Transition-metal dichalcogenide bilayers: Switching materials for spintronic and valleytronic applications

We report that an external electric field applied normal to bilayers of transition-metal dichalcogenides TX₂ (T = 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 values similar to those in the monolayers (145 meV for MoS₂,. . . , 418 meV for WSe₂) at saturation fields less than 500 mV ^-1. The band gap closure results in a semiconductor-metal transition at field strength between 1. 25 (WX₂) and 1. 50 (MoX₂) V ^-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.