Strong exciton regulation of Raman scattering in monolayer dichalcogenides

TL;DR

This paper presents a first-principles framework that incorporates excitonic effects to accurately predict resonant Raman intensities in monolayer dichalcogenides, revealing how excitons regulate Raman scattering amplitudes.

Contribution

It introduces a novel computational approach that accounts for excitonic effects beyond the Placzek approximation, improving the understanding of Raman responses in 2D materials.

Findings

Excitonic effects strongly influence Raman scattering amplitudes in MoS2.

The approach explains the absence of Raman signals near A and B excitons.

It reduces computational effort by requiring only one GW-BSE calculation per mode.

Abstract

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 wavefunction characters with profound effects on diverse modes of 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 resonant Raman response around the A and B excitons (which produce very strong signals in optical absorption), and also the pronounced strength of the resonant Raman response from the C exciton. Furthermore, this efficient perturbative approach reduces the number of GW- BSE calculations from two per Raman mode (in finite displacement) to one for all modes and affords natural extension to higher-order resonant Raman processes.