Quantum Properties And Doping Modificationof Topological Insulator Bi₂Se₃

Topology insulator Bi₂Se₃ defined as a hot topic in the fields of condensed matter physics and material science,has potential value for the development of basic research and spin electronics.Compared to other topological insulators,Bi₂Se₃ is a kind of strong topological insulator,because it does not only have a relatively large band gap but also have a simple electronic structure in the surface.However,the material of Bi₂Se₃ has many drawbacks,such as:different types of intrinsic defects,which let the Fermi level of the system not exist in the energy gap;the weak magnetism,small band gap and the low Curie temperature,which all affect the observation of the Quantum anomalous Hall effect Observation.Therefore,it is the main research goal to study the defects of the system and to find the topological insulating materials with stronger ferromagnetic and larger band gap that can realize the high temperature Quantum anomalous Hall effect.In this paper,we used the CASTEP software package and the first-principles method based on density functional theory?DFT?to calculate the electronic structure and the basic properties of the system.The methods of generalized gradient approximation and the plane wave with ultrasoft pseudopotential were used in the process of calculation.In view of the theory and application of the topology insulator Bi₂Se₃,some useful explorations and attempts were made.The concrete contents are as follows:The intrinsic structure and vacancy defects of Bi₂Se₃ were studied.Firstly,the intrinsic structure of Bi₂Se₃ was calculated and the results showed that Bi₂Se₃ was a kind of direct material with narrow band gap,and there was a strong covalent effect between Bi and Se.Then,the the vacancy defects including V_Se1,V_Se2 and V_Bi of Bi₂Se₃ were calculated and studied.The results showed that V_Se1 was the main source of material defects.The formation of V_Se1 was directly related with the 6p state of Bi atoms surrounding V_Se1.Finally,the different models of V_Se1?4×4×1,3×3×1,2×2×1?were calculated and we found that with the increase of vacancy concentration,the stability of the system was enhanced and the formation of defects was affected more by the surrounding Bi atoms.In order to obtain Bi₂Se₃ material with strong magnetic properties and superior performance,the supercell of Bi₂Se₃ was studied by using Cr and Fe with strong magnetic properties.Firstly,the supercells of Bi₂Se₃ were doped with Cr³⁺?high and low spin?and Fe³⁺?high,middle and low spin?.The magnetic properties of the system were studied when the material was doped.We found that the middle spin-doping system in Cr was stable and the Fe-high spin doping system was stable.Then,three systems with the concentration of high spin in Cr including 4.2%,8.4%and 12.5%were calculated respectively.We found that with the increase of the concentration of Cr,the bandgap of the system gradually increased,and the magnetic properties of the system gradually increased.It also found that the Cr-doped system was ferrimagnetic.Lastly,three systems with the concentration of high spin in Fe including 4.2%,8.4%and 12.5%were calculated respectively.The results showed that with the increase of the concentration of Fe,the magnetism properties of the system enhanced,and the doping system was ferromagnetic.In short,the system of Bi₂Se₃doped by elements of Cr and Fe had magnetism.The system is expected to become new materials in the fields of electronic components and spin devices.In this paper,the intrinsic structure,vacancy defects and the electronic structure and basic properties of the magnetic insulator Bi₂Se₃ were systematically studied,and some useful conclusions were drawn.These all laid a foundation for the further development and application of the future topology insulator research.