Helical Edge States and Quantum Phase Transitions in Tetralayer Graphene

TL;DR

This study reveals multiple topologically distinct helical edge states and quantum phase transitions in tetralayer graphene, demonstrating complex symmetry competition and rich quantum phases influenced by magnetic and electric fields.

Contribution

It provides the first experimental observation of two topologically distinct phases with helical edge states in tetralayer graphene and develops a theoretical model explaining their conductance.

Findings

Identification of insulating and two metallic phases with different numbers of helical edge states

Observation of phase transitions driven by magnetic and displacement fields

Correlation of conductance with spin-polarization plateaus

Abstract

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.