Formation and Control of Helical States in 2D Gases and Topological Insulators
Abstract
It has been realized that a p-wave order parameter can emerge in a synthetic superconductor constructed from a semiconductor and an s-wave superconductor, provided that fermion doubling is removed [1], [2]. In one dimension, the required electron spectrum consists of two counter-propagating modes with opposite spin orientations, so-called helical channels. Helical channels can be realized in nanowires with spin-orbit interactions in the presence of magnetic field [3], [4], topological insulators [5], at the edges of the quantum spin Hall system [6], or in the integer and fractional quantum Hall effect regimes [7]–[10]. This thesis will discuss the formation and control of helical states in different systems. The thesis will begin with a brief review of the quantum Hall effect and fractional quantum Hall effect, the theoretical and experimental study of ν = 2/3 edge states, and spin polarization of the ν = 2/3 state. Chapter 2 will discuss the investigation of transport properties of helical domain walls between incompressible spin-polarized ν = 2/3 and spin unpolarized ν= 2/3. Experimentally, the current carried by helical domain walls is found to be substantially smaller than the prediction of the naive model. The experimental results are compared with detailed Luttinger liquid theory and it is showed that inclusion of spin non-conserving tunneling process reconciles theory with experiment. Chapter 3 will discuss the magnetic and transport properties of EuSe/Bi2Se3, which may enable a control of helical states related to topological surface states in topological insulators by using magnetic proximity effect. A metamagnetic insulator EuSe, which consists of various magnetic phases, is proven to grow on Bi2Se3 in (001) direction. The magnetic and transport results indicate that the interfacial exchange between EuSe and Bi2Se3is antiferromagnetic, which may modify the surface states and result in even more nontrivial physics that are not observed before.
Degree
Ph.D.
Advisors
Wang, Purdue University.
Subject Area
Energy|Atomic physics|Condensed matter physics|Electromagnetics|Materials science|Physics
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