Electrical tuning of valley polarization in monolayer transition metal dichalcogenides

The valley degree of freedom in two-dimensional crystalline materials is another promising candidate for de-signing dissipationless information devices in addition to the spin degree of freedom. The primary challenge for the application of valley degree of freedom is to achieve a significant and controllable valley polarization. Here, we discover an electrical tunable mechanism for generating valley polarization caused by the backscattering induced by Rashba spin-orbit coupling in monolayer nanoribbon of transition-metal dichalcogenides. Since the Rashba spin-orbit coupling in monolayer transition-metal dichalcogenides can be induced by the external electric field, the valley polarization here can be dynamically controlled by means of the external gating. The results of the valley-dependent transport calculations show that a pronounced valley polarization can be generated when the external electric field-induced Rashba spin-orbit coupling is present in the central scattering region of the nanoribbon. Moreover, the efficiency and direction of the valley polarization can be controlled by tuning the Fermi energy and the strength of Rashba spin-orbit coupling. The calculated results also confirm that the valley polarization remains significant in the presence of atomic vacancy defects. This finding opens up a potential possibility for realizing electrical control of the valleytronics devices.