Study On Ternary Topological Insulators And New Two-dimensional Ferromagnetic Materials

In recent years,topological insulators have become one of the research hotspots in the field of spintronics due to their novel and unique properties.Different from the classical metals and insulators,topological insulators are the special materials in which the bulk phase is insulated while the surface state is metallic.The surface states of topological insulators are protected by the symmetry of time inversion,and the electrons in the surface states have the property of spin momentum locking,which inhibits the elastic scattering from impurity or defect in the lattice.The research on the topological insulators,of which are mainly based on bismuth selenide(Bi₂Se₃)and bismuth telluride(Bi₂Te₃),has greatly promoted the application of topological insulators in electronic spin devices.With the development of topological insulators,ternary topological insulators,such as Bi₂Se₂Te,Bi₂Se₂Te,Bi_2-xSb_xSe₃ and Bi_2-xSe_xTe₃,have gradually attracted the attention.These materials have good properties of bulk phase insulation,resulting in the increasing contribution of surface states to the electrical conductivity.While most of the studies focus on the properties of these ternary TIs,limited researches were performed to investigate the dynamic atomic stack of its crystal structure.In this paper,the variation of lattice structure with Te concentration in ternary topological insulator Bi₂Se_3-xTex system is studied.At the same time,in the lattice structure of topological insulators,these quintuple layers are weakly bonded to each other by the van der Waals force Therefore,topological insulators are good candidate for heterojunction structures.With the development of two-dimensional materials,the discovery of intrinsically two-dimensional ferromagnetic materials has aroused more attention.Because of the strong spin-orbit coupling in the topological insulators,the heterojunction structure composed of topological insulators and 2D ferromagnetic materials is of great significance in the application of spintronic devices.Molecular beam epitaxy(MBE)technology has unique advantages in the growth of high quality films and the precise control of the element stoichiometry ratio in films.At the same time,it provides an effective way for the preparation of heterojunction.In this paper,the structure and properties of binary and ternary topological insulators and two-dimensional ferromagnetic materials have been studied.High quality Bi₂Se₃ and Bi₂Te₃ thin films have been successfully prepared on Ga As(111)B substrates by molecular beam epitaxy.We have studied the structure and transport properties of Bi₂Se₃ and Bi₂Te₃ thin films.Meanwhile,the epitaxy Bi₂Se₃ films on mica and YIG substrates are also studied,which provides a theoretical reference for low power and optical devices based on topological insulators.In Bi₂Se_3-xTe_x ternary topological insulators,the lattice constants of the thin films along the a axis and c axis increase gradually with the increasing proportion of Te elements.Raman results show that the Raman peaks shift as a step-like way with the change of Te concentration in Bi₂Se_3-xTe_x thin films.When the Te concentration is within the range of 0<x<1,the Raman peak changes slightly,when 1<x<2,the peak changes sharply,when 2<x<3,the peak changes gently.We investigate the process by which Te atoms occupy positions in the preferred lattice during doping.In topological insulators,the atoms are arranged in the order of-A_VI⁽¹⁾-B_V-A_VI⁽²⁾-B_V-A_VI⁽¹⁾.Te atoms will occupy the atomic lattice position of the middle layer first,and then occupy the atomic positions of the other layers.Based on the dynamic study of Se and Te atoms in the lattice structure of Bi₂Se_3-xTe_x thin films,we propose a hypothesis about the position of Te and Se atoms in the lattice structure.This hypothesis can provide a theoretical basis for the precise determination of the arrangement position of Te and Se atoms in Bi₂Se_3-xTe_x thin films.At the same time,the phenomenon of weak anti-localization was observed in the magnetro-transport results of Bi₂Se_3-xTe_x thin film system.We have successfully prepared high quality two-dimensional magnetic material Cr Te₂ films with various thickness on Ga As(111)B substrates using molecular beam epitaxy.The magnetic properties of Cr Te₂films were investigated by low temperature magnetro-transport system.The results show that even when the thickness of Cr Te₂ film is down to atomic layers,Cr Te₂ also exhibits a strong out-of-plane easy-axis property.In the 4-monolayer thin film,the Curie temperature of Cr Te₂ is found to be 191 K,and the Curie temperature of Cr Te₂ film increases from 191 K to 205 K with the increase of the thickness of the film from 4 to 35 monolayers.When the temperature is 5 K,the coercivity of Cr Te₂ films decreases from 8200 Oe at 4 layers to 5120 Oe at 35 layers.The nonmonotonic variation of abnormal Hall resistance with temperature was found in all Cr Te₂ films.In addition,we have successfully prepared the Bi₂Te₃/Cr Te₂heterojunction structure,which lays a foundation for the further basic research of two-dimensional ferromagnetism,and also has important significance for the development of the application of spintronic devices.