Vibrational coherent control of intervalley scattering in monolayer WSe2

The past two decades have witnessed significant development in our ability to selectively excite specific vibrational modes as coherent phonons, providing an avenue towards engineering the functional properties of quantum materials with light [1,2]. In particular, the intervalley scattering mediated by coherent phonons in monolayer transition metal dichalcogenide family has inspired the great potential of manipulating valley polarization which concerns the realization of practical valleytronics [3]. However, most previous theoretical works based on the Fermi's golden rules deal with evolution of electrons and phonons adopting a quasi-thermal distribution [4] and fail to capture the nonthermal scattering dynamics when the system is driven far from equilibrium.
This talk aims at revealing the quantitative coherent-phonon-driven electronic intervalley transition upon photoexcitation in monolayer WSe2. We start with launching coherent lattice motion along the zone-boundary LA mode and show the non-equilibrium electronic configuration and electron population are strongly coupled with coherent phonon oscillation using the state-of-art time-dependent density functional theory (TDDFT). We observe a step-like electron transition from the optically bright K to dark Q state on a time scale ~ 400 fs in excellent agreement with previous experimental studies [5] and further a reversed Q-to-K transition which we refer to as coherent phonon induced Rabi oscillation. We discuss the coupled electron and lattice dynamics by constructing a two-level model and uncover the crucial role of nonadiabatic coupling effects. Finally, we propose a new strategy to effectively tune the intervalley scattering rates by varying the coherent phonon amplitude, which could be realized via nonlinear phononics that we hope to spark experimental investigations [6].

1 H. Hübener, et al., Nano Lett. 18 (2), 1535-1542 (2018)
2 S. Priyadarshi, et al., Phys. Rev. Lett. 128, 136402 (2022)
3 J. R. Schaibley, et al., Nat. Rev. Mater. 1, 16055 (2016).
4 Z. Jin, et al., Phys. Rev. B 90, 045422 (2014)
5 J. Madéo, et al., Science 370, 1199 (2020)
6 A. S. Disa, et al., Nat. Phys. 17, 1087–1092 (2021).