报告摘要：

Graphene, a single atomic layer of carbon, has been arguably one of the hottest topics in condensed matter physics recently. This worldwide enthusiasm has been fueled by its novel electronic, optical and mechanical properties discovered in the past few years. Most strikingly, the electrons in graphene behave like two-dimensional Dirac fermions, which lead to unusual quantum Hall effect in high magnetic fields. Such quantum effect persists even at room temperature owing to graphene’s exceptionally high quality.

These collective phenomena originate from graphene’s unique honey-comb lattice and its intricate interplay with impurities and defects. Scanning tunneling microscopy and spectroscopy (STS) offer ideal tools to study the local electronic properties of graphene with unparalleled spatial resolution. Graphene keeps giving us surprises at such atomic length scale. We observe an unexpected energy gap feature in the graphene tunneling spectrum that is pinned to the Fermi level. This feature coexists with an additional depression in the graphene density-of-states (DOS) that shifts energetically with gate voltage. Our analysis reveals that the pinned gap-feature is due to phonon-assisted inelastic tunneling, while the DOS depression directly marks the location of the graphene Dirac point. Our ability to locally probe the energetic location of the Dirac point via STM spectroscopy allows us to spatially map out electron density inhomogeneities in graphene with a resolution that is two orders of magnitude higher than previous experiments. We observe charge inhomogeneities that coexist with energy-dependent electronic interference patterns, and which can be directly correlated with nanometer-scale topographic features, thus revealing new impurity scattering behavior for Dirac fermions in graphene. These results are significant for understanding the sources of electron density inhomogeneity and scattering in graphene, as well as the microscopic mechanisms that determine graphene electron mobility.