A wide variety of van der Waals (vdW) materials, most famously graphene, can be mechanically isolated down to a single atomic layer. These atomically thin crystals can further be stacked together to create designer heterostructures with a wide variety of emergent properties. Furthermore, lattice mismatch and/or an interlayer twist between neighboring crystals leads to the emergence of a geometric interference pattern which acts as a long-wavelength synthetic superlattice. This so-called moire superlattice significantly reconstructs the structural and electronic properties of the composite material. In this talk I will discuss our investigation of monolayer graphene (a conductive monolayer sheet of carbon atoms arranged in a hexagonal lattice) encapsulated between two crystals of boron nitride (BN – insulating layers of alternating nitrogen and boron atoms arranged in a hexagonal lattice). In our BN-encapsulated graphene field effect transistor device, the rotational alignment between all three components – and the resulting moire superlattices – can be varied. While changing the rotation angle between layers and simultaneously measuring electronic transport in the graphene, we detect a tunable transition between the absence or presence of inversion symmetry in the overall heterostructure. Additionally, we find that smaller-than-one-degree deviations from perfect alignment of all three layers leads to detectable coexisting long-wavelength moire superlattice potentials. Our results demonstrate that the interplay between multiple moire patterns can be utilized to controllably tune crystal symmetry, and thus significantly modify the physical properties of the composite BN-encapsulated graphene heterostructure.
