Imaging moiré deformation and dynamics in twisted bilayer graphene

Abstract

In ‘magic angle’ twisted bilayer graphene (TBG) a flat band forms, yielding correlated insulator behavior and superconductivity. In general, the moiré structure in TBG varies spatially, influencing the overall conductance properties of devices. Hence, to understand the wide variety of phase diagrams observed, a detailed understanding of local variations is needed. Here, we study spatial and temporal variations of the moiré pattern in TBG using aberration-corrected Low Energy Electron Microscopy (AC-LEEM). We find a smaller spatial variation than reported previously. Furthermore, we observe thermal fluctuations corresponding to collective atomic displacements over 70 pm on a timescale of seconds. Remarkably, no untwisting is found up to 600 ∘C. We conclude that thermal annealing can be used to decrease local disorder. Finally, we observe edge dislocations in the underlying atomic lattice, the moiré structure acting as a magnifying glass. These topological defects are anticipated to exhibit unique local electronic properties.

Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2

“…While the excitonic phase in TMDCs is far more stable than that in semiconductor quantum wells, conventional probes of excitons run into several challenges when attempting to characterize the Mott transition. Spatially averaged techniques are prone to inhomogeneous broadening, a result of unavoidable nanoscale inhomogeneity, which emerges during fabrication of all van der Waals (vdW) materials. − Furthermore, techniques reliant on interband selection rules can easily present a distorted picture of the Mott transition, since only a tiny fraction of excitons, those that obey the selection rules, can contribute to the measured response. This restriction can easily obscure the complex interplay of intervalley scattering, − Auger recombination, , and interlayer tunneling within TMDCs. , …”

An Atomistic Insight into Moiré Reconstruction in Twisted Bilayer Graphene beyond the Magic Angle

“…It has been previously argued that, for a large twist angle, the gaining vdW energy cannot compensate for the decreasing intralayer elastic energy. 14,16,24 This results in no change of vdW stacking energy between rigid and relaxed structures, ultimately indicating the absence of reconstruction. However, our analysis of ILE over different θ values (Figure S6) clearly shows a small but relatively significant difference between the rigid and relaxed structures of higher θ TBGs.…”

“…Previous studies have attributed the minor atomic displacements of relaxed large θ TBGs to an insignificant change in the atomic registry of local domains, indicating the absence of reconstruction. [14][15][16]54 However, examining TBG systems from an atomistic perspective and employing our subdomain identification method, we show considerable changes in the local registries for large θ TBGs. We used the area fraction measurement to capture the structural changes in local domains of relaxed and unrelaxed geometries.…”

“…Consequently, the well-defined BLG stacking configurations of AA, AB, and SP types emerge and spatially vary throughout the superlattice. , Upon allowing atomic relaxation, an atomic-scale reconstruction occurs, and local stacked regions evolve to their true minimum local energy configuration. This process is known as atomic or moiré reconstruction. , Previous studies have reported this phenomenon for low angle TBGs, especially in the vicinity of or below the “magic angle” (θ m = 1.1°). , As the size of the MP shrinks with increasing θ and leaves less space for reconfiguration of atoms, experimental observation of moiré reconstruction becomes a challenge and is generally assumed to be absent for θ > 2°. ,, Since large angle TBGs contain the same atomic registry as small twist angles, it is unreasonable to expect moiré reconstruction to become absent suddenly. The interplay between the in-plane elastic energy and interlayer vdW energy is still expected to contribute to reconstruction at higher angles due to the same fundamental physics.…”

“… 14 , 15 As the size of the MP shrinks with increasing θ and leaves less space for reconfiguration of atoms, experimental observation of moiré reconstruction becomes a challenge and is generally assumed to be absent for θ > 2°. 14 , 16 , 17 Since large angle TBGs contain the same atomic registry as small twist angles, it is unreasonable to expect moiré reconstruction to become absent suddenly. The interplay between the in-plane elastic energy and interlayer vdW energy is still expected to contribute to reconstruction at higher angles due to the same fundamental physics.…”