Strain control of exciton and trion spin-valley dynamics in monolayer transition metal dichalcogenides
Abstract
The electron-hole exchange interaction is a fundamental mechanism that drives valley depolarization via intervalley exciton hopping in semiconductor multi-valley systems. Here, we report polarization-resolved photoluminescence spectroscopy of neutral excitons and negatively charged trions in monolayer MoSe2 and WSe2 under biaxial strain. We observe a marked enhancement(reduction) on the WSe2 triplet trion valley polarization with compressive(tensile) strain while the trion in MoSe2 is unaffected. The origin of this effect is shown to be a strain dependent tuning of the electron-hole exchange interaction. A combined analysis of the strain dependent polarization degree using ab initio calculations and rate equations shows that strain affects intervalley scattering beyond what is expected from strain dependent bandgap modulations. The results evidence how strain can be used to tune valley physics in energetically degenerate multi-valley systems.
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