Strong exciton regulation of Raman scattering in monolayer MoS2

The weakly screened electron-hole interactions in an atomically thin semiconductor not only downshift its excitation spectrum from a quasiparticle one, but also redistribute excitation energies and wave-function characters with profound effects on the diverse modes of the material response, including the exciton-phonon scattering processes accessible to resonant Raman measurements. Here, we develop a first-principles framework to calculate frequency-dependent resonant Raman intensities that includes excitonic effects and goes beyond the Placzek approximation. We show how excitonic effects in MoS2 strongly regulate Raman scattering amplitudes and thereby explain the puzzling near absence of a resonant Raman response around the A and B excitons (band-edge excitations which produce very strong signals in optical absorption), and also the pronounced strength of the resonant Raman response from the C exciton (a higher-energy excitation arising from parallel valence and conduction bands). Furthermore, this efficient perturbative approach reduces the number of GW plus Bethe-Salpeter-equation calculations from two per Raman mode (in finite displacement) to one for all modes and affords a natural extension to higher-order resonant Raman processes.