Valleytronics: Fundamental Challenges and Materials Beyond Transition Metal Chalcogenides.

Valleytronics, harnessing the valley degree of freedom in the momentum space, is a potential energy-efficient approach for information encoding, manipulation, and storage. Valley degree of freedom exists in a few conventional semiconductors, but recently the emerging 2D materials, such as monolayer transition-metal dichalcogenides (TMDs), are considered more ideal for valleytronics, due to the additional protection from spin-valley locking enabled by their inversion symmetry breaking and large spin-orbit coupling. However, current limitations in the valley lifetime, operation temperature, and light-valley conversion efficiency in existing materials encumber the practical applications of valleytronics. In this article, the valley depolarization mechanisms and recent progress of novel materials are systematically reviewed for valleytronics beyond TMDs. Valley physics is first reviewed and the factors determining the valley lifetime, including the intrinsic electron-electron and electron-lattice interactions, as well as extrinsic defect effects. Then, experimentally demonstrated and theoretically proposed valley materials are introduced which potentially improve valley properties through the changes of spin-orbit coupling, electronic interactions, time-reversal symmetry, structures, and defects. Finally, the challenges and perspectives are summarized to realize valleytronic devices in the future.