Triangular antiferromagnetism on the honeycomb lattice of twisted bilayer graphene

TL;DR

This paper explores the electronic properties of twisted bilayer graphene, proposing a triangular antiferromagnetic order that explains observed insulating and metallic states, including topological features.

Contribution

It introduces a novel antiferromagnetic order on the honeycomb lattice that accounts for experimental observations in twisted bilayer graphene.

Findings

Identification of a correlated insulator with an energy gap

Observation of a doubly-degenerate Fermi surface upon doping

Discussion of metallic states with spin fluctuations and topological order

Abstract

We present the electronic band structures of states with the same symmetry as the three-sublattice planar antiferromagnetic order of the triangular lattice. Such states can also be defined on the honeycomb lattice provided the spin density waves lie on the bonds. We identify cases which are consistent with observations on twisted bilayer graphene: a correlated insulator with an energy gap, yielding a single doubly-degenerate Fermi surface upon hole doping. We also discuss extensions to metallic states which preserve spin rotation invariance, with fluctuating spin density waves and bulk $\mathbb{Z}_2$ topological order.