Interactions between TMDCs and ferromagnetic EuS

Monolayer transition metal dichalcogenides are of great interest in valleytronics application. The broken inversion symmetry leads to two degenerate but inequivalent valleys K and K’, which provide unique opportunities for valley control through helicity of light. Lifting the valley degeneracy would enable the control of valley degree of freedom by an electric field. Recently we and others demonstrated that, by exploiting the exchange field from a ferromagnetic substrate, the valley splitting of TMDs such as WSe₂ can be strongly enhanced. In this work, we explore the different effects of exchange field on two types of TMDs: WSe₂ vs WS₂. For both materials, we observed enhanced valley splitting; however, the signs of the valley splitting are opposite for WS₂ and WSe₂. The slope of the valley splitting for the A exciton of WSe₂ is about 3meV/T, while WS₂ show a value of -5meV/T. We further used X-ray magnetic circular dichroism to probe the induced magnetic moment in TMDs. Finally we report our effort to tune the sign and magnitude of valley splitting by anion alloying of S and Se. The interactions between TMDCs and ferromagnetic substrates provide attractive tuning knobs for valley control.