Strain induced topological transitions in twisted double bilayer graphene

We theoretically study the band structures and the valley Chern numbers of the AB-AB and AB-BA stacked twisted double bilayer graphene under heterostrain effect. In the absence of heterostrain, due to the constrains by the spatial symmetries, the central two flat bands of the AB-AB are topological trivial bands, while in the AB-BA they have a finite Chern number. The heterostrain breaks all the point group symmetries and the constrains are lifted, hence the topological properties of the two arrangements can be tuned by different strain magnitudes epsilon and directions phi. The heterostrain has dissimilar impacts on the Chern numbers of the AB-AB and AB-BA, owing to their different band gaps, and these gaps can be modified by a vertical electric field. Our results show that the topological transitions for both arrangements occur in the epsilon range of 0.1%-0.4%, which can be realized in the graphene-based sample.