Measuring local moir\'e lattice heterogeneity of twisted bilayer graphene

TL;DR

This paper presents a new imaging method to map local heterogeneity in moiré lattices of twisted bilayer graphene, revealing that twist angle variations and local strain are comparably influential on electronic properties.

Contribution

The authors introduce a continuous mapping technique for moiré lattice inhomogeneities, applicable across various imaging methods and moiré materials, and quantify twist angle heterogeneity in TBG.

Findings

Twist angle variation is about 0.08° around 2.02° on large areas.

Twist angle heterogeneity and local strain have similar effects on electronic structure.

Method is applicable to different imaging techniques and moiré materials.

Abstract

We introduce a new method to continuously map inhomogeneities of a moir\'e lattice and apply it to large-area topographic images we measure on open-device twisted bilayer graphene (TBG). We show that the variation in the twist angle of a TBG device, which is frequently conjectured to be the reason for differences between devices with a supposed similar twist angle, is about 0.08{\deg} around the average of 2.02{\deg} over areas of several hundred nm, comparable to devices encapsulated between hBN slabs. We distinguish between an effective twist angle and local anisotropy and relate the latter to heterostrain. Our results imply that for our devices, twist angle heterogeneity has a roughly equal effect to the electronic structure as local strain. The method introduced here is applicable to results from different imaging techniques, and on different moir\'e materials.