Observation of exciton-exciton interaction mediated valley Depolarization in Monolayer MoSe2 Journal Article

Author(s): Mahmood, Fahad; Alpichshev, Zhanybek; Lee, Yi-Hsien; Kong, Jing; Gedik, Nuh
Article Title: Observation of exciton-exciton interaction mediated valley Depolarization in Monolayer MoSe2
Abstract: The valley pseudospin in monolayer transition metal dichalcogenides (TMDs) has been proposed as a new way to manipulate information in various optoelectronic devices. This relies on a large valley polarization that remains stable over long time scales (hundreds of nanoseconds). However, time-resolved measurements report valley lifetimes of only a few picoseconds. This has been attributed to mechanisms such as phonon-mediated intervalley scattering and a precession of the valley pseudospin through electron-hole exchange. Here we use transient spin grating to directly measure the valley depolarization lifetime in monolayer MoSe2. We find a fast valley decay rate that scales linearly with the excitation density at different temperatures. This establishes the presence of strong exciton-exciton Coulomb exchange interactions enhancing the valley depolarization. Our work highlights the microscopic processes inhibiting the efficient use of the exciton valley pseudospin in monolayer TMDs.
Keywords: 2D materials, exciton interactions, Transition metal dichalcogenides, valley dynamics, valley polarization
Journal Title: Nano Letters
Volume: 18
Issue 1
ISSN: 1530-6992
Publisher: American Chemical Society  
Date Published: 2018-01-10
Start Page: 223
End Page: 228
DOI: 10.1021/acs.nanolett.7b03953
Notes: The authors thank Furkan Cagri, Alex Frenzel, Liang Fu, Joe Orenstein, Edbert Sie, Darius Torchinsky, and Di Xiao for useful discussions. This work is supported by the U.S. Department of Energy, BES DMSE (experimental setup and data acquisition), and from the Gordon and Betty Moore Foundation’s EPiQS Initiative grant GBMF4540 (manuscript writing). Y.H.L acknowledges support from AOARD grant (cofunded with ONRG) FA2386-16-1-4009 and Ministry of Science and Technology (MOST-105-2112-M-007-032-MY3 and MOST-106-2119-M-007-023-MY3).
Open access: yes (repository)