Mapping Dirac Quasiparticles near a Single Coulomb Impurity on Graphene

TL;DR

This study directly measures how Dirac fermions in graphene respond to a single Coulomb impurity, revealing electron-hole asymmetry and enabling extraction of graphene's intrinsic dielectric constant, thus testing fundamental relativistic quantum predictions.

Contribution

First experimental observation of Dirac fermions' response to a single Coulomb impurity in graphene, validating theoretical models and measuring the material's dielectric constant.

Findings

Electron-hole asymmetry in Coulomb response observed

Intrinsic dielectric constant of graphene measured as 3.0 ± 1.0

Significant electron-electron interactions inferred from small dielectric constant

Abstract

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. Surprisingly, many key theoretical predictions for this ultra-relativistic regime have yet to be tested in a laboratory. Graphene, a 2D material in which electrons behave like massless Dirac fermions, 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, the primary factor determining the strength of electron-electron interactions in this material. 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 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: {\epsilon}_g= 3.0 \pm 1.0. This small value of {\epsilon}_g indicates that microscopic electron-electron interactions can contribute significantly to graphene properties.