| The nontrivial topological nature of the electronic bands in the three-dimensional topological insulators causes their gapless surface state in their bulk band gap.This surface state has potential applications in further electronic devices because the Dirac electrons in the state manifest forbidden back scattering,helical spin texture and other exotic properties.However,the real topological surface state(TSS)transport is difficult to achieve in the Bi-based,binary compounds because their bulk states are dominant in the transport.In this thesis we used the devices based on quaternary BiSbTeSe2 which has been optimized so that their transport signals are dominated by the TSS at low temperature.We observed the Quantum Hall(QH)effect and the nonlocal resistance of the TSS in the mesoscopic BiSbTeSe2 devices.By controlling the back gate voltage,magnetic field and temperature,we found that the QH effect survives up to 30 K.In this thesis I give a theoretical analyisis of the TSS and an explanation of the characteristics of different QH plateaus.The QH effect was also observed in the devises whose top surface of the topological insulator was decorated by magnetic Co clusters.We analyzed the QH effect by using the renormalization group(RG)flow.The quantization trajectory can be obtained from the RG flow.Compared with the RG flow before and after the magnetic clusters decoration,we found an anomolous quantization trajectory.To explain this result,we put forward a delayed Landau level hybridization model.In this model,the magnetic clusters open a Zeeman gap on the top surface of the topological insulator through antiferromagnetic coupling,and shift the zero Landau level to the top of the gap.Such model successfully explains the anomalous quantization trajectory in our experiments and can be used to estimate the value of the gap which is at least 4.8 meV.We carried out several experiments and confirmed that the results are repeatable.We also compared the QH effect in the sample decorated by non-magnetic Cu clusters.We found that the results are consistent with the previous conclusion.In addition to the QH effect,the universal conductance fluctuations(UCFs)of the TSS were also studied.By modulating the magnetic field and gate pressure,we obtained the dependence of the UCF of the TSS on magnetic field and Fermi energy,respectively.We found that the root mean square of UCF amplitude has a(?)-times reduction with the increase of magnetic field in both of the electron region and the hole region,and the critical magnetic field is about 0.1 T.Accroding to the random matrix theory,we find that such an obversation is in conformity with the expectation from the view of symmetry,and this reduction is due to the time-reversal symmetry broken. |
