On possible deep subsurface states in topological insulators: The PbBi4Te7 system

Theoretical studies of the bulk and surface electronic structures of PbBi4Te7 are presented. The PbBi4Te7 compound has a layered structure of five-layer (Bi2Te3) and seven-layer (PbBi2Te4) blocks alternating along the hexagonal axis. Analysis of the spin-orbit-induced inversion of the band gap edges indicates that this compound is a three-dimensional topological insulator. The topological properties of this compound are mainly determined by the PbBi2Te4 blocks. The Dirac cone is formed on the PbBi4Te7(0001) surface near the Gamma point for any block (either Bi2Te3 or PbBi2Te4) forming the surface. It is shown that the Dirac state can be localized not only on the surface but also deeply beneath it.