Nature Phys. http://doi.org/kdh (2013)

Molybdenum disulphide (MoS2) has a layered structure reminiscent of graphite that makes it an ideal dry lubricant. Latterly there has also been increasing interest in its electronic properties, as these share large similarities with, and may even offer some tantalizing advantages over, those of graphene. The key difference between the two is that MoS2 is characterized by a honeycomb structure in which there are two atoms with different symmetry. This offers an additional degree of freedom in the form of quantized momentum valleys in the electronic structure, which can be selectively excited by circularly polarized light. Xiaodong Xu and colleagues now show this difference in symmetry can also be harnessed in bilayer MoS2, by demonstrating that the circularly polarized photoluminescence can be continuously tuned using an electric field. A direct consequence of this is that the magnetic moment associated with the valley also changes sign from a positive to a negative value.