Imaging moir\'e deformation and dynamics in twisted bilayer graphene

TL;DR

This study uses advanced microscopy to analyze the spatial and temporal variations of moiré patterns in twisted bilayer graphene, revealing minimal variation, thermal fluctuations, and the presence of topological defects, which impact local electronic properties.

Contribution

We provide detailed spatial and temporal analysis of moiré patterns in TBG, demonstrating reduced variation, thermal fluctuation behavior, and identifying edge dislocations that influence electronic properties.

Findings

Lower spatial variation than previously reported

Thermal fluctuations cause collective atomic displacements under 70pm

Presence of edge dislocations affecting local electronic properties

Abstract

In twisted bilayer graphene (TBG) a moir\'e pattern forms that introduces a new length scale to the material. At the 'magic' twist angle of 1.1{\deg}, this causes a flat band to form, yielding emergent properties such as correlated insulator behavior and superconductivity [1-4]. In general, the moir\'e structure in TBG varies spatially, influencing the local electronic properties [5-9] and hence the outcome of macroscopic charge transport experiments. In particular, to understand the wide variety observed in the phase diagrams and critical temperatures, a more detailed understanding of the local moir\'e variation is needed [10]. Here, we study spatial and temporal variations of the moir\'e pattern in TBG using aberration-corrected Low Energy Electron Microscopy (AC-LEEM) [11,12]. The spatial variation we find is lower than reported previously. At 500{\deg}C, we observe thermal fluctuations of the moir\'e lattice, corresponding to collective atomic displacements of less than 70pm on a time scale of seconds [13], homogenizing the sample. Despite previous concerns, no untwisting of the layers is found, even at temperatures as high as 600{\deg}C [14,15]. From these observations, we conclude that thermal annealing can be used to decrease the local disorder in TBG samples. Finally, we report the existence of individual edge dislocations in the atomic and moir\'e lattice. These topological defects break translation symmetry and are anticipated to exhibit unique local electronic properties.