Spatially resolving density-dependent screening around a single charged atom in graphene

Due to the relativistic nature of its charge carriers, graphene has very unique screening properties. We have explored the screening of an individual Ca ion as a function of Dirac quasiparticle density in graphene by combining scanning tunneling microscopy (STM) with a gate-tunable graphene device. We find that the screening length in graphene is very gate-tunable, decreasing with increasing charge carrier density. Comparing our experimental results to tight-binding calculations provides new insight into electron-electron interactions in graphene, as well as the fundamental behavior of relativistic fermions in the presence of charged impurities.