Quasiparticle scattering in three-dimensional topological insulators near the thickness limit

In the ultrathin regime, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ films feature two surfaces (with each surface being a two-dimensional Dirac-fermion system) with complicated spin textures and a tunneling term between them. We find in this regime that the quasiparticle scattering is completely different compared with the thick-film case and even behaves differently at each thickness. The thickness-dependent warping effect and tunneling term are found to be the two main factors that govern the scattering behaviors. The interband backscattering that signals the existence of a tunneling term is found to disappear at four quintuple layers by the step-edge reflection approach. A four-band model is presented that captures the main features of the thickness-dependent scattering behaviors. Our work clarifies that the prohibition of backscattering guaranteed by symmetry in topological insulators breaks down in the ultrathin regime.