Transport Propertiesof Topological Insulatorsingate Electric Fieldand Pulsedmagnetic Field

Three-dimensional topological insulators(TIs)arenovel quantum materialsin whichthe bulk is an insulator with nontrivial band topology but the surface possesses Dirac-like metallicstates protected by the time reversal symmetry(TRS). In this thesis, we presenttransport studiesofthree-dimensional TI thin films and single crystals ingate electric field and pulsedmagnetic field.Breaking the TRS in TIs is expected to spawn exotic topological quantum effects and novel spintronic applications. One way to achieve this goal is to create spontaneous ferromagnetic order by means of magnetic doping.The intricate interplay between topological protection and brokenTRS may lead to highly unconventional behavior that can find unique applications.We first study the magnetic properties of TIthin films at the critical point of topological phase transition. The magnetic quantum phase transition accompanied by a sign reversal of anomalous Hall effect can be actively controlled by anin situgate electric field. In the well-defined topologically nontrivial phase, we find that the ferromagnetism can be strengthened by increasing carrier density in both the electron- and hole-dopedregimes.The magnetoresistance(MR) of TI thin films with spontaneously broken TRS is also investigated. We observe an unusually complex evolution of MR when the Fermi level(EF) is tuned across the Dirac point by gate electric field. In particular, MR tends to be positive when EFlies close to the Dirac point but becomes negative at higher energies. This trend is opposite to that expected from the Berry phase picture for localization, but is intimately correlated with the gate-tuned magnetic order. We show that the underlying physics is the competition between the topology-induced weak antilocalization and magnetism-induced negative MR.Another TRS-breaking approach is to apply a strong magnetic field perpendicular to the topological surface states(SSs). Quantum oscillations in strong magnetic field is a powerful technique for mapping out the Landau Level spectrum and revealing the Zeeman effect of topological SSs.We report quantum oscillation studies on the Bi2Te3-xSx TI single crystals in pulsed magnetic field up to 92 T. The surface and bulk quantum oscillations can be disentangled by combined Shubnikov-de Haasand de Hass-van Alphen oscillations. At high magnetic field beyond the bulk quantum limit, the Zeeman effect of topological SSs is clearly demonstrated and the associatedg factor is extracted.Furthermore,a complex and systematic evolution of MR with applied gate voltages is observed in the Sb2Te3 thin film.The low field sharp increase of MR,which is characteristic of topology-induced weak antilocalization, gradually evolves to the parabolic behavior as bulk carriers dominate the transport process. The competition between the bulk and SSs leads to the linear MR observed at intermediate magnetic field. With bulk carriers reduced by gate electric field, the high field MR becomes negative when the surface quantum Hall regime is approached.Our resultsreveal the intrinsic transport properties of the topological SSs under the influence of gate electric field andpulsed magnetic field. They also provide crucial informationfor the application of TIs in microelectronic and spintronic devices.