Unexpected symmetry and AA stacking of bilayer silicene onAg(111)

Abstract

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 $\mathrm{Ag}(111)$ surface with significant differences to the monolayer case. We find $(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})R{30}^{\ensuremath{\circ}}$ bilayer silicene on $(\sqrt{19}\ifmmode\times\else\texttimes\fi{}\sqrt{19})R23.{4}^{\ensuremath{\circ}}$ silver substrate to be the most stable. The calculated STM images, however, exhibit a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ensuremath{\circ}}$ 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.