Helical trilayer graphene: a moiré platform for strongly-interacting topological bands – Part 1

Topological bands in two dimensional materials can be promoted by broken xy-inversion (C_2z) symmetry. Existing examples include materials where the constituent layers break C_2z on their own. Here, we explore helical trilayer graphene (HTG), three graphene layers, each preserving C_2z on their own, twisted in sequence by the same angle. Unlike alternating-twist trilayer graphene, this forms two moiré patterns with different orientations. Although HTG is globally C_2z-symmetric, lattice relaxation leads to large periodic domains in which C_2z is broken on the moiré scale. Using magnetotransport, we observe the anomalous Hall effect – a clear signature of topological bands. At a magic angle of θ_m ≈ 1.8°, we uncover a robust phase diagram of correlated and magnetic states. Our results bring to light the importance of local symmetries on length scales comparable to the inter-particle distance, n^-1/2.