Topological Insulator And Magnetically Doped Topological Insulator Thin Films By Molecular Beam Epitaxy
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Topological Insulator And Magnetically Doped Topological Insulator Thin Films By Molecular Beam Epitaxy
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Author : Shuang Li
language : en
Publisher:
Release Date : 2013
Topological Insulator And Magnetically Doped Topological Insulator Thin Films By Molecular Beam Epitaxy written by Shuang Li and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2013 with categories.
Searching for energy dissipation-less systems has become increasingly important for low power electronic devices. Topological insulators, a new topological state of quantum matter, have recently been proposed as an emerging material for use in low power electronics, because of the unique transport along its topologically protected edge/surface states. In addition, it has been predicted that the incorporation of magnetic elements into topological insulators could lead to the quantum anomalous Hall state, which is a truly dissipation-less system. However, the material quality of topological insulator thin films remains as a major stumbling block for exploring the novel physics of topological insulators and their proposed applications. In the first part of this thesis, I will first describe an advanced thin film deposition technique, molecular beam epitaxy (MBE) and the mini-MBE system we designed and built for topological insulator thin film growth. Then I will briefly illustrate some basic principles and sample preparation methods for a variety of characterization techniques we used for the material property investigation. In the second part of this thesis, I will present the growth and characterization of topological insulator bismuth telluride thin films grown by a two-step MBE process developed as part of this research. By optimizing the growth recipe and particularly developing the two-step growth method, defect densities were significantly reduced and higher crystal and surface quality bismuth telluride thin films were achieved. The existence of a topological surface state on our bismuth telluride thin films was also confirmed. The Fermi level of our bismuth telluride thin film was tuned to very close to the bulk gap region. The successful growth of centimeter-sized, uniform, high quality topological insulator thin films provides an excellent platform for both fundamental studies of the properties of topological insulators and fabrications of mesoscopic devices. Finally, I will report on the first successful growth of gadolinium substituted bismuth telluride thin films with high Gd concentrations by MBE. We systematically investigated the crystal structure, band structure, magnetic, and electronic properties of gadolinium substituted bismuth telluride thin films. The topological surface state was found to remain intact by Gd substitution into bismuth telluride. Although ferromagnetic behavior in gadolinium substituted bismuth telluride thin films was not observed above 2K by both magnetic and magneto-transport measurements, gadolinium substituted bismuth telluride thin films were found to have a Curie susceptibility due to the paramagnetic Gd ions with an atomic magnetic moment of 6.93 Bohr magneton per Gd ion, which suggests that it is possible to realize dissipation-less transport with a small external magnetic field or with a ferromagnetic layer on top of gadolinium substituted bismuth telluride thin films.
Transport Studies Of The Electrical Magnetic And Thermoelectric Properties Of Topological Insulator Thin Films
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Author : Jinsong Zhang
language : en
Publisher: Springer
Release Date : 2016-04-18
Transport Studies Of The Electrical Magnetic And Thermoelectric Properties Of Topological Insulator Thin Films written by Jinsong Zhang and has been published by Springer this book supported file pdf, txt, epub, kindle and other format this book has been release on 2016-04-18 with Science categories.
This book presents the transport studies of topological insulator thin films grown by molecular beam epitaxy. Through band structure engineering, the ideal topological insulators, (Bi1−xSbx)2Te3 ternary alloys, are successfully fabricated, which possess truly insulating bulk and tunable conducting surface states. Further transport measurements on these ternary alloys reveal a disentanglement between the magnetoelectric and thermoelectric properties. In magnetically doped topological insulators, the fascinating quantum anomalous Hall effect was experimentally observed for the first time. Moreover, the topology-driven magnetic quantum phase transition was Systematically controlled by varying the strength of the spin-orbital coupling. Readers will not only benefit from the description of the technique of transport measurements, but will also be inspired by the understanding of topological insulators.
Binary And Rare Earth Doped Topological Insulator Thin Films
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Author : Sara Elizabeth Harrison
language : en
Publisher:
Release Date : 2015
Binary And Rare Earth Doped Topological Insulator Thin Films written by Sara Elizabeth Harrison and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2015 with categories.
Topological insulators (TIs) are a newly discovered class of electronic materials which are characterized by an insulating bulk band gap and metallic conducting edge/surface states. Their novel electronic band structure arises from strong spin-orbit coupling that leads to bulk energy band inversion which necessitates the formation of metallic states at their physical boundaries with dissimilar materials. These metallic edge/surface states have intriguing spin-momentum locking properties and a very robust nature, due to scattering protection by time-reversal symmetry (TRS), which make them interesting from both a fundamental science perspective as well as for their potential use in future generation electronic and spintronic applications. Recently, the discovery of the three-dimensional (3D) TIs in the bismuth telluride family of materials has provided an exciting new direction for TI research. The surface states on these 3D TIs are detectable at room temperature which eases the harsh requirements previously needed to study TIs and increases their potential for use in practical applications. As commercially successful thermoelectric materials, the use of widely accessible bulk crystals of the bismuth telluride family of 3D TIs has enabled early studies of their topological surface states. However, a prerequisite for realizing many proposed TI applications is the synthesis of high crystalline quality thin films which necessitates efforts in thin film materials engineering. In addition, a new area of TI materials research has also recently emerged around breaking the TRS in 3D TIs by inducing ferromagnetism through magnetic doping. This approach is predicted to provide a promising route for realizing exotic physical states, such as the recently discovered quantum anomalous Hall state. However, exploring magnetically induced phenomena has been experimentally challenging which has prompted the search for alternative TI systems through fundamental magnetic doping studies. This dissertation focuses on the growth and characterization of binary and rare earth-doped bismuth telluride thin films. All samples were fabricated using molecular beam epitaxy (MBE) and their structural, electronic, and magnetic properties were characterized using a comprehensive set of surface- and bulk-sensitive analytical techniques. The development of a new two-temperature step MBE growth process for bismuth telluride thin films is presented. The two-step method is shown to yield films of high crystallinity with significantly improved material properties over films grown using other growth recipes. This growth technique served as the starting platform for other studies presented in this work, including investigations into surface preparation techniques for ex situ grown TI thin films and magnetic doping studies with rare earth elements. Major shortcomings of conventional preparation techniques for preserving or restoring the surface of air exposed TI films are also presented. Commonly employed sputter cleaning is shown to be incompatible with TI samples that are prone to severe oxidation such as magnetically doped TIs. Se- and Te-capping layer studies provide new evidence that this commonly employed technique is ineffective at preserving the as-grown properties of bismuth telluride thin films. Alternatively, the efficacy of in situ cleaving for preparation of clean binary and rare earth-doped TI surfaces is demonstrated. Finally, the first experimental work on MBE-grown Dy-doped bismuth telluride thin films is presented. X-ray studies reveal that large concentrations of Dy, ranging from 0% to 35.5% (in % of the Bi sites), can be incorporated into the host bismuth telluride crystal lattice without the formation of secondary phases. A subset of films in the doping series are shown to maintain a high degree of crystallinity with evidence for substitutional doping of Dy and the absence of intercalation in the van der Waals gaps. Electronic band structure measurements show that there is a critical Dy doping concentration above which evidence for a sizable gap (tens of meV) in the surface state is detected. Bulk magnetometry reveals paramagnetic behavior down to low temperatures for all samples in the doping series. The use of rare earth dopants introduces the highest magnetic moments into a TI system, which could have a transformative potential for TI-based applications in the future.
Topological Insulators
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Author : Ke He
language : en
Publisher: Elsevier Inc. Chapters
Release Date : 2013-11-23
Topological Insulators written by Ke He and has been published by Elsevier Inc. Chapters this book supported file pdf, txt, epub, kindle and other format this book has been release on 2013-11-23 with Science categories.
Material is a key to the experimental observation of novel quantum phenomena predicted in topological insulators. In this chapter, we review the recent theoretic and experimental efforts devoted to improving the existing topological insulator materials and exploring new topological insulators. The emphasis is on growth and engineering of the properties of topological insulator thin films by molecular beam epitaxy for realization of various quantum effects.
Advanced Topological Insulators
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Author : Huixia Luo
language : en
Publisher: John Wiley & Sons
Release Date : 2019-03-12
Advanced Topological Insulators written by Huixia Luo and has been published by John Wiley & Sons this book supported file pdf, txt, epub, kindle and other format this book has been release on 2019-03-12 with Technology & Engineering categories.
This book is the first pedagogical synthesis of the field of topological insulators and superconductors, one of the most exciting areas of research in condensed matter physics. Presenting the latest developments, while providing all the calculations necessary for a self-contained and complete description of the discipline, it is ideal for researchers and graduate students preparing to work in this area, and it will be an essential reference both within and outside the classroom. The book begins with the fundamental description on the topological phases of matter such as one, two- and three-dimensional topological insulators, and methods and tools for topological material's investigations, topological insulators for advanced optoelectronic devices, topological superconductors, saturable absorber and in plasmonic devices. Advanced Topological Insulators provides researchers and graduate students with the physical understanding and mathematical tools needed to embark on research in this rapidly evolving field.
Topological Insulator Systems With Magnetism
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Author : Joon Lee
language : en
Publisher:
Release Date : 2014
Topological Insulator Systems With Magnetism written by Joon Lee and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2014 with categories.
This dissertation describes topological insulator systems hybridized with magnetism. The Dirac surface states induced by strong spin-orbit interaction can be modified by breaking time-reversal symmetry that protects the surface state. We study the modified surface states of topological insulators by introducing magnetism by doping magnetic atoms or interfacing a magnetic layer to the surface. Also, we explore potential spintronics applications of topological insulators by utilizing magnetic tunnel junctions to evidence the inherently spin-polarized texture of the topological insulator surface state. For this dissertation research, single crystalline topological insulator thin films grown by molecular beam epitaxy have been employed. From the motivation of breaking time-reversal symmetry in the surface state, the first experiments study the structural, magnetic, and magneto-transport properties of a magnetically doped, three-dimensional topological insulator, bismuth telluride doped with Mn. We observed ferromagnetism with a Curie temperature up to 17 K in films with ~2-10% Mn concentrations. The observed ferromagnetism is independent of carrier density in the Mn-doped bismuth telluride films, suggesting that it is not mediated by charge carriers. The next topological insulator system with magnetism is a hybrid topological insulator/ferromagnet heterostructure as a new approach for topological insulator hybrid systems using a dilute magnetic semiconductor Ga1-xMnxAs. A highly resistive Ga1-xMnxAs with out-of-plane magnetic anisotropy is cleanly interfaced with a topological insulator Bi2-xSbxTe3-ySey by molecular beam epitaxy. Magneto-transport measurements on a top-gated heterostructure device show a crossover from positive magneto-conductance to negative magneto-conductance as well as a systematic emergence of an anomalous Hall effect as the temperature is lowered or as the chemical potential approaches the Dirac point. The results are possibly interpreted as the modification of the surface state at the interface by the adjacent, ferromagnetic Ga1-xMnxAs layer. The last topological insulator system with magnetism is a topological insulator channel with a magnetic tunnel junction on it. We seek a potential role of topological insulators in spintronics as generators of carrier spin polarization. Electrical detection of the inherent spin polarization of the topological insulator surface state was demonstrated using a permalloy/Al2O3 magnetic tunnel junction on a (Bi,Sb)2Te3 channel. The observed hysteretic spin signals occurring at the magnetic switching field of the ferromagnet permalloy layer can be interpreted as the projection of the current-induced spin polarization on a topological insulator surface onto the magnetization of the ferromagnet via tunneling.
Quantized Phenomena Of Transport And Magneto Optics In Magnetic Topological Insulator Heterostructures
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Author : Masataka Mogi
language : en
Publisher: Springer Nature
Release Date : 2022-05-07
Quantized Phenomena Of Transport And Magneto Optics In Magnetic Topological Insulator Heterostructures written by Masataka Mogi and has been published by Springer Nature this book supported file pdf, txt, epub, kindle and other format this book has been release on 2022-05-07 with Computers categories.
This book presents experimental studies on emergent transport and magneto-optical properties in three-dimensional topological insulators with two-dimensional Dirac fermions on their surfaces. Designing magnetic heterostructures utilizing a cutting-edge growth technique (molecular beam epitaxy) stabilizes and manifests new quantization phenomena, as confirmed by low-temperature electrical transport and time-domain terahertz magneto-optical measurements. Starting with a review of the theoretical background and recent experimental advances in topological insulators in terms of a novel magneto-electric coupling, the author subsequently explores their magnetic quantum properties and reveals topological phase transitions between quantum anomalous Hall insulator and trivial insulator phases; a new topological phase (the axion insulator); and a half-integer quantum Hall state associated with the quantum parity anomaly. Furthermore, the author shows how these quantum phases can be significantly stabilized via magnetic modulation doping and proximity coupling with a normal ferromagnetic insulator. These findings provide a basis for future technologies such as ultra-low energy consumption electronic devices and fault-tolerant topological quantum computers.
Magnetic Topological Insulator And Quantum Anomalous Hall Effect
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Author : Xufeng Kou
language : en
Publisher:
Release Date : 2015
Magnetic Topological Insulator And Quantum Anomalous Hall Effect written by Xufeng Kou and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2015 with categories.
The engineering of topological surface states is a key to realize applicable devices based on topological insulators (TIs). Among various proposals, introducing magnetic impurities into TIs has been proven to be an effective way to open a surface gap and integrate additional ferromagnetism with the original topological order. In this Dissertation, we study both the intrinsic electrical and magnetic properties of the magnetic TI thin films grown by molecular beam epitaxy. By doping transition element Cr into the host tetradymite-type V-VI semiconductors, we achieve robust ferromagnetic order with a strong perpendicular magnetic anisotropy. With additional top-gating capability, we realize the electric-field-controlled ferromagnetism in the magnetic TI systems, and demonstrate such magneto-electric effects can be effectively manipulated, depending on the interplays between the band topology, magnetic exchange coupling, and structural engineering. Most significantly, we report the observation of quantum anomalous Hall effect (QAHE) in the Cr-doped (BiSb)2Te3 samples where dissipationless chiral edge conduction is realized in the macroscopic millimeter-size devices without the presence of any external magnetic field, and the stability of the quantized Hall conductance of e2/h is well-maintained as the film thickness varies across the 2D hybridization limit. With additional quantum confinement, we discover the metal-to-insulator switching between two opposite QAHE states, and reveal the universal QAHE phase diagram in the thin magnetic TI samples. In addition to the uniform magnetic TIs, we further investigate the TI/Cr-doped TI bilayer structures prepared by the modulation-doped growth method. By controlling the magnetic interaction profile, we observe the Dirac hole-mediated ferromagnetism and develop an effective way to manipulate its strength. Besides, the giant spin-orbit torque in such magnetic TI-based heterostructures enables us to demonstrate the current-induced magnetization switching with the critical current density much lower than other heavy metal/magnet systems. Our work on the magnetic TIs and their heterostructures thus unfolds new avenues for novel multifunctional nano-electronics and non-volatile spintronic applications.
Emergent Transport Properties Of Magnetic Topological Insulator Heterostructures
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Author : Kenji Yasuda
language : en
Publisher: Springer Nature
Release Date : 2020-09-07
Emergent Transport Properties Of Magnetic Topological Insulator Heterostructures written by Kenji Yasuda and has been published by Springer Nature this book supported file pdf, txt, epub, kindle and other format this book has been release on 2020-09-07 with Computers categories.
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.
Exploring The Three Dimensional Quantum Anomalous Hall Effect And Topological Superconductivity In Topological Insulator Heterostructures
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Author : Ruoxi Zhang
language : en
Publisher:
Release Date : 2023
Exploring The Three Dimensional Quantum Anomalous Hall Effect And Topological Superconductivity In Topological Insulator Heterostructures written by Ruoxi Zhang and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2023 with categories.
Topological materials exhibit unique properties that make them robust against local defects and perturbations. These properties stem from the distinctive band structure compared to conventional materials, which are characterized by different topological invariants. In this thesis, we study two phenomena that arise in epitaxial topological insulator (TIs) films/heterostructures. The first phenomenon is the quantum anomalous Hall (QAH) effect. The QAH effect requires no external magnetic field and possesses non-dissipative chiral edge states that are resistant to local disorders. The second phenomenon is the topological superconducting (TSC) states. The TSC state hosts quasiparticle excitations, including Majorana zero modes (MZMs) and chiral Majorana edge modes (CMEMs). These excitations have potential applications in fault-tolerant topological quantum computations. The first experimental observation of the QAH effect was realized in molecular beam epitaxy (MBE)-grown magnetically doped TI thin films, which offer the advantages of scalability and reproducibility. However, the introduction of magnetic dopants also leads to higher disorder density in TI thin films. To overcome this limitation, we employed MBE-grown magnetically doped TI/TI/magnetically doped TI sandwich heterostructures to separate the magnetic dopants from the TI bulk. By employing this method, we successfully realized high Chern number QAH states, Chern domain walls, and hundred-nanometer-thick QAH samples. These results reveal new phases of matter and the underlying physics of the QAH phase transition induced by interlayer coupling. The second half of the thesis describes our effort in the TSC state in QAH insulators and TIs with induced superconductivity. The first project in this effort focuses on the search for CMEMs, which are predicted to emerge in QAH/superconductor hybrid structures. We examined a prior transport experiment that claimed the realization of CMEMs by measuring the two-terminal resistance. We improved the experimental design by fabricating Josephson junction and tunneling junction devices based on Bi2Te3 and (Bi,Sb)2Te3, and obtained transport results that suggest the dominance of Dirac surface states in vortex generation in the junction area.