Mapping Dirac quasiparticles near a single Coulomb impurity on graphene Citation

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. The response of Dirac fermions to a Coulomb potential is predicted to differ significantly from the behavior of non-relativistic electrons seen in traditional atomic and impurity systems 1-3. Surprisingly, many key theoretical predictions for this ultra-relativistic regime have yet to be tested in a laboratory 4-12. Graphene, a 2D material in which electrons behave like massless Dirac fermions 13-15 , provides a unique opportunity to experimentally test such predictions. The response of Dirac fermions to a Coulomb potential in graphene is central to a wide range of electronic phenomena and can serve as a sensitive probe of graphene's intrinsic dielectric constant 6,8 , the primary factor determining the strength of electron-electron interactions in this material 16. 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 tunneling microscopy and spectroscopy were used to fabricate tunable charge impurities on graphene and to measure how they are screened by Dirac fermions for a Q = +1|e| impurity charge state. Electron-like and hole-like Dirac fermions were observed to respond very differently to tunable Coulomb potentials. Comparison of this electron-hole asymmetry to theoretical 2 simulations has allowed us to test basic predictions for the behavior of Dirac fermions near a Coulomb potential and to extract the intrinsic dielectric constant of graphene: 3.0 1.0 g ε = ±. This small value of g ε indicates that microscopic electron-electron interactions can contribute significantly to graphene properties. Our experiment was performed using a scanning tunneling microscope (STM) in UHV at T = 4.8K to probe back-gated devices consisting of CVD-grown graphene 17 placed on top of boron nitride (BN) flakes 18 on a SiO 2 /Si surface (see Supplementary Materials for methods). Utilization of BN substrates significantly reduces the charge inhomogeneity of graphene 19,20 , thus allowing us to probe the intrinsic graphene electronic response to individual charged impurities. The charged impurities probed in this work were cobalt trimers constructed on graphene by atomically manipulating cobalt monomers with the tip of an STM 21 (cobalt atoms were deposited via e-beam evaporation onto low temperature graphene samples). Figs.1 a-f show the process of manipulating three cobalt monomers to create a single Co trimer on graphene (the detailed interior structure of the Co trimer cannot …