Intrinsic gap and temperature collapse of the electric conductivity in bilayer graphene*

Recent experiments have reported signatures of electron-hole scattering in the electric conductivity of suspended bilayer graphene near charge neutrality. According to these experiments, plots of the electric conductivity as a function of μ/k_BT (chemical potential scaled with temperature) obtained for different temperatures in the range of 10K<T<50K collapse on a single curve independent of T. This puzzling observation has been taken [arXiv:1905.09835] as an indication that the relevant scattering mechanism (besides electron-hole scattering) is not electron-impurity but electron-phonon scattering. Here we demonstrate that the collapse can be explained without invoking electron-phonon scattering by taking into account the fact that the suspended bilayer graphene is not a truly gapless system. In the presence of a small gap the intrinsic Coulomb resistivity acquires a temperature dependence that compensates for the temperature dependence of the impurity resistivity. Our theory produces excellent agreement with the observed conductivity collapse over the full reported range of temperatures, with a gap of 5 mev.