Abstract
Large spin–orbit coupling in combination with circular dichroism allows access to spin-polarized and valley-polarized states in a controlled way in transition metal dichalcogenides. The promising application in spin-valleytronics devices requires a thorough understanding of intervalley coupling mechanisms, which determine the lifetime of spin and valley polarizations. Here we present a joint theory–experiment study shedding light on the Dexter-like intervalley coupling. We reveal that this mechanism couples A and B excitonic states in different valleys, giving rise to an efficient intervalley transfer of coherent exciton populations. We demonstrate that the valley polarization vanishes and is even inverted for A excitons, when the B exciton is resonantly excited and vice versa. Our theoretical findings are supported by energy-resolved and valley-resolved pump-probe experiments and also provide an explanation for the recently measured up-conversion in photoluminescence. The gained insights might help to develop strategies to overcome the intrinsic limit for spin and valley polarizations.
Highlights
Large spin–orbit coupling in combination with circular dichroism allows access to spinpolarized and valley-polarized states in a controlled way in transition metal dichalcogenides
The theoretical approach is based on the density matrix formalism, providing microscopic access to the time-resolved and energy-resolved dynamics of micrDoscopEic quantities, such as the microscopic polarization pij 1⁄4 ayj ai, which is a measure for the transition probability between the states i and j20
We focus on the coherent exciton dynamics, where we include dephasing and the formation of incoherent excitons via phonon-scattering constants calculated on a microscopic footing[22]
Summary
Large spin–orbit coupling in combination with circular dichroism allows access to spinpolarized and valley-polarized states in a controlled way in transition metal dichalcogenides. We present a joint theory–experiment study shedding light on the Dexter-like intervalley coupling We reveal that this mechanism couples A and B excitonic states in different valleys, giving rise to an efficient intervalley transfer of coherent exciton populations. In previous theoretical and experimental studies, it has already been shown that in addition to relatively slow intervalley spin-flip scattering processes[1,2,3,4,5] there is an efficient intervalley coupling mechanism via Coulomb exchange processes[6,7,8,9,10,11,12,13,14,15] This Coulomb-induced dipole–dipole interaction couples resonant excitonic states in K and K′ valleys, giving rise to a decay of valley polarization on a picosecond time scale. The theoretical predictions are supported by measurements of the valley polarization in energy-resolved and valley-resolved femtosecond two-color pump-probe experiments, and provide a microscopic explanation for recently published photoluminescence experiments measuring an up-conversion from A to B excitons in the opposite valley[19]
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