Cr-doped Bi _X Sb _Y Te ₃ , Bi ₂ Se < Fe - Doped Bi ₂ Se ₃ Transport Properties

Topological insulators are novel electronic materials which, like traditional insulators, have a band gap in the bulk state, but have protected conducting states on their surfaces. Surface states are related to the spin-orbital interactions and time reversal symmetry. Two-dimensional (2D) topological insulator is a quantized spin Hall insulator. The surface states of3D topological insulator support novel spin-polarized2D Dirac fermions. New states can occur on the surfaces of3D topological insulators due to an induced band gap. The surfaces of3D topological insulator with a magnetic gap can give rise to the novel quantized spin Hall state which can give rise to the topological magnetoelectric effect. A superconducting energy gap can lead to states which can support the Majorana fermions. These peculiar properties make topological insulators as one of the most promising candidates for spintronics and topological quantum computation in the future. Therefore, topological insulators have attracted great attentions since their discovery.In this paper we study the transport properties of topological insulators, including: the origin of ferromagnetism in Cr doped BixSbyTe3; the study of linear magneto-resistance in films of Bi2Se3and the transport properties of Fe doped Bi2Se3. The purposes of these studies are to grow single crystals of high quality; detect the surface states in transport measurements by decreasing the thickness of the samples; study the magnetic properties in transition elements doped topological insulators and the linear magneto-resistance in topological insulator Bi2Se3.The main results are shown blew:(1) We perform the systematic resistivity measurements of ferromagnetic p-type Cro.2Sb1.8-xBixTe3bulk crystals. From measurements of temperature and magnetic field dependent resistivity, the exchange interaction Jpd between localized3d electrons and conductive holes in Cro.2Sb1.8-xBixTe3crystals was determined. The observed hump around the Curie temperature and intense MR can be explained using the magnetic impurity scattering model. The Jpd decreases with the increase of the Bi doping level, which is also important in making the change in the Curie temperature for different Bi concentrations. Our results demonstrate that the magnetic impurity scattering and phonon scattering play important roles in the resistivity, and weak localization becomes important for samples with a high density of defects. Our results clearly show that the ferromagnetism in Cro.2Sb1.8-xBixTe3crystals is from the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction.(2) By now the studies of topological insulator have concentrated on the prediction of new types of topological insulators, the determination of the topological states and the fundamental questions related to the peculiar spin-polarized states. Linear magneto-resistance can make topological insulator as one of the candidates for sensor applications. We have grown the single crystals and cleaved mechanically the crystals to get thin films with thickness ranging from50to200nm to make the Hall bar devices. At low temperature the Hall resistance is nonlinear and the transverse magneto-resistance is linear versus magnetic fields. When temperature is blew40K and magnetic fields change from0to5T, the magneto-resistance changes linearly with the magnetic fields and changes little with the temperature. The magnitude of the linear magneto-resistance can reach60%at T=2K and has no sign of saturation when magnetic field is at5T. By analyzing the data we conclude the linear magneto-resistance in topological insulator Bi2Se3is related to the mobility of the samples. The linear magneto-resistance found in topological insulators is important for practical applications, such as magnetic sensors.(3) Ferromagnetism has been found in3d transition elements (V, Cr, Mn, Fe) doped Bi2Se3, Bi2Te3, Sb2Te3single crystals and films. Among these magnetic topological insulators, massive Dirac fermions were only found in Fe, Mn doped Bi2Se3using angle-resolved photoemission spectroscopy (APERS). Though the band gap near the Dirac point is large (60meV), the bulk magnetic properties in FexBi2-xSe3are still debated. Here, we conduct the transport measurements of Fe doped Bi2Se3. The results show that our samples are of no ferromagnetism.