In WSe2, phonon scattering moves excitons into momentum-dark states that do not couple valleys, quenching intervalley exchange coupling and extending valley lifetimes to nanoseconds, unlike in MoSe2 where lifetimes are hundreds of femtoseconds.
Phonon-assisted Photoluminescence from Dark Excitons in Monolayers of Transition Metal Dichalcogenides
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abstract
The photoluminescence (PL) spectrum of transition metal dichalcogenides (TMDs) shows a multitude of emission peaks below the bright exciton line and not all of them have been explained yet. Here, we study the emission traces of phonon-assisted recombinations of momentum-dark excitons. To this end, we develop a microscopic theory describing simultaneous exciton, phonon and photon interaction and including consistent many-particle dephasing. We explain the drastically different PL below the bright exciton in tungsten- and molybdenum-based materials as result of different configurations of bright and dark states. In good agreement with experiments, we show that WSe$_2$ exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark excitons.
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cond-mat.mes-hall 1years
2019 1verdicts
CONDITIONAL 1representative citing papers
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Quenching of Intervalley Exchange Coupling in the Presence of Momentum-Dark States in TMDCs
In WSe2, phonon scattering moves excitons into momentum-dark states that do not couple valleys, quenching intervalley exchange coupling and extending valley lifetimes to nanoseconds, unlike in MoSe2 where lifetimes are hundreds of femtoseconds.