Unique Spin-Valley Properties in 2D Transition Metal Dichalcogenide Monolayers and Heterostructures
Wen-Hao Chang1*
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
* presenting author:Wen-Hao Chang, email:whchang23@gmail.com
A robust valley polarization is a key prerequisite for exploiting valley pseudospin to carry information in next-generation electronics and optoelectronics. Monolayer transition metal dichalcogenides (TMDs) with inherent spin-valley coupling offer a unique platform to develop such valleytronic devices. However, realization of robust valley polarization at room temperature remains challenging. Here, I will demonstrate our recent progress on the generation of long-lived valley-polarized holes in monolayer WSe2 and WSe2/MoSe2 heterostructures by optical pumping. Using time-resolved Kerr rotation spectroscopy, we observe a very long-lived valley polarization (~1 ns) in monolayer WSe2 at low temperatures, which is much longer than the trion recombination lifetime (~10-20 ps). A model considering the transfer of valley pseudospin from photocarriers to resident holes can explain the long-lived valley polarizations. The optically initialized valley holes remains robust even at room temperature, which opens up the possibility to realize room-temperature valleytronics based on TMDs. In WSe2/MoSe2 vertical heterostructures, the interlayer carrier transfer and the formation of interlayer exciton further stabilize the valley polarizations. In particular, we found that the spin-valley properties of the interlayer excitons is stacking orientation dependent. The interlayer charge transfers prefer to conserve the spin, while the momentum conservation is relaxed by the large energy mismatch. We show that the stacking symmetry play a critical role in the interlayer spin transfer.

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Keywords: 2D materials, Transition Metal Dichalcogenide, Spin-valley physics