GW calculations for Bi2Te3 and Sb2Te3 thin films: Electronic and topological properties

Employing the GW method, we discuss the electronic and topological properties of Bi2Te3 and Sb2Te3 thin films consisting of one to six quintuple layers (QLs). Although both bulk materials are three-dimensional topological insulators, the two-dimensional topological phases of their thin films differ. We find the nontrivial quantum spin Hall phase, together with a sizable band gap of 0.13 eV, for a Bi2Te3 film of 2 QL thickness, whereas the 2 QL Sb2Te3 film hosts a topologically trivial band structure. All our GW results are in excellent agreement with experiments. This concerns the dispersions of the highest valence bands and lowest conduction bands around (Gamma) over bar, the band gaps of thin films, and, in particular, the dispersion of the topological surface state and its energetic position relative to the bulk bands. A crucial technical issue of our study is that we go beyond the "one-shot" GW approach by diagonalizing the GW Hamiltonian. This yields improved quasiparticle wave functions and band structures. The physical origin of the respective off-diagonal elements in the GW Hamiltonian is analyzed in detail. Without the additional diagonalization, the GW band structures are unphysical for many film thicknesses.