Observation of spatially-varying Fermi velocity in strained-graphene directly grown on hexagonal boron nitride

Electronic properties of graphene can be controlled by using elastic strain. Theories predicted two major effects of strain, pseudo-magnetic field and spatially-varying Fermi velocity effects in graphene, but only the former was confirmed by experiments. Here, we report the experimental evidences of the latter, observed in strained graphene weakly coupled to a hexagonal boron nitride (BN) using scanning tunneling microscopy (STM) and spectroscopy (STS). V-shaped STS spectra showed spatially-varying slopes at lines across the strained ridges, whereas they showed constant slopes at lines along the long directions of ridges. The observed slope variations in the former were explained with the reduced Fermi velocity which is linear with the uniaxial strain less than 5% in agreement with theoretical predictions. Spatially-varying Fermi velocity that is controllable by local strain can provide an accessible component for the electronic engineering of graphene.