Study Of Electronic Structure And Magnetic Properties Of Topological Insulators Sb₂Te₃and Bi₂Se₃Films

In recent years, Topological insulators have become one of the research hotspot andfrontier of physics because it shows a lot of exotic quantum properties. It is possible to realizea number of new fundamental physical phenomena such as electron transport of weaklocalization effect, anomalous quantum hall effect, topology magnetoelectric effect andmagnetic monopoles, Majorana fermion in topological insulators. Therefore it has importantand potential significance and application prospects in spintronic materials and low energyconsumption spintronic devices, fault-tolerant quantum communication, quantum computing,etc. Owing to exotic physical properties and potential technological applications in spintronicsand quantum computing, topological insulator (TI) has been at the core of a very activeresearch area.The three-dimensional TIs like Bi2Te3, Bi2Se3and Sb2Te3etc. have become the modelmaterials due to their exceptional properties of possessing relatively large bulk band gaps andone single Dirac cone at the Dirac point in Brillouin zone. However, the currently availableBi2Te3, Bi2Se3and Sb2Te3materials always show unacceptably high bulk conductivityintroduced by the strong native defects, where the surface state is buried in the bulk band.This challenges the realization of the striking phenomena and possibility of being functionalcomponents of TI in electronic devices. Therefore, how to inhibit the intrinsic defects ofthree-dimensional topological insulator material and realize the modulation of the electronicstructure became an urgent problem. In this paper, the first-principles calculations areperformed to systematically investigate the electronic and magnetic structures of topologicalinsulator films. We obtained some meaningful results. It is expected that the researchconclusions can explain some experimental phenomena and their physical meaning, and alsoprovide a theoretical guidance for the practical application of topological insulators.1. The helical surface states of the S-covered topological insulator Sb2Te3(0001).The effects of S atom surface adsorption and substitution on the helical surface states ofSb2Te3are studied by the density-functional theory with spin–orbit coupling being taken intoaccount self-consistently. It is found that S atoms play the role of surface passivation whenadsorbed on both surfaces of a6QL Sb2Te3film in symmetrical configuration. The linear dispersion of the surface states is found to be preserved in this situation. For a film withasymmetric S atom adsorption, the degeneracy of the surface states from the two sides of afilm is broken. The Dirac Cones of top and bottom surface shift to the point. Comparedwith S symmetric adsorption, asymmetric S substitution rebuilds the mirror symmetry to acertain extent and preserves the Dirac cone. The results verify that the topological surfacestates have immunity to nonmagnetic impurity. This supports the idea that the topologicalinsulator surface electronic states are dominated by its structural symmetry and the effect ofthe asymmetric environment of topological insulator Sb2Te3films should thus be considered.2. Effect of the topological surface states and magnetism of topological insulators Bi2Se3film with magnetic doping.We studied the effects of Cr (Mn) adsorption and doping on the electronic and magneticproperties of Bi2Se3topological insulators employing spin orbit coupling (SOC)self-consistently. Cr (Mn) atom induces a spin-polarization with total net magnetic momentsof2.157B(1.730B). There is a p-d hybridization between the Cr3d（Mn3d）states and thenearest neighbor Se4p （Bi6p）states. The magnetic interactions break the local time reversalsymmetry, where the top surface of the Dirac cone disappears. While, the Dirac cone of thebottom surface is still alive and shifts downward.with no gap opening has been found at theDirac point of the bottom surface. The Fermi levels shift up to the band gap. It is a possibilityof achieving the insulating massive Dirac fermion state in Bi2Se3films. Meanwhile, ourresults show that asymmetrically covered with magnetic ions in the Bi2Se3ultrathin film canavoid the interaction between the top and bottom surface of film to preserves a gapless(original gap) and isolated surface state.3. Effect of the topological surface states of topological insulators Bi2Se3film withTl-doping.We studied the electronic structure of Bi2Se3thin films with Tl impurities by employingspin-orbit coupling (SOC) self-consistently. Tl adsorption induces a giant Rashba spinsplitting in the quantum well states of Bi2Se3film, and modulates the Dirac Cone. The upperand lower surface states of the film become different and the degeneracy of the two Dirac cones is lifted. The Dirac cone of the top surface shifts down to the valence band and theDirac cone of the bottom surface shifts to the gap. The Fermi levels shift up to the conductionband. While an ideal and isolated Dirac cone is realized in slab geometry of Bi2Se3withappropriate Tl substitutions of Bi atoms and the Fermi level is pinned at the Dirac point. Thedoping of thallium atom tunes the electronic structure of topological insulator. Our studiesprovide a promising way to tune the electronic band structure of the topological insulator bydoping thallium atom in well-known binary TI Bi2Se3.