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Optical and transport gaps in gated bilayer graphene

We discuss the effect of disorder on the band gap measured in bilayer graphene in optical and transport experiments. By calculating the optical conductivity and density of states using a microscopic model in the presence of disorder, we demonstrate that the gap associated with transport experiments is smaller than that associated with optical experiments. Intrinsic bilayer graphene has an optical conductivity in which the energy of the peaks associated with the interband transition are very robust against disorder and thus provide an estimate of the band gap. In contrast, extraction of the band gap from the optical conductivity of extrinsic bilayer graphene is almost impossible for significant levels of disorder due to the ambiguity of the transition peaks. The density of states contains an upper bound on the gap measured in transport experiments, and disorder has the effect of reducing this gap which explains why these experiments have so far been unable to replicate the large band gaps seen in optical measurements.