Giant Atomic Swirl in Graphene Bilayers with Biaxial Heterostrain

F Mesple and NR Walet and GT de Laissardiere and F Guinea and D Dosenovic and H Okuno and C Paillet and A Michon and C Chapelier and VT Renard, ADVANCED MATERIALS, 35 (2023).

DOI: 10.1002/adma.202306312

The study of moire engineering started with the advent of van der Waals heterostructures, in which stacking 2D layers with different lattice constants leads to a moire pattern controlling their electronic properties. The field entered a new era when it was found that adjusting the twist between two graphene layers led to strongly-correlated- electron physics and topological effects associated with atomic relaxation. A twist is now routinely used to adjust the properties of 2D materials. This study investigates a new type of moire superlattice in bilayer graphene when one layer is biaxially strained with respect to the other-so-called biaxial heterostrain. Scanning tunneling microscopy measurements uncover spiraling electronic states associated with a novel symmetry-breaking atomic reconstruction at small biaxial heterostrain. Atomistic calculations using experimental parameters as inputs reveal that a giant atomic swirl forms around regions of aligned stacking to reduce the mechanical energy of the bilayer. Tight-binding calculations performed on the relaxed structure show that the observed electronic states decorate spiraling domain wall solitons as required by topology. This study establishes biaxial heterostrain as an important parameter to be harnessed for the next step of moire engineering in van der Waals multilayers. Biaxial heterostrain in bilayer graphene leads to a new atomic lattice relaxation to minimize the stacking energy. Elastic energy is stored in solitons hosting topological states. This demonstrates that heterostrain can be an additional tuning parameter in moire materials opening new possibilities for moire engineering.image

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