Visualization of thickness dependent inter-layer backscattering of the topological surface state in Bi2Te3 thin films

In topological insulator thin films, the tunneling between the top and bottom surface states can break the spin helicity of Dirac cone and open a gap. Consequently, the forbidden backscattering for intact Dirac cone will be enabled, which serves as microscopic evidence of interlayer coupling. Here using low-temperature STM, we systematically investigate thickness dependence of quasiparticle interference (QPI) near step edges and point defects in Bi₂Te₃ thin films. A clear backscattering signal along ΓM is observed for 2QL film but vanished in 4 QL film, in strong agreement with the Dirac cone gap evolution extracted form QPI dispersion. Meanwhile, the backscattering along ΓK direction persists up to 7 QL and reveals rich scattering features. Furthermore, the reopened backscattering channels is found to obey an r^-0.5 power-law decay, similar to conventional surface state. Our study provides, for the first time, a direct microscopic visualization of the gap opening mechanism in the Dirac cone of topological insulator thin films.