Two-dimensional Transition Metal Dichalcogenides (TMDs) have been widely studied because of the peculiar electronic band structure and the strong excitonic effects [1]. In these materials the large spin-orbit coupling lifts the spin degeneracy of the valence (VB) and the conduction band (CB) giving rise to the A and B interband excitonic transitions. In monolayer WS2, the spins of electrons in the lowest CB and in the highest VB at K/K’ point of the Brillouin zone are antiparallel resulting in an intravalley dark exciton state at a lower energy than the bright exciton, see left panel of Fig.1. On the one hand, the presence of dark excitons has been revealed indirectly from the observation of anomalous quenching of the PL emission at low temperature in single-layer WS2 [2]; on the other hand, however, the intravalley spin-flip process is assumed to occur on a significantly long time scale, which is usually neglected in theoretical models describing exciton intra or inter valley scattering processes [3].

© 2019 IEEE

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