(Bi₂Se₃)_m(Bi₂)_n Series Of Materials First Principles Study

Topological insulators (TIs) have attracted a lot of attention in condensed matters physics and material science. Topological insulators (TIs) are recently discovered quantum states characterized by a full gap in the bulk and gapless edge or surface states which are protected by time-reversal symmetry. Among them, the Bi2Se3 family of materials has attracted a lot of attention due to a larger bulk gap and a simpler surface spectrum consisting of a single Dirac cone.Recently, ultrathin Bi films have also been theoretically predicted to be an elemental two-dimensional TI.For an infinitely adaptive series (Bi2Se3)m(Bi2)n, it is derived from an ordered stacking of Bi2Se3 QL and Bi2 BL building blocks. Therefore, investigation of physical properties for infinitely adaptive series (Bi2Se3)m(Bi2)n, is an interesting topic. In this paper, we study the geometry structure, electronic structures and phonon dynamics of (Bi2Se3)m(Bi2)n. The Topological surface states of infinitely adaptive series (Bi2Se3)m(Bi2)n with different termination and different thicknesses are also discussed within the density functional theory.The research work contains two aspects:1. Phonon dynamics in infinitely adaptive series (Bi2Se3)m(Bi2)n.Base on the density perturbation functional theory, the evolution of phonon structures with different ratio of m:n is analyzed. The shift of phonon frequency of Raman modes observed experimentally for Bi-rich sample is reproduced well by our calculations. Due to the difference of electronegativity between Se and Bi atoms, the charge from the inserted Bi2 BL is mostly transferred to the Bi2Se3 QL.So the van der Waals interaction is replaced by the Coulomb attractive interaction between two building blocks with the insertion of Bi2 BL into Bi2Se3. We think that this is the reason for the shift of phonon frequency of Raman modes. By calculating the projected phonon densities of states (PDOS), the large mismatch between Bi2Se3 QL and Bi2 BL along the normal direction of hexagonal plane implies that thermoelectric performance for slightly Bi-rich sample might have a significant improvement due to the reduction of thermal conductance.2. Topological surface states of infinitely adaptive series (Bi2Se3)m(Bi2)n. The surface states of Bi4Se3 and BiSe films with different terminated terraces have been studied by thedensity-functional theory. Our calculations show that the evolution of the surface states from hybridization gaps for thin films to vanishing gaps for thicker films and the topological nature of the surface states remains robust for thicker films. However, the topological surface states show different Kramers point energies and dispersions for different terminations. For both Bi4Se3 and BiSe films with Bi2-termination, the surface band shows the nearly linear dispersion and strong Rashba spin splitting. For different ratio of m:n, there are two inequivalent QL surfaces with Bi2Se3-termination. For the surface QL which is adjacent to Bi2 BL (Bi2Se3 QL), the topological surface band exhibits a nonlinearly dispersion without (with) Rashba spin splitting. The origin of Rashba spin splitting is due to spin-orbit coupling and the charge transfer from the Bi2 BL to the QL slab.Our results show that the topological surface state in infinitely adaptive series (Bi2Se3)m(Bi2)n can be tuned by changing the ratio of m:n or by changing termination.