A Theoretical Investigation Of Two-dimensional Topological Insulators Based On Group ? Elements

Protected by time-reversal symmetry, low energy scattering of the edge states in two-dimensional (2D) TIs (also called quantum spin Hall insulators) is enjoined, resulting in dissipationless transport edge channels, which are quite promising for application in spintronics and quantum computations, and are now attracting interests of material science and theoretical condensed matter physics. To achieve this perpose, many works predicted various of materials with topological properties, but only HgTe/CdTe and InAs/GaSb quantum wells can achieve QSH effect at ultralow temperature. Searching for large-gap QSH insulators is the key to increase the operating temperature.The topological gaps of QSH insulators are origin from spin orbital coupling (SOC), so the system with strong SOC effect is more likely to be a large-gap QSH insulator. As we all know, SOC comes from relativistic effect, with the coupling strength proportional to electronic momentum and electric field strength. The electric field strength around a heavey atom is extremely strong, while the electronic momentum is also quite high. As a result, the SOC are always strong in the materials with heavy atoms. Therefore we concentrated our mind on Ge/Sn materials. By first-principle calculations, we demonstrated two stable large-gap QSH insulators, so called dumbbell stanane and dumbbell Sn6Ge4H4. This thesis contains fellowing sections:The first chapter is an introduction of this thesis, containing the background and the allurement of the research. First of all is the introduction of Spintronics and SOC, coming out with a conclusion that heavy atoms own stronger SOC effect compared with light atoms like C, N, O. Following is the paper research of QSH effect and QSH insulators, containing both theory works and experimental works. We indicated the obstacles in the further application of QSH insulators. At the last part, the research review for dumbbell-like group IV materials and group IV compounds, including experimental and theoretical works.Chapter two presents the calculation method this thesis based on, first-pinciples calculations. In this chapter, the basic theories and approaches of ab initio calculations and density function theory (DFT) are briefly introduced. What's more, introduction of the software we used in the DFT calculations, VASP codes is also given at the last part.In Chapter three, using first-principles calculations, we demonstrate that the stable hydrogenated stanene with a dumbbell-like structure (DB stanane) has large topological nontrivial band gaps of 312 meV (? point) and 160 meV for bulk characterized by a topological invariant of Z2=1, due to the s-px,y band inversion. Helical gapless edge states appear in the nanoribbon structures with high Fermi velocity comparable to that of graphene. The nontrivial topological states are robust against the substrate effects. The realization of this material is a feasible solution for applications of QSH effect at room temperature and beneficial to the fabrication of high-speed spintronics devices.In Chapter 4, we proved that high temperature QSH effect may be reached in a hydrogenated germanium-tin (Sn6Ge4H4) dumbbell (DB) structure, where spin-orbit coupling (SOC) opens a bulk band gap of 166 meV. The topological nontriviality is related to the band inversion of s-px,y of Sn atoms induced by surface hydrogenation and can be characterized by a topological invariant of Z2=1. This work offers a promising candidate material for achieving the long-desired room-temperature QSH effect.The last section, Chapter 5 is the conclusion of this whole thesie.