Transition-metal dichalcogenide bilayers: switching materials for spin-   and valleytronic applications
  We report that an external electric field applied normal to bilayers of transition-metal dichalcogenides TX2, M = Mo, W, X = S, Se, creates significant spin-orbit splittings and reduces the electronic band gap linearly with the field strength. Contrary to the TX2 monolayers, spin-orbit splittings and valley polarization are absent in bilayers due to the presence of inversion symmetry. This symmetry can be broken by an electric field, and the spin-orbit splittings in the valence band quickly reach similar values as in the monolayers (145 meV for MoS2... 418 meV for WSe2) at saturation fields less than 500 mV A-1. The band gap closure results in a semiconductor-metal transition at field strength between 1.25 (WX2) and 1.50 (MoX2) V A-1. Thus, by using a gate voltage, the spin polarization can be switched on and off in TX2 bilayers, thus activating them for spintronic and valleytronic applications. 
Nourdine Zibouche, Pier Philipsen, Agnieszka Kuc, Thomas Heine
cond-mat.mtrl-sci