  The band structure of transition metal dichalcogenides (TMDCs) with valence
band edges at different locations in the momentum space could be harnessed to
build devices that operate relying on the valley degree of freedom. To realize
such valleytronic devices, it is necessary to control and manipulate the charge
density in these valleys, resulting in valley polarization. While this has been
demonstrated using optical excitation, generation of valley polarization in
electronic devices without optical excitation remains difficult. Here, we
demonstrate spin injection from a ferromagnetic electrode into a heterojunction
based on monolayers of WSe2 and MoS2 and lateral transport of spin-polarized
holes within the WSe2 layer. The resulting valley polarization leads to
circularly polarized light emission which can be tuned using an external
magnetic field. This demonstration of spin injection and magnetoelectronic
control over valley polarization provides a new opportunity for realizing
combined spin and valleytronic devices based on spin-valley locking in
semiconducting TMDCs.
