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

Li-Qiao Xia; Sergio C. de la Barrera; Aviram Uri; Aaron Sharpe; Yves H. Kwan; Ziyan Zhu; Kenji Watanabe; Takashi Taniguchi; David Goldhaber-Gordon; Liang Fu; Trithep Devakul; Pablo Jarillo-Herrero

doi:10.1038/s41567-024-02731-6

paper

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

arXiv:2310.12204 · doi:10.1038/s41567-024-02731-6

Abstract

Quantum geometry of electronic wavefunctions results in fascinating topological phenomena. A prominent example is the intrinsic anomalous Hall effect (AHE) in which a Hall voltage arises in the absence of an applied magnetic field. The AHE requires a coexistence of Berry curvature and spontaneous time-reversal symmetry breaking. These conditions can be realized in two-dimensional moiré systems with broken $x y$ -inversion symmetry ( $C_{2 z}$ ) that host flat electronic bands. Here, we explore helical trilayer graphene (HTG), three graphene layers twisted sequentially by the same angle forming two misoriented moiré patterns. Although HTG is globally $C_{2 z}$ -symmetric, surprisingly we observe clear signatures of topological bands. At a magic angle $θ_{m} \approx 1. 8^{\circ}$ , we uncover a robust phase diagram of correlated and magnetic states using magnetotransport measurements. Lattice relaxation leads to large periodic domains in which $C_{2 z}$ is broken on the moiré scale. Each domain harbors flat topological bands with valley-contrasting Chern numbers $\pm (1, - 2)$ . 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 $ν = 1$ , a time-reversal symmetric phase appears beyond a critical electric displacement field, indicating a topological phase transition. Finally, hysteresis upon sweeping $ν$ points to first-order phase transitions across a spatial mosaic of Chern domains separated by a network of topological gapless edge states. 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.

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

Abstract

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