Valley light-emitting transistor

In crystalline solids, it is often the case that the Fermi surface consists of multiple pockets at well-separated degenerate band extrema (that is, valleys) in momentum space. The valley index constitutes a discrete degree of freedom of carrier, just like spin. Exploiting valley in addition to spin will make future electronics more versatile. Two-dimensional (2D) transition metal dichalcogenides, a new class of direct-gap semiconductors,1, 2 have provided an appealing laboratory to explore valley-electronics, because of the discovery of a valley optical selection rule that allows optical control and detection of valley polarization.3 Iwasa from University of Tokyo, and Riken and his team have now demonstrated in 2D WSe2 the first electric control of valley-dependent optical emission.4