Charged Impurity Scattering in Graphene

TL;DR

This paper experimentally investigates how charged impurities affect graphene's conductivity, confirming theoretical predictions about impurity scattering, asymmetry, and the minimum conductivity's dependence on impurity density and potential inhomogeneity.

Contribution

It provides experimental evidence on charged impurity effects in graphene, including impurity density variation and the resulting impact on conductivity and minimum conductivity.

Findings

Charged impurity scattering produces linear conductivity with predicted magnitude.

Asymmetry between attractive and repulsive impurity scattering is observed.

Minimum conductivity occurs at non-zero impurity density, not at neutrality point.

Abstract

Since the experimental realization of graphene1, extensive theoretical work has focused on short-range disorder2-5, ''ripples''6, 7, or charged impurities2, 3, 8-13 to explain the conductivity as a function of carrier density sigma_(n)[1,14-18], and its minimum value sigma_min near twice the conductance quantum 4e2/h[14, 15, 19, 20]. Here we vary the density of charged impurities nimp on clean graphene21 by deposition of potassium in ultra high vacuum. At non-zero carrier density, charged impurity scattering produces the ubiquitously observed1, 14-18 linear sigma_(n) with the theoretically-predicted magnitude. The predicted asymmetry11 for attractive vs. repulsive scattering of Dirac fermions is observed. Sigma_min occurs not at the carrier density which neutralizes nimp, but rather the carrier density at which the average impurity potential is zero10. Sigma_min decreases initially with nimp, reaching a minimum near 4e2/h at non-zero nimp, indicating that Sigma_min in present experimental samples does not probe Dirac-point physics14, 15, 19, 20 but rather carrier density inhomogeneity due to the impurity potential3, 9, 10.