Multiphonon Raman scattering mediated by the exciton states in monolayer transition metal chalcogenides

In the frame of Huang-Rhys's model, we study multiple orders of Raman scattering in monolayer transition metal chalcogenides (TMDC) based on the intrinsic excitons coupled with three types of phonon modes, including longitudinal optical (LO) phonon, surface optical (SO) phonon, and longitudinal acoustic (LA) phonon. We find that multiphonon structures display multiple order overtones at equal interval energies of $\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LO}}$ and $\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{SO}}$, respectively, and intensities of overtones depend on the obtained values of Huang-Rhys factor, which can be modulated by the strength of exciton-optical phonons coupling, cutoff wave vector of optical phonon modes, and large exciton Bohr radius. Moreover, we propose the combined multiphonon processes of $p\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LO}}+m\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{SO}}$ and $p\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LO}}(m\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{SO}})+\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LA}}$. These combined processes with weaker intensity could be used to analyze the fine structure of multiphonon Raman spectra in experiments. Our theoretical results indicate that the strong exciton effect in monolayer TMDC provides an excellent platform for exploring multiphonon Raman scattering.