Experimental characterization and simulation of quasi-particle-interference in the Bi-bilayer topological insulator

Topological insulators are a new class of materials with a gapless surface state where spin and momentum are locked. A Bi-bilayer is predicted to be a 2D-topological insulator and well suited for scanning probe techniques that can be utilized to probe the topological edge states. Unfortunately there are only a few substrates that allow the growth of Bismuth in the rhombohedral structure, which is essential for the formation of the bilayer. Here we present a combined experimental and theoretical study of the quasi-particle interference (QPI) in the Bi-bilayer grown on the 3D-topological insulator Bi$_2$Se$_3$. Fourier-transform-scanning-tunneling-spectroscopy reveals additional features in QPI in comparison to a bare Bi$_2$Se$_3$ surface, indicating the development of new surface states below and above the Fermi energy. Via a comparison of measured QPI-patterns and simulated QPI-patterns based on DFT calculations, the bands participating in electron scattering are identified. DFT calculations further reveal a large influence of the bilayer-substrate-distance on the resulting band structure.