Topological bands and correlated states in helical trilayer graphene

The intrinsic anomalous Hall effect (AHE) indicates nonzero Berry curvature of electronic wavefunctions in solids and spontaneous time-reversal symmetry breaking. These conditions can be realized in two-dimensional moiré systems with broken xy-inversion symmetry (C_2z symmetry) that host flat electronic bands.

In this talk, I will present our recent work on helical trilayer graphene (HTG) – three graphene layers, each twisted in sequence by the same angle. Unlike alternating-twist trilayer graphene, this results in two moiré patterns with different orientations. Although HTG is globally C_2z-symmetric, surprisingly, we observe clear signatures of topological bands. Using magnetotransport measurements, we uncover a robust phase diagram of correlated and magnetic states at a magic angle θ_m ≈ 1.8°. We explain our observations in terms of lattice relaxation that leads to large periodic domains in which C_2z symmetry is broken on the moiré scale. Each domain harbors flat topological bands with valley-contrasting Chern numbers ±(1, -2). We find correlated states at integer electron fillings per moiré unit cell ν = 1, 2, 3 and fractional fillings 2/3, 7/2, with the AHE arising at ν = 1, 3 and 2/3, 7/2. At ν = 1, the absence of an AHE beyond a critical electric displacement field indicates a topological phase transition. Finally, our picture of domains with varying Chern numbers is supported by the observation of hysteretic electrical switching of the magnetic state.

We establish HTG as an important platform that realizes ideal conditions for exploring strongly interacting topological phases and, due to its emergent moiré-scale symmetries, demonstrates a novel way to engineer topology in a multi-moiré system.