Impact of point defects on electronic properties of topological materials

Topological insulators and topological Weyl semimetals are both novel classes of quantum materials. They are characterized by the surface states arising from the nontrivial topology of band structure. The surface state of topological insulators exists in the band gap, which originates from the band inversion due to the strong spin-orbit interaction. The topological Weyl semimetals are characterized by the surface Fermi arcs that connects the projections of two Weyl nodes of opposite chiralities and unusual magnetotransport properties induced by the chiral anomaly. These topological materials not only introduce novel fundamental physics concepts and properties, but also provide a new route to the development of technological applications of electronics, spintronics and topological quantum computation. Defects and impurities are inevitably present in all functional materials including topological materials. An important theme in the research of topological materials is to investigate and control defects. The goal of this doctoral thesis is to address the impact of point defects on the electronic properties of topological materials from several different perspectives. First, we present the experimental studies on the topological insulator Bi2Se3 single crystals and thin films. The prominent native defects in Bi2Se3 single crystals are clearly identified. In particular, Se vacancies and Se interstitial defects are responsible for the spatial fluctuation and the bulk conduction. Through the optimization of synthesis process, the formation of native defects is effectively suppressed in the Bi2Se3 single crystal sample, which is approaching the intrinsic limit with the Fermi level inside the band gap. We also investigate the long-lasting p-type doping problem in Bi2Se3 thin films. We find that the obstacles are the excess n-type defects from the interface between sample and substrate and from the air exposure. By properly protecting the surface of thin films and applying buffer layer, we have achieved p-type Bi2Se3 thin films via compensation doping with calcium. Next, we study the the topological phase transition in (Bi1-xInx)2Se3. We present compelling microscopic evidence of an inhomogeneous topological phase transition in this system, which is characterized by non-uniform spatial distributions of local band gap and topological surface states. The nanoscale inhomogeneity originates from effective suppression of local spin-orbit coupling and band topology by In defects. Our observations demonstrate that strong disorders can play a significant role in the topological phase transition. Then we investigate the distributions of electronic defect states of magnetic dopants vanadium(V) and chromium(Cr) in the topological insulator (Bi,Sb)2Te3, which is the parent compound to achieve the quantum anomalous Hall effect. The spectroscopic imaging shows very diverse distributions of local defect states of V and Cr. These unique spectroscopic features of the two dopants serve as electronic fingerprints to study the Cr/V-codoped topological insulator thin films. Finally we study the type-II topological Weyl semimetal WTe2. We directly visualize the quasiparticle interference patterns of surface states emerging around point defects via scanning tunneling spectroscopy. The spin-preserved joint density of state maps are computed via density functional theory calculations The good agreement between the calculations and the experiments provides solid evidence of the presence of surface states on the type-II topological Weyl semimental WTe2.Ph.D.Includes bibliographical reference