Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

We study the electronic band structure in the K/K′ valleys of the Brillouin zone of monolayer WSe₂ and MoSe₂ by optical reflection and photoluminescence spectroscopy on dual-gated field-effect devices. Our experiment reveals the distinct spin polarization in the conduction bands of these compounds by a systematic study of the doping dependence of the A and B excitonic resonances. Electrons in the highest-energy valence band and the lowest-energy conduction band have antiparallel spins in monolayer WSe₂ and parallel spins in monolayer MoSe₂. The spin splitting is determined to be hundreds of meV for the valence bands and tens of meV for the conduction bands, which are in good agreement with first-principles calculations. These values also suggest that both <i>n</i>- and <i>p</i>-type WSe₂ and MoSe₂ can be relevant for spin- and valley-based applications.