Modulation of Fermi velocities of Dirac electrons in single layer graphene by moiré superlattice

Study of electronic properties of graphene on an anisotropic crystal substrate including boron nitride (BN) has raised significant interest recently due to the application of graphene based vertical hetero-devices. We have performed scanning tunneling microscopy (STM) and scanning tunneling spectroscopy studies of single-layer graphene on hexagonal BN (h-BN) substrates with an applied perpendicular magnetic field at low temperature. Different periodic moiré superlattices can be resolved with STM, and their quantized Landau levels in high magnetic field are investigated. The renormalized Fermi velocities of massless Dirac fermions in graphene are revealed to show dependent on the moiré superlattice. Density functional theory calculation verifies that the interlayer interaction between graphene and h-BN is stronger with smaller twisting angle between lattices of graphene and h-BN, thus, leading to a reduction in the velocity of charge carriers. Our results should provide valuable insight of electronic properties and device performance of graphene on crystal substrates.