Identifying the effect of hBN alignment on twisted bilayer graphene using scanning tunneling microscopy*

When two graphene layers are stacked with a small twist angle, a moire superlattice arises that modifies the electronic band structure. Specifically, at the magic angle (~1.1deg), the bands become flat. In these flat bands, various correlated electronic states emerge. In some samples, correlated insulating behaviors are observed at integer fillings, accompanied by superconducting states upon doping. In other samples, the quantum anomalous Hall effect replaces the correlated insulator at ¾ filling while superconductivity is absent. Despite the lack of direct experimental evidence, it has been suggested that the latter phase is produced by the alignment between graphene and the hBN substrate. Using a scanning tunneling microscope (STM), we extract the hBN alignment information from graphene-hBN moire lattice superimposed on the graphene-graphene moire lattice. Comparing spectra from different samples, we explore the effect of hBN alignment on the electronic properties of twisted bilayer graphene.