Theoretical Study On Electronic Structure And Coupling Effect Of Surface State In Few Layers Bi₂Se₃ Topological Insulator

In this paper,the first-principle calculation method based on density functional theory is used to investigate the related properties of Bi₂Se₃ thin films,containing electronic structure,magnetic,optical properties and thickness dependence.The main research results are as follows:1.The coupling interaction between the bottom and top surface states and the corresponding influence on transport and optical properties of Bi₂Se₃ thin films with1⁸ quintuple layers?QL?.In this part,the geometric parameters and structural stability of the topological insulator Bi₂Se₃ thin films are studied,and then the electronic structure and optical properties of each system are calculated and analyzed.It is found that the coupling between the upper and lower surfaces of the film shows a significant spatial and thickness dependence,and this dependence is analyzed from the hybridization between the surface states,which explains the experimentally reported transport properties on thickness-dependent non-uniformity.In addition,the optical properties based on the electronic structure also have a significant thickness dependence.The calculation results show that with the increase of thickness,the multi-peak structure of the virtual dielectric function generated in low and high energy region respectively undergoes red shift and blue shift.The red shift trend is consistent with the recently reported experimental observations,and the blue shift is first studied.The results of this study provide a detailed understanding of the transport and optical properties of Bi₂Se₃ thin-film devices.2.The influence of different types of Se vacancy and Mn substitution,and interaction between them on the electronic structure in 1QL Bi₂Se₃ thin films.This part deals with the electronic structure of four types of 1QL Bi₂Se₃ thin films with Se/Bi vacancy,magnetic element Mn substitution for Bi/Se,multiple Se vacancies,Mn substitution for Bi and Se vacancy.The results include the formation energy,electronic structure,and magnetic moments.It is found that Se vacancy leads to orbital hybridization and introduce gap states but no magnetic properties.On the contrary,Bi vacancy introduces magnetic moments but not gap states.Mn substitution for Bi has similar electronic structure to the Bi vacancy,but the former Fermi level is closer to the top of valence band.In two cases of more concentrated and more dispersed multiple Se vacancies,the former may have a greater impact on the electronic structure.When Mn substitution for Bi and Se vacancy both exist,if Se vacancy is not adjacent to Mn,vacancy has less impact on the system and Fermi level is closer to band gap,due to the Se defects affecting the bonding of Mn and Se.In this work,the electronic structure,optical and magnetic properties of Bi₂Se₃ thin films are obtained together with those of the doped Bi₂Se₃ thin films.The obtained results are generally consistent with some experimental results and deepen the understanding of the relevant experimental results.