Infrared magneto-spectroscopy in the Hofstadter butterfly regime of graphene-boron nitride moire superlattices*

Graphene stacked on hexagonal boron nitride forms a moire superlattice system. Rotational angles close to zero degrees provide ideal moire length scales, of order ten nanometers, enabling experimental access to the fractal Hofstadter spectrum at lab scale magnetic fields. Using infrared magneto-spectroscopy, we report observations of cyclotron resonance transitions in a graphene-hexagonal boron nitride sample, having a relative alignment between the layers of ~0.9 degrees. The device shows strong satellite Dirac peaks in electronic transport that indicate it is in the Hofstadter regime. We observe splittings of the cyclotron resonance lines beyond what is seen in intrinsic monolayer graphene, likely to arise from the fractal energy levels.