Modulation On Band Structures Of Two Dimentional Topological Insulators

Topological insulators(TIs) are new states of quantum matter, whose discoveries make a breakthrough for both material science and fundamental condense matter. They are promising sources for potential applications in quantum computing and low-energy spintronics devices. Such states are characterized by an insulating bulk and metallic edge(or surface) states protected by time-reversal symmetry, which is fully different from traditional insulators and metals. Such metallic edge(or surface) states is different from which is produced by dangling bond or surface reconstruction, and it is irrelevant to doping and chemical adsorption, but it mainly depended upon topological band structures. It is a crucial issue to explore large-gap topological insulators because of their especially topological properties. Therefore, search for large-gap quantum spin hall(QSH) insulator and effective approaches to tune QSH states were investigated in our works.On the one hand, Dirac point materials played an important role in the exploration of two-dimensional(2D) topological insulators, therefore, to explore 2D large-gap topological insulators focus on searching for Dirac point materials. δ-graphyne and α-graphyne decorated with H or F were investigated by first-principle calculations in this work. It was found that the existence of Dirac point depended upon total π-electron conjugation systems, i. e., the π bond was constituted by alternate single and double bonds in whole system, like graphene. It absolutely didn’t rely on the triple bond between two Carbon atoms.On the other hand, chemical functional groups decorated target materials and applying external strain in the target materials were studied to tune QSH states. Silicene and Sn films decorated with chemical functional groups(X =-H,-F,-Cl,-Br,-I) were investigated by the first-principle calculations. Under external strains, the topological properties of decorated silicene and Sn film, and the nature of band inversion was investigated to explore the quantum phase transition. Our results showed that the phase transition was observed under a critical strain, and which clearly asserted that applying external stain is a potential approach for phase transition in conventional materials.