In-plane critical magnetic fields in magic-angle twisted trilayer graphene

TL;DR

This paper explains why magic-angle twisted trilayer graphene exhibits exceptionally high in-plane critical magnetic fields, attributing it to a specific symmetry that persists under magnetic fields, unlike in bilayer graphene.

Contribution

It identifies the role of $ ext{C}_2 ext{M}_h$ symmetry in enabling high in-plane critical magnetic fields in twisted trilayer graphene.

Findings

Symmetry $ ext{C}_2 ext{M}_h$ persists in trilayers under magnetic fields.

Displacements between layers affect the critical magnetic field.

Electric fields break the symmetry, reducing the critical magnetic field.

Abstract

It has recently been shown that superconductivity in magic-angle twisted trilayer graphene survives to in-plane magnetic fields that are well in excess of the Pauli limit, and much stronger than the in-plane critical magnetic fields of magic-angle twisted bilayer graphene. The difference is surprising because twisted bilayers and trilayers both support the magic-angle flat bands thought to be the fountainhead of twisted graphene superconductivity. We show here that the difference in critical magnetic fields can be traced to a $\mathcal{C}_2 \mathcal{M}_{h}$ symmetry in trilayers that survives in-plane magnetic fields, and also relative displacements between top and bottom layers that are not under experimental control at present. An gate electric field breaks the $\mathcal{C}_2 \mathcal{M}_{h}$ symmetry and therefore limits the in-plane critical magnetic field.