  The valley degree of freedom of electrons in solids has been proposed as a
new type of information carriers beyond the electronic charge and spin. Recent
experimental demonstrations of the optical orientation of the valley
polarization and generation of the valley current through the valley Hall
effect in monolayer MoS2 have shown the potential of two-dimensional
semiconductor transition metal dichalcogenides for valley based electronic and
optoelectronic applications. The valley Hall conductivity in monolayer MoS2, a
non-centrosymmetric crystal, however, cannot be easily tuned, presenting a
challenge for valley-based applications. Here we report the control of the
valley Hall effect in bilayer MoS2 transistors through a gate. The inversion
symmetry present in bilayer MoS2 was broken by the gate applied electric field
perpendicular to the plane. The valley polarization near the edges of the
device channels induced by the longitudinal electrical current was imaged by
use of Kerr rotation microscopy. The polarization is out-of-plane, has opposite
sign for the two edges, and is strongly dependent on the gate voltage. The
observation is consistent with the symmetry dependent Berry curvature and
valley Hall conductivity in bilayer MoS2. Our results are another step towards
information processing based on the valley degree of freedom.
