The valley degree of freedom of electrons in solids has been proposed as anew type of information carriers beyond the electronic charge and spin. Recentexperimental demonstrations of the optical orientation of the valleypolarization and generation of the valley current through the valley Halleffect in monolayer MoS2 have shown the potential of two-dimensionalsemiconductor transition metal dichalcogenides for valley based electronic andoptoelectronic applications. The valley Hall conductivity in monolayer MoS2, anon-centrosymmetric crystal, however, cannot be easily tuned, presenting achallenge for valley-based applications. Here we report the control of thevalley Hall effect in bilayer MoS2 transistors through a gate. The inversionsymmetry present in bilayer MoS2 was broken by the gate applied electric fieldperpendicular to the plane. The valley polarization near the edges of thedevice channels induced by the longitudinal electrical current was imaged byuse of Kerr rotation microscopy. The polarization is out-of-plane, has oppositesign for the two edges, and is strongly dependent on the gate voltage. Theobservation is consistent with the symmetry dependent Berry curvature andvalley Hall conductivity in bilayer MoS2. Our results are another step towardsinformation processing based on the valley degree of freedom.
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