In twisted bilayer graphene (TBG) a moire 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 moire 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 moire variation is needed [10]. Here, we study spatial and temporal variations of the moire 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 moire 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 moire lattice. These topological defects break translation symmetry and are anticipated to exhibit unique local electronic properties.
تحميل البحث