Abstract: The advent of atomically thin 2D materials, such as graphene and transition metal dichalcogenide (TMD) semiconductors, has boosted the development of valleytronics. In particular, excitonic valley degree of freedom in TMDs such as monolayer MoS₂ can be effectively addressed by optical means. Here we experimentally show a counter-intuitive improvement of valley polarization in MoS₂ induced by scattering. We report seven- and twelve-folds of valley polarization enhancement due to thermally activated and charge doping induced scattering respectively. This interesting effect is attributed to the reduction of valley pseudospin precession arising from rapid modulation of exciton momentum and concomitant exchange interaction field, which is analogous to motional narrowing in nuclear magnetic resonance spectroscopy. Our work advances understanding of valley depolarization mechanisms in TMD atomic layers and illustrates a novel approach for controlling and improving valleytronic devices.

Biography: Dr. Jun Yan is an Associate Professor of Physics at the University of Massachusetts Amherst, USA. He attended graduate school at Columbia University in the city of New York, NY, USA, and earned his Ph.D. in 2009. He held a postdoc position from 2009 to 2012 at the University of Maryland, College Park before joining UMass Amherst as a tenure-track faculty. His current research focus is mainly on the optical and optoelectronic properties of atomically thin crystals and quantum materials.