Observation of perfect valley coherence in monolayer MoS$_2$ through giant enhancement of exciton coherence time

In monolayer transition metal dichalcogenide semiconductors, valley coherence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manipulation of exciton a highly formidable task. Using monolayer MoS$_2$ sandwiched between top and bottom graphene, here we demonstrate perfect valley coherence by observing 100% degree of linear polarization of excitons in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a combination of calculation (solutions of Bethe-Salpeter and steady-state Maialle-Silva-Sham equations) and choice of systematic design of experiments using four different stacks with varying screening and exciton lifetime. To the best of our knowledge, this is the first report in which the valley coherence timescale has been significantly enhanced beyond the exciton radiative lifetime in monolayer semiconductors.