Determination of minimum conductivity of graphene from contactless microwaves measurements

We report a non-direct method to determine the minimum conductivity (σ_min) of graphene. The proposed technique is based on the prior characterization of graphene sheet impedance at microwave frequencies using waveguides, and on the subsequent extraction of σ_min from the measured scattering rate. This approach presents some advantages as compared with standard DC characterization methods, as it is inherently contactless and does not require graphene patterning. Our experimental study estimates a σ_min in the range of 2e²/h-4e²/h for different samples. We also compute the density of charged impurities of our samples, and show that they are related to graphene conductivity at its Dirac point.