  Recent transport experiments have demonstrated that the rhombohedral stacking
trilayer graphene is an insulator with an intrinsic gap of 6meV and the Bernal
stacking trilayer one is a metal. We propose a Hubbard model with a moderate
$U$ for layered graphene sheets, and show that the model well explains the
experiments of the stacking dependent energy gap. The on-site Coulomb repulsion
drives the metallic phase of the non-interacting system to a weak surface
antiferromagnetic insulator for the rhombohedral stacking layers, but does not
alter the metallic phase for the Bernal stacking layers.
