Ultrafast dynamics of spin relaxation in monolayer WSe and the WSe/graphene heterojunction.

Excitonic devices based on two-dimensional (2D) transition metal dichalcogenides (TMDCs) can combine spintronics with valleytronics due to its special energy band structure. In this work, we studied the generation and relaxation processes of spin/valley polarized excitons dynamics in monolayer WSe and its van der Waals (vdW) heterojunction with graphene using a circularly polarized femtosecond pump-probe system. The spin/valley depolarization dynamics of the A exciton in monolayer WSe is found to exhibit a biexponential decay. The fast relaxation process is due to the ultrafast intervalley electron-hole spin-flip exchange coupling and electron-phonon scattering. And the slow relaxation process originates from the recombination and relaxation of the trion states. Graphene has an electron extraction effect on WSe, which prevents the formation of trions. Therefore, the spin/valley depolarization process of the A exciton in the heterojunction exhibits only a fast relaxation process. In both monolayer WSe and its heterojunction with graphene, B/A' excitons exhibit a negative spin/valley polarization which is mainly due to two-photon absorption and excited Bose scattering. Our work systematically studied the spin/valley depolarization dynamics of excitons and revealed possible mechanisms of their differences in isolated 2D WSe and vdW heterojunctions.