▎ 摘　　要

The response of Dirac fermions to a Coulomb potential is predicted to differ significantly from how non-relativistic electrons behave in traditional atomic and impurity systems(1-3). Surprisingly, many key theoretical predictions for this ultra-relativistic regime have not been tested(4-12). Graphene, a two-dimensional material in which electrons behave like massless Dirac fermions(13,14), provides a unique opportunity to test such predictions. Graphene's response to a Coulomb potential also offers insight into important material characteristics, including graphene's intrinsic dielectric constant(6,8), which is the primary factor determining the strength of electron-electron interactions in graphene(15). Here we present a direct measurement of the nanoscale response of Dirac fermions to a single Coulomb potential placed on a gated graphene device. Scanning tunnelling microscopy was used to fabricate tunable charge impurities on graphene, and to image electronic screening around them for a Q = +1 vertical bar e vertical bar charge state. Electron-like and hole-like Dirac fermions were observed to respond differently to a Coulomb potential. Comparing the observed electron-hole asymmetry to theoretical simulations has allowed us to test predictions for how Dirac fermions behave near a Coulomb potential, as well as extract graphene's intrinsic dielectric constant: epsilon(g) = 3. 0 +/- 1.0. This small value of epsilon(g) indicates that electron-electron interactions can contribute significantly to graphene properties.