Imaging non-circular cyclotron orbits in a graphene/hBN superlattice

When graphene is rotationally aligned with an underlying hexagonal boron nitride (hBN) substrate, a superlattice naturally forms. This drastically alters graphene’s electronic band structure, leading to minibands whose structure enables us to electrostatically tune the Fermi-surface size and shape. A striking consequence of altering the Fermi-surface shape is that electrons are no longer expected to follow circular cyclotron motion in the presence of a magnetic field. Transverse electron focusing (TEF) measurements have shown signatures of these non-circular orbits in a graphene/hBN superlattice, however, a direct probe has been lacking.
Here, we image the shape of cyclotron orbits in an hBN/graphene superlattice by performing TEF measurements in a cryogenic scanning gate microscope (SGM). We study a hall-bar device with collimating contacts and use a home-built multi-terminal lock-in technique to efficiently determine where electrons are electrostatically deflected by the SGM tip. With this system, we resolve both circular and triangular orbits, in good agreement with expectations from band structure calculations.