Long valley relaxation time of free carriers in monolayer WSe2

Monolayer transition metal dichalcogenides (TMDs) feature a valley degree of freedom, giant spin-orbit coupling, and spin-valley locking. These exotic natures have stimulated efforts of exploring potential applications in conceptual spintronics, valleytronics, and quantum computing. Among all the exotic directions, a long relaxation time of spin and/or valley polarization is critical. The present valley dynamics studies concentrate on the band edge excitons which predominate the optical response due to an enhanced Coulomb interaction in two dimensions. The valley relaxation time of free carriers remains ambiguous. In this Rapid Communication, we use time-resolved Kerr rotation spectroscopy to probe the valley dynamics of excitons and free carriers in monolayer tungsten diselenide. The valley relaxation time of free carriers is found around 2 ns at 70 K, about three orders of magnitude longer than the excitons of about 2 ps, and 15 times larger than that of trions (130 ps). The extended valley relaxation time of free carriers evidences that an exchange interaction dominates the valley relaxation in optical excitations. The pump-probe spectroscopy also reveals an exciton binding energy of 0.60 eV in monolayer WSe₂.