  Electrons in 2-dimensional crystals with a honeycomb lattice structure
possess a new valley degree of freedom (DOF) in addition to charge and spin.
Each valley is predicted to exhibit a Hall effect in the absence of a magnetic
field whose sign depends on the valley index, but to date this effect has not
been observed. Here we report the first observation of this new valley Hall
effect (VHE). Monolayer MoS2 transistors are illuminated by circularly
polarized light which preferentially excites electrons into a specific valley,
and a finite anomalous Hall voltage is observed whose sign is controlled by the
helicity of the light. Its magnitude is consistent with theoretical predictions
of the VHE, and no anomalous Hall effect is observed in bilayer devices due to
the restoration of crystal inversion symmetry. Our observation of the VHE opens
up new possibilities for using the valley DOF as an information carrier in
next-generation electronics and optoelectronics.
