Spin- and valley-coupled electronic states in monolayer WSe₂ on bilayer graphene

We have fabricated a high-quality monolayer WSe₂ film on bilayer graphene by epitaxial growth and revealed the electronic states by spin- and angle-resolved photoemission spectroscopy. We observed a direct energy gap at the Brillouin-zone corner in contrast to the indirect nature of gap in bulk WSe₂, which is attributed to the lack of interlayer interaction and the breaking of space-inversion symmetry in monolayer film. A giant spin splitting of ∼0.5 eV, which is the largest among known monolayer transition-metal dichalcogenides, is observed in the energy band around the zone corner. The present results suggest a high potential applicability of WSe₂ to develop advanced devices based with the coupling of spin- and valley-degrees of freedom.