Surface hopping simulations show valley depolarization driven by exciton-phonon resonance

Abstract

Resonances between excitonic transitions and nuclear coordinates have been shown to drive a variety of excited-state dynamical phenomena in molecular systems. Here, we report mixed quantum--classical simulations showing similar resonances to primarily contribute to valley depolarization in monolayer MoS$_2$. The applied simulation framework combines reciprocal-space surface hopping with microscopic models of the quasiparticle band structure, electron--hole interactions, and carrier--phonon interactions, parametrized against ab initio calculations. This enables low-cost excited-state dynamics simulations that are microscopic, non-Markovian, and non-perturbative in the carrier--phonon interaction. The framework furthermore retains explicit information on transient phonon occupancies, through which we show a resonance between the dominant optical phonon branch and the lowest exciton band to largely drive valley depolarization, by activating a Maialle--Silva--Sham mechanism. Resulting valley polarization times are consistent with experimental measurements across temperatures.