Electronic Structure Of Topological Insulator And Magnetic Topological Insulator

Topological insulator is a novel quantum matter, which is insulating in bulk and conductive near the surface. This metallic edge state is protected by time reversal symmetry from scattering by non-magnetic defects. The robustness of the surface states has great potential in the application of spintronics and quantum device. Scanning tunneling microscopy（STM） and spin-polarized STM（SPSTM） are among the most effective and successful techniques in the field of surface science and surface magnetism during the past decades. Taking the advantage of high surface sensitivity, STM and SPSTM are ideal tools in investigating systems with rich electronic and spin properties on the surface, such as topological insulator.In this work, we investigated the growth of two dimensional topological insulator Bi（111） on three dimensional topological insulators and observed the one dimensional topological edge state by STM. We also revealed the particular surface magnetic anisotropy of magnetically doped topological insulator. In detail, our work includes three parts:（1） The ultra-thin Bi film on three dimensional topological insulators Bi₂Se₃ and Bi₂Te₃. We have successfully grown Bi（111） ultra-thin film on the surface of three dimensional topological insulators. Combining the precise real time lattice measurement by reflective high energy electron diffraction（RHEED） and microscopic observation by STM, we investigated the growth of the first few bilayers of Bi（111）. First, we studied the growth of Bi（111） thin film on Bi₂Se₃. The lattice constant of bulk Bi（111） surface is 4.54 ?, and that of Bi₂Se₃ is 4.13?. The lattice mismatch between them is up to 9.9%. The first Bi（111） bilayer（BL） is highly compressed to 4.21 ?, and the following second and third BL relaxed to 4.35 ? and 4.39 ? rapidly. The lattice relaxation becomes small from the fourth BL. During the formation of second BL, a structural phase transition from Bi（110） to Bi（111） was observed. A quasi-hexagonal structure distortion on the second and third BL Bi（111） with the lattice constant around 9,9nm and 6.3nm respectively was also found. The lattice constant of Bi₂Te₃ is 4.38 ?, which matches the Bi（111） lattice much better than Bi₂Se₃. An improved quality of the first Bi Bilayer on Bi₂Te₃ was observed with straight step edge and large saturation coverage. No structural phase transition was found in this case. The growth of Bi（111） on Bi₂Te₃ is very similar to that of >4 BLs on Bi₂Se₃.（2） The energy and spatial distribution of one dimensional topological edge state of Bi（111） BL. Since Bi（111） BL on Bi₂Te₃ show very high quality, we choose this system to study the two dimensional topological properties of Bi（111）. Through tunneling spectra measurement based on STM, we obtained the spatial and energy distribution of the electron states near the island edges and inside the islands of 1BL and 2BL Bi（111） thin film. Combining angle-resolved photoemission spectroscopy and first principle calculation, we confirmed the observed one dimensional edge state is the characteristic edge states of the two dimensional topological insulator. The energy distribution of toplogical edge states covers the energy gap of Bi（111） and extends lower than the energy gap, and the space distribution is restricted in 1²nm interval near the step edges. In this way, we confirmed the theoretical prediction and description of the energy bands and topological edge states in Bi（111） BL.（3） Magnetic anisotropy of topological surface states. Through measuring the surface magnetism by SPSTM and bulk magnetism by vibrating sample magnetometer（VSM）, we investigated the magnetism of magnetically doped topological insulator Cr0.05Sb1.95Te3, which is grown by the melting method. The bulk measurement shows the ferromagnetic easy axis is normal to the（111） plane with the coercive force of 50 Gs, and the hard axis lies in the（111） plane with the saturation field of 1T which is observed by STSTM as well. In addition, the surface measurement by SPSTM also shows a ferromagnetic easy-axis in the（111） plane with the coercive force of 500 Gs. The energy resolved spin polarization reveals that the magnetic properties on the surface are different from the bulk. The easy-axis of the bulk is normal to the（111） plane, while that of the surface states is in the（111） plane. The magnetic anisotropy and spin polarization of topological surface states are essential to understand the magnetism of magnetically doped topological insulator.