Valley degree of freedom in two-dimensional van der Waals materials

Layered materials can possess valleys that are indistinguishable from one another except for their momentum. These valleys are individually addressable in momentum space at the K and K ' points in the first Brillouin zone. Such valley addressability opens up the possibility of utilizing the momentum states of quasi-particles as a completely new paradigm in both quantum and classical information processing. This review focuses on the physics behind valley polarization and discusses carriers of valley degree of freedom (VDF) in layered materials. We then provide a detailed survey of simple spectroscopic techniques commonly utilized to identify and manipulate valley polarization in van der Waals layered materials. Finally, we conclude with recent developments in the manipulation of VDF for device applications and associated challenges.