  Exploiting the valley degree of freedom to store and manipulate information
provides a novel paradigm for future electronics. A monolayer transition metal
dichalcogenide (TMDC) with broken inversion symmetry possesses two degenerate
yet inequivalent valleys, offering unique opportunities for valley control
through helicity of light. Lifting the valley degeneracy by Zeeman splitting
has been demonstrated recently, which may enable valley control by a magnetic
field. However, the realized valley splitting is modest, (~ 0.2 meV/T). Here we
show greatly enhanced valley spitting in monolayer WSe2, utilizing the
interfacial magnetic exchange field (MEF) from a ferromagnetic EuS substrate. A
valley splitting of 2.5 meV is demonstrated at 1 T by magneto-reflectance
measurements. Moreover, the splitting follows the magnetization of EuS, a
hallmark of the MEF. Utilizing MEF of a magnetic insulator can induce magnetic
order, and valley and spin polarization in TMDCs, which may enable valleytronic
and quantum computing applications.
