▎ 摘　　要

We numerically calculate the conductivity sigma of an undoped graphene sheet (size L) in the limit of a vanishingly small lattice constant. We demonstrate one-parameter scaling for random impurity scattering and determine the scaling function ss(sigma)=dln sigma/dlnL. Contrary to a recent prediction, the scaling flow has no fixed point (ss > 0) for conductivities up to and beyond the symplectic metal-insulator transition. Instead, the data support an alternative scaling flow for which the conductivity at the Dirac point increases logarithmically with sample size in the absence of intervalley scattering-without reaching a scale-invariant limit.