Strain fields in twisted bilayer graphene

TL;DR

This paper introduces a novel Bragg interferometry technique using 4D STEM to map atomic displacement and strain fields in twisted bilayer graphene, revealing complex relaxation regimes and strain effects influencing electronic properties.

Contribution

It provides the first detailed atomic-scale mapping of strain and relaxation in twisted bilayer graphene, uncovering two distinct structural relaxation regimes and strain accumulation patterns.

Findings

Discovered short-range disorder and strain fluctuations in twisted bilayer graphene.

Identified two regimes of structural relaxation differing from previous models.

Mapped strain tensor fields revealing anisotropic strain accumulation.

Abstract

Van der Waals heteroepitaxy allows deterministic control over lattice mismatch or azimuthal orientation between atomic layers to produce long wavelength superlattices. The resulting electronic phases depend critically on the superlattice periodicity as well as localized structural deformations that introduce disorder and strain. Here, we introduce Bragg interferometry, based on four-dimensional scanning transmission electron microscopy, to capture atomic displacement fields in twisted bilayer graphene with twist angles < 2{\deg}. Nanoscale spatial fluctuations in twist angle and uniaxial heterostrain are statistically evaluated, revealing the prevalence of short-range disorder in this class of materials. By quantitatively mapping strain tensor fields we uncover two distinct regimes of structural relaxation -- in contrast to previous models depicting a single continuous process -- and we disentangle the electronic contributions of the rotation modes that comprise this relaxation. Further, we find that applied heterostrain accumulates anisotropically in saddle point regions to generate distinctive striped shear strain phases. Our results thus establish the reconstruction mechanics underpinning the twist angle dependent electronic behaviour of twisted bilayer graphene, and provide a new framework for directly visualizing structural relaxation, disorder, and strain in any moir\'e material.