Mapping Dirac Quasiparticles near a Single Coulomb Impurity on Graphene

We have locally mapped the response of charge carriers to a single Coulomb potential placed on a gated graphene device. Scanning tunneling microscopy and spectroscopy were used to fabricate a tunable charge impurity and to measure how Dirac fermions screen it. By mapping spatial variation in the electronic structure of graphene we have directly probed the strength of screened electronic interactions, obtaining a value of epsilon = 3 for the intrinsic graphene dielectric constant. This small value suggests that microscopic electron-electron interactions contribute significantly to intrinsic graphene properties.