Strain-Modulated Perfect Valley Precession and Valley Transistor in Graphene

We propose the realization of perfect valley precession by use of strain, which gives rise to an effective vector potential and moves two valleys in opposite directions in momentum space. It is shown that the spatial precession period is independent of both the Fermi energy and the transverse wave vector. This is crucial to perfect precession in a practical device with multiple transverse wave vectors or modes. To theoretically demonstrate the perfect valley precession, we investigate the conductivity of a Kekul\'e graphene superlattice/graphene/Kekul\'e graphene superlattice junction that acts as a valley transistor. Due to the valley precession, the conductivity exhibits perfect oscillations with the strain and the conductivity minima are very close to zero. Our finding provides a strategy to design ideal valleytronics and straintronics devices.