Electrical Transport Studies of 3D Topological Insulator Bi2Se3 and Related Specific Structures

The electrical transport properties of 3D strong topological insulators Bi2Se3 and related specific hybrid structures have been investigated in this thesis. Topics of topological insulators studied include the linear positive and negative magnetoresistance in Bi2Se3 thin flakes, the superconducting proximity effect in Bi2Se3/Nb heterojuctions and the magnetotransport properties of epitaxial Bi2Se3/Sb2Se3 bilayer thin films. Recently intriguing magneto-transport properties, such as linear magnetoresistance and negative magnetoresistance observed in topological semimetals, have attracted attention because of their close association with the novel physics of Weyl fermions in quantum field theory. However, these unconventional magneto-transport phenomena have also been observed in systems other than Weyl semimetals; their origins could be various and are still under debate. We studied the magnetotransport properties of topological insulator Bi2Se3 flakes with different thickness grown by chemical vapor deposition. A non-saturating positive MR is observed in perpendicular fields (B⊥I), while an evident negative MR is observed in parallel fields (B//I). Meanwhile, two types of carriers are extracted from the nonlinear Hall resistance measurements. These features are pronounced at low temperature and gradually decrease as the temperature increases. The conductivity fluctuation in the accumulation layer at the surface/interface is believed to be responsible for the observed non-saturating linear transverse and linear negative magnetoresistance. The superconducting proximity effect in topological insulators has been a hot topic in recent years due to the possibility for the detection of Majorana bond states. In our Bi2Se3/Nb heterojunctions, transport measurements show that the junction resistance continuously decreases with several ripples below the Nb superconducting transition temperature. Meanwhile, a zero-bias conductance peak and several conductance dips are observed in the low-temperature differential conductance spectrum. Besides, the emergence of ripples in R(T) curves are found to be tied to the conductance dips in the differential conductance spectrum, and these ripples are closely related to the evolution of the zero-bias conductance peak. These phenomena are explained in terms of the multiple superconducting phases induced in Bi2Se3 flake by the superconducting proximity effect due to the transparent interface in our device. Moreover, an unusual superconducting phase with a higher superconducting transition temperature than that of the pristine Nb substrate is observed in the junction. The appearance of this superconducting phase can be attributed to the enhancement of local superconductivity of Nb because of the “coherence proximity effect”. Theoretical work has predicted that topological states could migrate from strong topological insulator Bi2Se3 into ordinary insulator Sb2Se3 in Bi2Se3/Sb2Se3 hybrid structures. The recent ARPES study of Bi2Se3/Sb2Se3 thin films successfully detected induced Dirac cone topological surface states. We studied the electrical transport properties of these epitaxial Bi2Se3/Sb2Se3 thin films with different Sb2Se3 thicknesses. The weak anti-localization effect observed can be tuned by adjusting the thickness of the Sb2Se3 layer, this phenomenon indicates an introduction of an additional coherent channel in samples with thin Sb2Se3 capping layer. This coherent channel vanishes when the Sb2Se3 is too thick. Besides, an in-plane anisotropic magnetoresistance is detected and is systemically studied with varying measuring conditions. The temperature-dependent behavior is explained by the competition between surface and bulk anisotropic magnetoresistance.