| Topological insulators are materials with gapped bulk states coexist with unique gapless surface states. The linear E-K dispersion surface states (SSs) are protected by the time-reversal symmetry (TRS). The spin direction of Dirac fermions on the surface states are locked with their moments because the spin-orbit coupling. The direct elastic back scattering of nonmagnetic impurities and defects are forbidden. Recently, scientists found that some traditional materials such as Bi2Te3 and Bi2Se3 are topological insulators. However, most of the times their Fermi levels are not inside the band gap as anticipant, the transport properties are dominated by bulk conduction which shadows the surface states. Thus, the bulk insulating behavior is needed for device applications. To achieve this purpose, alloys of the binary topological insulators, such as Bi2Se2Te, Bi2Te2Se, Bi2-xSbxSe3, Bi2-xSbxTe3, have been studied widely and shown suppressed bulk carrier density and enhancement of the surface conductance. Recently, the quaternary topological insulators Bi2-xSbxTe3-ySey (BSTS) were reported to show amazing bulk-insulating properties in which their active energy is quite higher than most of the binary topological insulators. The mixed chalcogens in BSTS samples are benefit for reducing defects. Therefore, the higher surface states contribution in whole transport properties could be expected.In our work, we performed a systematic study of the magneto-transport and the crystal structure around the optimized concentration suggested by Ando et al. A correlation between the physical properties and crystal structures has been revealed. In addition, we observe segregation of other composition phase which will introduce much higher carrier concentration.We also found the nonlinear Hall Effect in quaternary topological insulator Bi1.5Sb0.5Te1.8Se1.2·By using a multi-channel model, we decomposed the whole conductance into three conducting channels which are the surface states, the impurity band and the bulk at various temperatures, and observed mobility and the carrier density temperature relationship of all three channels.We also explored the transport properties of BSTS nanoflakes, and found some cusp near zero fields and some fluctuation of magnetic resistance. The cusp has been known as weak anti-localization (WAL), and such fluctuation can be repeated and the amplitude reduces as temperatures rise, which can be identified as the universal conductance fluctuations (UCF). Through further analysis, we can demonstrate the 2D nature of WAL effect and UCF effect. |
