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This book reveals unique transport phenomena and functionalities in topological insulators coupled with magnetism and superconductivity. Topological insulators are a recently discovered class of materials that possess a spin-momentum-locked surface state. Their exotic spin texture makes them an exciting platform for investigating emergent phenomena, especially when coupled with magnetism or superconductivity. Focusing on the strong correlation between electricity and magnetism in magnetic topological insulators, the author presents original findings on current-direction-dependent nonreciprocal resistance, current-induced magnetization reversal and chiral edge conduction at the domain wall. In addition, he demonstrates how the coupling between superconductivity and topological surface state leads to substantial nonreciprocal resistance. The author also elucidates the origins of these phenomena and deepens readers’ understanding of the topologically nontrivial electronic state. The book includes several works which are published in top journals and were selected for the President’s Award by the University of Tokyo and for the Ikushi Prize, awarded to distinguished Ph.D. students in Japan.

### Chapter 1. Introduction

Abstract
Topological insulator is a material with an insulating bulk and conductive surface states. The topological surface state is characterized by a unique coupling between electron spin and momentum, namely spin-momentum locking. To begin with the thesis, we introduce the theoretical background and experimental discovery of topological insulators in the historical order. We then explain emergent physical properties resulting from the interplay between spin-momentum locking, magnetism, and superconductivity, such as quantum anomalous Hall effect.
Kenji Yasuda

### Chapter 2. Experimental Methods

Abstract
In this chapter, we summarize the experimental methods used in this thesis. We epitaxially grew the topological insulator heterostructure using molecular beam epitaxy and fabricated the thin film into devices. We then characterized the films with electrical measurement and a magnetic force microscope at low temperatures.
Kenji Yasuda

### Chapter 3. Quantum Hall Physics in Magnetic Topological Insulators

Abstract
Magnetic topological insulator, where spontaneous magnetization is introduced by magnetic doping, realizes quantum anomalous Hall effect (QAHE). The chiral edge state (CES) at the edge of QAHE is a promising candidate for a dissipationless electronic channel under zero magnetic field. The realization temperature of QAHE was, however, extremely low below one hundred mK. To overcome this limitation, we fabricated a magnetic modulation-doped topological insulator and realized QHE and QAHE above 2 K. We utilized the high-temperature QAHE to prove and manipulate CES at the magnetic domain wall employing magnetic force microscope.
Kenji Yasuda

### Chapter 4. Spintronic Phenomena in Magnetic/Nonmagnetic Topological Insulator Heterostructures

Abstract
In this chapter, we demonstrate various interesting spintronic functionalities of magnetic topological insulators. The basic idea is to couple spin-momentum locked surface conduction electrons with localized spins, or magnetism. The coupling between them enables electric control of magnetism and vice versa. We experimentally find nonlinear nonreciprocal transport, i.e., diode-like effect, current-induced magnetization switching, skyrmion formation, and its current-induced motion. We show that the efficiency of these effects is much higher than that of conventional spintronic materials due to the strong entanglement of spin and momentum at the surface state.
Kenji Yasuda

### Chapter 5. Transport Property of Topological Insulator/Superconductor Interface

Abstract
Superconducting proximity coupled topological surface state is theoretically expected to exhibit topological superconductivity. In this chapter, to study the unique characteristics of the superconductivity, we focus on the interfacial superconductivity of Bi$$_2$$Te$$_3$$/FeTe. The nonreciprocal measurement is shown to be largely enhanced below the superconducting temperature, which is attributed to the interplay between spin-momentum locking and superconductivity.
Kenji Yasuda

### Chapter 6. Summary

Abstract
We conclude the thesis and discuss future perspectives.
Kenji Yasuda