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Navigating Impurities in Graphene

Above: Differential conductance maps of graphene near a cobalt trimer in different charge configurations showing variation in local electronic structure. Below: Spatial variation of differential conductance in experiment (left) is reproduced well by simulations (right).

MSD researchers Michael Crommie, Alex Zettl and coworkers have directly imaged how electrons respond to a charged impurity placed on electrically isolated graphene. The results shed light on the origins of graphene’s extraordinary mechanical and electronic properties.

Whereas electrons in "normal" materials (such as silicon) orbit charged impurities, electrons in graphene are predicted to simply bounce off weakly charged impurities. The researchers tested these predictions by using atomic manipulation to construct cobalt trimers on a layer of graphene: three-atom clusters whose charge can be toggled on and off through a back-gate electrode. They used a scanning-tunneling microscope to map the response of both electron-like and hole-like Dirac quasiparticles to the Coulomb potential created by the trimers. By comparing the observed electron-hole asymmetry to theoretical simulations, the team not only tested theoretical predictions for how Dirac fermions behave near a Coulomb potential, but also extracted graphene’s previously ambiguous dielectric constant.

Crommie and coworkers verified theoretical predictions for the subcritical impurity regime and found graphene's dielectric constant small enough to indicate that electron electron interactions contribute significantly to its properties – fundamental information for understanding how electrons move through graphene.

“Mapping Dirac quasiparticles near a single Coulomb impurity on graphene.” Y. Wang, V. W. Brar, A. V. Shytov, Q. Wu, William Regan, H.-Z. Tsai, A. Zettl, L. S. Levitov and M. F. Crommie. Nature Physics 8, 653–657 (2012). DOI: 10.1038/nphys2379