Sublattice site dependence of local electronic states in superstructures of CO built on a Cu(111) surface
AbstractA two-dimensional electron gas interacting with an external periodic potential attracts attention as a designable artificial material to explore topological phases. Here, to introduce a periodic potential into a Shockley state, superstructures of CO molecules have been fabricated on a Cu(111) surface by atom manipulation with a low-temperature scanning tunneling microscope. Local electronic states have been investigated in relation to specific locations on the CO triangular lattice. All tunneling spectra of the lattice exhibit a reduction at the bottom energy of the surface-state band, which reflects absorption of surface-state electrons into the bulk. For an (8×8) CO structure, spectra measured at positions corresponding to two equivalent triangular sublattices of artificial graphene have different features near the Fermi level. This sublattice site dependence is not observed for a (6×6) structure. First-principles calculations for a (4×4) structure have reproduced the local density of states that depend on the sublattice sites. These results can be understood that coupling between the surface state and the bulk is strengthened via scattering by the adsorbates, and that the external periodic potential is perturbed by the second layer of the Cu(111) substrate. The periodicity of the external potential appears to be the key parameter which dominates the equivalence of pseudospin states in the artificial graphene.