▎ 摘　　要

It is a fact that the minimal conductivity sigma(0) of most graphene samples is larger than the well-established universal value for ideal graphene 4e(2)/pi h; in particular, larger by a factor greater than or similar to pi. Despite intense theoretical activity, this fundamental issue has eluded an explanation so far. Here we present fully atomistic quantum mechanical estimates of the graphene minimal conductivity where electron-electron interactions are considered in the framework of density-functional theory. We show the conclusive evidence of the dominant role on the minimal conductivity of charged impurities over ripples, which have no visible effect. Furthermore, in combination with the logarithmic scaling law for diffusive metallic graphene, we elucidate the origin of the ubiquitously observed minimal conductivity in the range 8e(2)/h > sigma(0) greater than or similar to 4e(2)/h.