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In monolayer transition metal dichalcogenide semiconductors, valley coherencedegrades rapidly due to a combination of fast scattering and inter-valleyexchange interaction. This leads to a sub-picosecond valley coherence time,making coherent manipulation of exciton a highly challenging task. Usingmonolayer MoS2 sandwiched between top and bottom graphene, here we demonstratefully valley coherent excitons by observing ~100% degree of linear polarizationin steady state photoluminescence. This is achieved in this unique designthrough a combined effect of (a) suppression in exchange interaction due toenhanced dielectric screening, (b) reduction in exciton lifetime due to a fastinter-layer transfer to graphene, and (c) operating in the motional narrowingregime. We disentangle the role of the key parameters affecting valleycoherence by using a combination of calculation (solutions of Bethe-Salpeterand Maialle-Silva-Sham equations) and a careful choice of design of experimentsusing four different stacks with systematic variation of screening and excitonlifetime. To the best of our knowledge, this is the first report in which theexcitons are found to be valley coherent in the entire lifetime in monolayersemiconductors, allowing optical readout of valley coherence possible.

Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time