| Recently, topological insulators (TIs) have become hot topics in the research of condensed matter. The topological insulator shows an insulator energy band in the bulk and a metallic energy band on the surface. Different from the Schroedinger two-dimensional electron gas, this surface state (SS) has many novel features as follows. Firstly, the SS consists of massless Dirac Fermions that have linear dispersion relation. Secondly, the SS’s electrons states are protected by the time-reversal-symmetry (TRS) and thus immune to the back scattering from the nonmagnetic impurities and crystal structure defects. Thirdly, carriers in these surface states have their spin locked at a right-angle to their momentum (spin-momentum locking). Theoretical studies indicate that the topological insulators exhibit some novel phenomena. For example, the long range magnetic order established on the surface states can lead to the breaking of TRS, resulting in a gap opening at the Dirac point that makes the surface Dirac fermion massive. Furthermore, if Ep can be tuned into this surface state gap, an insulat rac fermion state is formed; this state may support many striking topological phenomena, such as the image magnetic monopole induced by a point charge, the half quantum Hall effect on the surface with a Hall conductance of e2/2h. The Majorana Fermions will be founded on the surface if a topological superconductor can be formed. During my Ph. D times, however, we are often disappointed in the topological insulator and magnetic topological insulators researches. One is the low ratio of the topological surface state over the whole conductance. Another one is the low charge mobility in the transition metal magnetically elements-doped magnetic topological insulators. This is why my Ph. D works are given as:"growth, doping and transport control of Bi2X3.based three dimensional topological insulators". And my works can be summarized as follows.(1) We have succeeded in growing the Bi2X3-based three topological insulator materials, such as the Bi2Te3, Bi2Se3, Cu-doped Bi2Te3 and the Sm-doped Bi2Se3, using the Melting method in a home-built furnace. Compared with the classical crystal growth methods, such as: the Bridgman method, the Floating Zone method, the Arc method and so on, the Melting method has many advantages:the easier to control and the cheaper to own. Most importantly, the high quality 3DTI materials that often have a low melting point which is lower than 900℃ that is the quartz tube’s limited temperature, can be easily obtained by this method in a very short time. For example, the binary 3DTIs Bi2Te3 and Bi2Se3 and their doped ones are prepared, on which the topological transports are investigated. The Cu-doped Bi2Te3 samples have been grown to carry out the transport investigation during the ageing process. And the rare earth element Sm-doped Bi2Se3 samples are obtained and have been confirmed to high carrier mobility magnetic topological insulators.(2) The two dimensional electron transport provided by the TSS is observed in bulk crystals of aged (Bi0.9Cu0.1)Te3.06, as demonstrated by measurements of the WAL effect and SDH oscillations. The mobility of the bulk carriers is suppressed by four orders of magnitude during the ageing process. Both the STM and the electrical measurements support a Fermi level inside the bandgap. The ageing method therefore leads to an optimized band-insulating TI crystal and appeals to a free-of-IB crystal. STM visualizes the novel defect features of Cu dopants and their dynamics during the ageing process, based on which the details of the ageing process are further revealed by ab initio calculations. These calculations suggest that there exists a diffusion barrier at the interface of the Bi2Te3 QLs. During the ageing process, Cu atoms freely migrate inside the QLs and frequently hit the barrier. The dopant atoms will also form clusters in between the QLs, leaving disorder within the QLs. This leads to a pronounced mobility suppression of the bulk electrons, finally allowing the observation of the TSS-related electron transport in bulk crystal samples.(3) We have successfully prepared a new type of DMS-based MTI (SmxBi1-x)2Se3. It shows an anisotropic ferromagnetic phase for x=0.05, which exhibits a Curie temperature of around 50 K and a typical coercive field of 500 Oe. XMCD analysis shows the polarized spins originate from the Sm atoms. Our evidence points to a robust ferromagnetic coupling, as further supported by the first-principles calculations. The higher Curie temperature, limited carrier density and satisfactory mobility suggest that it is a high-mobility candidate of MTIs, which acts as an ideal platform for the robust topological edge transport and magnetoelectric effect. |
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