Giant pseudo-magnetic fields, valley polarization and topological channels by nanoscale strain engineering of monolayer graphene*

We report the use of nearly strain-free PECVD-grown graphene ^[1] to induce controllable strain and pseudo-magnetic fields (B_s) by placing graphene on periodic nanostructure arrays ^[2]. We fabricated these arrays using focused ion beam and electron-beam lithography. These nanostructures were covered by a monolayer h-BN followed by a monolayer graphene, we found that graphene appeared to wrinkle up along the nanostructures. Each wrinkle results in four parallel channels of alternating positive and negative pseudo-magnetic fields, which are natural topological channels for valley-polarized propagation. Properly designed arrays of nanostructures could induce the desirable B_S values and spatial distributions, which can function as a valley splitter to separate valley-unpolarized currents, or a valley propagator to guide valley-polarized currents. To enable valleytronic applications, we pattern strained graphene with these topological channels into Hall bar geometry to study the valley Hall effect.

^[1] D. A. Boyd et al. Nat. Comm. 6, 6620 (2015).
^[2] N.-C. Yeh et al. Acta Mech. Sin. 32, 497 (2016).