Abstract
Monolayers of transition metal dichalcogenides are promising materials for valleytronic applications, since they possess two individually addressable excitonic transitions at the non-equivalent $K$ and $K'$ points with different spins, selectively excitable with light of opposite circular polarization. Here, it is of crucial importance to understand the elementary processes determining the lifetime of these optically injected valley excitons. In this study, we perform microscopic calculations based on a Heisenberg equation of motion formalism to investigate the efficiency of the intervalley coupling in the presence (W based TMDCs) and absence (Mo based TMDCs) of energetically low lying momentum-dark exciton states. While we predict a valley exciton lifetime on the order of some hundreds of fs in the absence of low lying momentum-dark states we demonstrate a strong quenching of the valley lifetime in the presence of such states.
Highlights
Monolayers of transition metal dichalcogenides (TMDCs) possess a variety of excitonic excitations with large binding energy and oscillator strength, which enabled the extensive investigation of exciton physics in these atomically thin materials [1,2,3,4,5,6,7]
MoSe2 and WSe2, we are able to investigate the influence of energetically low-lying momentum-dark states to the intervalley exchange coupling (IEC) dynamics
We have presented a microcopic theory investigating the impact of momentum-dark exciton states on the IEC in monolayer TMDCs
Summary
Monolayers of transition metal dichalcogenides (TMDCs) possess a variety of excitonic excitations with large binding energy and oscillator strength, which enabled the extensive investigation of exciton physics in these atomically thin materials [1,2,3,4,5,6,7]. The slow component of the spin-polarization lifetime decreases as a function of temperature in WSe2 [29], WS2 [31,32], and MoS2 [33,35], which was reported for the fast component in MoS2 [33] Another experimentally accessible observable to study the temporal relaxation of spin-polarized excitons is the degree of polarization of the emitted light after circular excitation [spin ↑ after σ+ excitation, cf Fig. 1(a)] of the material [30,36,37,38,39,40,41] for stationary luminescence or after pulsed excitation.
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