Unexpected symmetry and AA stacking of bilayer silicene on Ag(111)

We report results of ab initio density functional theory including van der Waals interaction for the formation of a silicene bilayer on top of the Ag(111) surface with significant differences to the monolayer case. We find (2 root 3 x 2 root 3) R30 degrees bilayer silicene on (root 19 x root 19) R23.4 degrees silver substrate to be the most stable. The calculated STM images, however, exhibit a (root 3 x root 3) R30 degrees symmetry. This translational symmetry, the resulting lattice spacing, and the height of the topmost monolayer agree with recent experimental findings. The band structure of the complete adsorbate system shows conical linear bands near the Fermi level due to the hybridization of adsorbate and substrate states.