Electric transport studies on topological insulators

Recently, a new type of topological condensed matter system characterized by Z2 topological invariant, which is different from the Chern number topological invariant in quantum Hall system, has attract huge attention in condensed matter physics.This new phase is called topological insulator.Topological insulators are different from ordinary insulators in the sense that it is composed of an insulating bulk state and of an massless spin-helical Dirac cone formed two-dimensional surface states.Helicity means that the spin-up electrons propagate in one direction while spin-down electrons propagate in the opposite direction.A fantastic prediction of this material is that the surface state is their insensitivity to any non-ferromagnetic impurities scattering and disorder.Thus, the carriers on the surface are predicted to have a long mean free path.Shortly after the theoretical prediction of 3D topological insulators in real materials, a number of surface spectroscopy measurements, such as spin and angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy, have been used to detect the topologically nontrivial surface state in three-dimensional topological insulator Bi1-xSbx, Bi2Se3, Bi2Te3, etc.This thesis presents transport experiments performed on topological insulators systems of Bi2Se3, Ca: Bi2Se3 and Bi1.5Sb0.5Te1.8Se1.2.We first performed the temperature and magnetic dependent measurements of Bi2Se3 and discovered SUMMARY VII Shubonikov-de Haas oscillations which verify the quality of single crystal.Due to the large bulk conduction contributions, we fabricate Bi1.5Sb0.5Te1.8Se1.2single crystals whose bulk conductions are strongly suppressed.A "Spin-valve" effect has been observed in the CoFe(5nm)/ Bi1.5Sb0.5Te1.8Se1.2structure and it may be originated from the interactions between ferromagnetic layer and helical surface states of topological insulator.Moreover, we prepared the nanodevice based on Bi1.5Sb0.5Te1.8Se1.2nanoflake and observed the two channel conduction mode in the samples.Weak-antilocalization effect and ambipolar field effect were also verified and studied in detail based on the nanodevice by the low temperature electrical transport measurements.Interestingly, the superconducting transition was obtained in the pure Bi2Se3 at the temperature below 4K.We propose that the defects formed during the single crystal growth induce such transitions.