Materials: Exciton-phonon coupling in resonant Raman and in luminescence spectroscopy*

2D and layered materials are known to exhibit very pronounced excitonic effects due to the confinement of electrons and holes in a layer and due to the weak dielectric screening of the electron-hole interaction. A theoretical description of optical spectra that include vibrations must therefore take into account exciton-phonon coupling in order to obtain meaningful results. We present our methods for the calculation of exciton-phonon coupling via a finite displacement [1] and via a diagrammatic approach [2,3], both using many-body perturbation theory.

We present results for the phonon-assisted luminescence spectrum of bulk hexagonal boron nitride (hBN), where the combination of indirect band gap and strong excitonic effects leads to a complex peak structure due to coupling of excitons with various phonon branches [1]. Furthermore, we present calculations of resonant Raman intensities with the combined inclusion of both excitonic and non-adiabatic effects. In bulk hBN, which has high phonon-frequencies due to the light atoms, we demonstrate the emergence of strong quantum interference between different excitonic resonances due to non-adiabatic effects. In MoS₂ and MoTe₂, our calculations explain the observed different intensity dependences of the A₁' and E' modes on the energy of the exciting laser [3].

[1] F. Paleari, H.P.C. Miranda, A. Molina-Sánchez, L. Wirtz, Phys. Rev. Lett. 122, 187401 (2019)
[2] F. Paleari, PhD thesis, University of Luxembourg (2019)
[3] S. Reichardt and L. Wirtz, Sci. Adv. 6, eabb5915 (2020)