Heterostrain engineering on twisted graphene bilayer around the first magic angle

Abstract: Very recently, twisted graphene bilayer (TGB) around the first magic angle 1.1{\deg} has attracted much attention for the realization of exotic quantum states, such as correlated insulator behavior and unconventional superconductivity. Here we elaborately studied a series of TGBs around the first magic angle engineered by heterostrain, where each layer is strained independently. Our experiment indicated that a moderate heterostrain enables the structural evolution from the small-angle TGB ( ~ 1.5{\deg}) to the strained magic-angle TGB ({\theta} ~ 1.1{\deg}), exhibiting the characteristic low-energy flat bands. The heterostrain can even drive the system into highly strained tiny-angle TGBs ({\theta} << 1.1{\deg}) with large deformed tetragonal superlattices, where a unique network of topological helical edge states emerges. Furthermore, the predicted domain wall modes, which are strongly localized and result in hexagon-triangle-mixed frustrated lattice derived from Kagome lattice, are observed in the strained tiny-angle TGBs.