Topologically Protected Helical States in Minimally Twisted Bilayer Graphene

The ability to create arbitrary stacking configurations of layered two-dimensional materials has opened the way to the creation of designer band structures. Twisted bilayer graphene is one of the simplest examples of such a van der Waals heterostructure where the electronic properties of the composite material can be fundamentally tuned with twist angle. The angle between the two graphene layers in the heterostructure produces a moiré pattern which affects its electronic properties. Using scanning tunneling microscopy and spectroscopy, we have investigated these minimally twisted bilayer graphene heterostructures. As the twist angle between the layers decreases there are fundamental changes in the electronic properties. We have found that the degeneracy of the low energy bands increases at twist angles below about 1 degree. For even smaller twist angles, a series of domain walls form connecting the AA sites in the moiré pattern. When an electric field is applied to these very small twist angle samples, the AB and BA regions develop band gaps while topologically protected states emerge along the domain walls. In this talk, we will discuss the fabrication of these minimally twisted heterostructures as well as our latest scanning probe microscopy results.