Landau quantization and the thickness limit of topological insulator thin films of Sb ₂Te ₃

We report the experimental observation of Landau quantization of molecular beam epitaxy grown Sb ₂Te ₃ thin films by a low-temperature scanning tunneling microscope. Different from all the reported systems, the Landau quantization in a Sb ₂Te ₃ topological insulator is not sensitive to the intrinsic substitutional defects in the films. As a result, a nearly perfect linear energy dispersion of surface states as a 2D massless Dirac fermion system is achieved. We demonstrate that four quintuple layers are the thickness limit for a Sb ₂Te ₃ thin film being a 3D topological insulator. The mechanism of the Landau-level broadening is discussed in terms of enhanced quasiparticle lifetime.