LEEM imaging of the moiré pattern of twisted bilayer graphene

The discovery that magic angle twisted bilayer graphene (MABLG) is a superconductor, yields the promise of exciting new solid state physics [1]. However, the influence of inhomogeneity of twist angle, strain and defects on charge transport properties in these exfoliated, torn and stacked flakes remains an important open question.
Here, we demonstrate that Low Energy Electron Microscopy (LEEM) can directly image MABLG moiré patterns on the full device scale, identifying specific areas of the magic twist angle. This has enabled efficient Nano-ARPES measurements confirming the existence of flat conduction bands [2]. Furthermore, we compare monolayer-on-monolayer to bilayer-on-bilayer graphene, mapping the moiré pattern at 2 nm resolution over large areas of several micrometers. Using this data, local variations in twist angle and strain are extracted by geometric phase analysis [3]. The direct observability of these properties establishes the potential of LEEM to this field of physics.

[1] Y. Cao, et al., Nature 556.7699 (2018): 43-50.
[2] S. Lisi, et al., Nat. Phys. (2020) doi:10.1038/s41567-020-01041-x
[3] T. Benschop, et al., arXiv:2008.13766