Phonon-assisted inter-valley scattering determines ultrafast exciton dynamics in MoSe$_2$ bilayers

TL;DR

This study reveals that phonon-assisted inter-valley scattering significantly influences ultrafast exciton dynamics in MoSe₂ bilayers, with faster decoherence and distinct relaxation channels compared to monolayers, advancing understanding of valley physics in TMDCs.

Contribution

It provides the first detailed microscopic analysis of exciton decoherence and relaxation mechanisms in MoSe₂ bilayers, highlighting the role of phonon emission in inter-valley scattering.

Findings

Faster exciton decoherence in bilayers than monolayers.

Identification of coherent and incoherent relaxation channels.

Phonon emission facilitates scattering from K to Γ and Λ valleys.

Abstract

While valleys (energy extrema) are present in all band structures of solids, their preeminent role in determining exciton resonances and dynamics in atomically thin transition metal dichalcogenides (TMDC) is unique. Using two-dimensional coherent electronic spectroscopy, we find that exciton decoherence occurs on a much faster time scale in MoSe$_2$ bilayers than that in the monolayers. We further identify two population relaxation channels in the bilayer, a coherent and an incoherent one. Our microscopic model reveals that phonon-emission processes facilitate scattering events from the $K$ valley to other lower energy $\Gamma$ and $\Lambda$ valleys in the bilayer. Our combined experimental and theoretical studies unequivocally establish different microscopic mechanisms that determine exciton quantum dynamics in TMDC monolayers and bilayers. Understanding exciton quantum dynamics provides critical guidance to manipulation of spin/valley degrees of freedom in TMDC bilayers.