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 VHE opens up new possibilities for using the valley DOF as an information carrier in next-generation electronics and optoelectronics. 
 
 
The charge and spin degrees of freedom (DOF) of electrons are at the heart of modern electronics. They form the basis for a wide range of applications such as transistors, photodetectors and magnetic memory devices. Interestingly, electrons in 2-dimensional (2D) crystals that have a honeycomb lattice structure possess an extra valley DOF (1) in addition to charge and spin. This new DOF has the potential to be used as an information carrier in nextgeneration electronics (2-6). Valley-dependent electronics and optoelectronics based on semimetallic graphene, a representative 2D crystal, have been theoretically proposed (2-5), but the presence of inversion symmetry in the crystal structure of pristine graphene makes both optical and electrical control of the valley DOF very difficult.  

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