Enhanced Valley Polarization in WS 2 /LaMnO 3 Heterostructure.

Monolayer transition metal dichalcogenides have attracted great attention for potential applications in valleytronics. However, the valley polarization degree is usually not high because of the intervalley scattering. Here, a largely enhanced valley polarization up to 80% in monolayer WS 2 under nonresonant excitation at 4.2 K is demonstrated using WS 2 /LaMnO 3 thin film heterostructure, which is much higher than that for monolayer WS 2 on SiO 2 /Si substrate with a valley polarization of 15%. Furthermore, the greatly enhanced valley polarization can be maintained to a high temperature of about 160 K with a valley polarization of 53%. The temperature dependence of valley polarization is strongly correlated with the thermomagnetic curve of LaMnO 3 , indicating an exciton-magnon coupling between WS 2 and LaMnO 3 . A simple model is introduced to illustrate the underlying mechanisms. The coupling of WS 2 and LaMnO 3 is further confirmed with an observation of two interlayer excitons with opposite valley polarizations in the heterostructure, resulting from the spin-orbit coupling induced splitting of the conduction bands in monolayer transition metal dichalcogenides. The results provide a pathway to control the valleytronic properties of transition metal dichalcogenides by means of ferromagnetic van der Waals engineering, paving a way to practical valleytronic applications.