Heterostrain engineering on twisted graphene bilayer around the first magic angle

Electronic properties of twisted graphene bilayer (TGB) are exquisitely sensitive to the twisted angle {\theta} between the layers1-17, which enables the realization of exotic quantum states in this unique family of materials. For example, correlated insulator behavior and unconventional superconductivity are observed very recently in the TGB13,14 around the first magic angle {\theta}~1.1{\deg}, attributing to the existence of low-energy flat bands in which electron-electron interactions dominate the electronic states of the system2-4,16-17. Here we studied effects of heterostrain, where each layer is strained independently, on the TGB around the first magic angle and demonstrated that both the structure and electronic properties of the TGB can be changed dramatically by a slight in-plane heterostrain. Our experiment indicated that a moderate heterostrain could drive an unstrained TGB with {\theta} ~1.5{\deg} into the strained magic-angle TGB, exhibiting the characteristic low-energy flat bands. By further increasing the heterostrain, the triangular moir\'e pattern in the TGB even becomes deformed tetragonal superlattice. Then, the inversion symmetry-broken gaps locally in AB/BA-stacked regions are observed and a network of topological helical edge states emerges in domain walls between the gapped AB and BA regions.