Bulk and Edge Properties of Twisted Double-Bilayer Graphene

TL;DR

This paper demonstrates that twisted double bilayer graphene exhibits a topologically non-trivial phase with metallic edge states and insulating bulk, confirmed through combined experimental measurements and theoretical analysis.

Contribution

It provides the first combined experimental and theoretical evidence of topological edge states in twisted double bilayer graphene at charge neutrality.

Findings

Metallic edge transport observed alongside insulating bulk.

Quantitative analysis of edge state scattering using Landauer-Buttiker formalism.

Signatures consistent with a topologically non-trivial phase with Chern number two.

Abstract

The emergence of controlled, two-dimensional moir\'e materials has uncovered a new platform for investigating topological physics. Twisted double bilayer graphene (TDBG) has been predicted to host a topologically non-trivial gapped phase with Chern number equal to two at charge neutrality, when half the flat bands are filled. However, it can be difficult to diagnose topological states using a single measurement because it is ideal to probe the bulk and edge properties at the same time. Here, we report a combination of chemical potential measurements, transport measurements, and theoretical calculations that show that twisted double bilayer graphene can host metallic edge transport while simultaneously being insulating in the bulk. A Landauer-Buttiker analysis of measurements on multi-terminal samples allows us to quantitatively assess edge state scattering. We interpret these results as signatures of the predicted topological phase at charge neutrality.