Heterostrain engineering on twisted graphene bilayer around the first magic angle

TL;DR

This study demonstrates how heterostrain engineering in twisted graphene bilayers around the first magic angle can induce structural evolution, flat bands, and topological edge states, revealing new quantum phenomena.

Contribution

It provides experimental evidence that heterostrain can tune the electronic and topological properties of twisted bilayer graphene near the magic angle.

Findings

Heterostrain enables transition from small-angle to magic-angle TGBs with flat bands.

Large deformed superlattices with topological edge states are observed.

Localized domain wall modes consistent with Kagome lattice are identified.

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.