Interfacing 2D and 3D Topological Insulators: Bi(111) Bilayer onBi2Te3

We report the formation of a bilayer Bi(111) ultrathin film, which is theoretically predicted to be in a two-dimensional quantum spin Hall state, on a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ substrate. From angle-resolved photoemission spectroscopy measurements and ab initio calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one- and two-dimensional edge states are coexisting at the surface.