Helical Edge States and Quantum Phase Transitions in Tetralayer Graphene

Shi Che, Yanmeng Shi, Jiawei Yang, Haidong Tian, Ruoyu Chen, Takashi Taniguchi, Kenji Watanabe, Dmitry Smirnov, Chun Ning Lau, Efrat Shimshoni, Ganpathy Murthy, Herbert A. Fertig

Published 2020-06-16Version 1

Helical conductors with spin-momentum locking are promising platforms for Majorana fermions. Here we report observation of two topologically distinct phases supporting helical edge states in charge neutral Bernal-stacked tetralayer graphene in Hall bar and Corbino geometries. As the magnetic field B and out-of-plane displacement field D are varied, we observe a phase diagram consisting of an insulating phase and two metallic phases, with 0, 1 and 2 helical edge states, respectively. These phases are accounted for by a theoretical model that relates their conductance to spin-polarization plateaus. Transitions between them arise from a competition among inter-layer hopping, electrostatic and exchange interaction energies. Our work highlights the complex competing symmetries and the rich quantum phases in few-layer graphene.