Dissecting intervalley coupling mechanisms in monolayer transition metal dichalcogenides

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Optics

Background:

• Monolayer transition metal dichalcogenides (TMDs) exhibit spin-valley locking, enabling valley degree of freedom control in optically excited carriers.
• Understanding intervalley coupling processes in 1L-TMDs is crucial but currently lacks a unified picture.

Purpose of the Study:

• To investigate exciton valley polarization dynamics in monolayer tungsten diselenide (1L-WSe2).
• To dissect individual intervalley coupling mechanisms and their role in exciton dynamics.
• To provide a comprehensive understanding of valley depolarization and spin-flip mechanisms.

Main Methods:

• Broadband helicity-resolved transient absorption spectroscopy.
• Microscopic simulations.
• Experimental data analysis combined with theoretical modeling.

Main Results:

• Phonon-assisted scattering plays a critical role in the rapid decay of A exciton circular dichroism.
• The B exciton exhibits dichroism formation with opposite polarity, influenced by phonon-assisted scattering.
• A momentum-dark Dexter process, activating intervalley exchange, drives valley depolarization.
• Efficient single electron spin-flip mechanisms were identified.

Conclusions:

• This work elucidates the complex interplay of intervalley coupling mechanisms in 1L-TMDs.
• The findings advance the fundamental understanding of exciton dynamics and valley polarization in these materials.
• The study highlights the importance of phonon-assisted scattering and Dexter processes in controlling valley properties.