Imaging supermoiré relaxation in helical trilayer graphene: Part I

In twisted van der Waals materials, the weak interlayer bonding allows for atomic-scale lattice relaxation driven by the competing energetics of local stacking configurations and strain. This can have profound effects on the electronic structure. Helical trilayer graphene (HTG), which consists of three graphene layers sequentially twisted by equal angles, is a model system in this context because it is predicted to undergo relaxation on both moiré and supermoiré (moiré-of-moiré) length scales. In this talk, I will describe imaging of HTG that reveals periodic modulations in the electronic structure at supermoiré length scales. Our measurements indicate reconstruction into domains of uniform moiré periodicity that locally break C_2z symmetry and host isolated flat bands. We show that domain size and shape can be tuned by strain while maintaining the same local properties within them. I will discuss how our results clarify the interplay between local atomic stacking, strain, and electronic states. Furthermore, the local C_2z symmetry breaking we observe and the near ideal quantum geometry of the flat bands make HTG a promising platform to realize interaction-driven topological states.