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Author: Jue Jiang
Publisher:
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Languages : en
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The magnetic topological insulator (TI) and the superconductor look different from each other, however, they share a similar electrical transport property of a profound significance: zero resistance. Their potential in the future low-power-consumption applications is beyond measure, therefore, the research attention on TI has been dramatically expanding since its debut in 2009, and the study of superconductivity keeps inspiring people of generations in the past 100 years. The realization of the non-dissipative channel in magnetic TI requires the broken time-reversal-symmetry by ferromagnetic dopants. The engineering of ferromagnetism, in turn, induces new topological phenomena. In this dissertation, we show that by fabricating a magnetic TI/pure TI/magnetic TI sandwich structure, rigorous quantum anomalous Hall (QAH) effect could be realized along with axion insulator state or topological Hall effect, depending on the sample structure. In Cr-doped/non-doped/V-doped TI heterostructures, QAH effect emerges when the magnetizations of the Cr-doped and V-doped magnetic layers are parallel, while an axion insulator state with zero Hall resistance and insulating longitudinal resistance appears when magnetization alignment is anti-parallel; In an Cr-doped/non-doped/Cr-doped TI structure, by tuning the chemical potential, QAH effect crossovers to topological Hall effect, where the electron spins form topologically non-trivial spin textures.Superconductivity, on the other hand, would be destroyed in a ferromagnet due to the decoupling of a Cooper pair by the exchange coupling. Therefore, a spin-singlet Cooper pair is not able to survive in a ferromagnet more than a few nanometers. In this dissertation, however, we show that in a ferromagnetic Ni nanowire (500 nm wide and 40 nm thick), by simply adding a thin Cu buffer layer with natural oxidation between superconducting/ferromagnetic interface, an unusual long-range superconducting proximity effect (up to 136 nm) emerges. Strong evidence points to the Cu oxides for providing a noncollinear magnetic profile that is crucial to the induction of spin-triplet. The spin-triplet pairing can have two electrons with the same spin direction, and thus immune to the exchange coupling of the ferromagnetic nanowire. The experiments of magnetic TI heterostructures and spin-triplet superconductivity presented in this dissertation would inspire more relating studies and pave the way for next-generation energy-efficient spintronic and electronic applications.
Author: Jue Jiang
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
The magnetic topological insulator (TI) and the superconductor look different from each other, however, they share a similar electrical transport property of a profound significance: zero resistance. Their potential in the future low-power-consumption applications is beyond measure, therefore, the research attention on TI has been dramatically expanding since its debut in 2009, and the study of superconductivity keeps inspiring people of generations in the past 100 years. The realization of the non-dissipative channel in magnetic TI requires the broken time-reversal-symmetry by ferromagnetic dopants. The engineering of ferromagnetism, in turn, induces new topological phenomena. In this dissertation, we show that by fabricating a magnetic TI/pure TI/magnetic TI sandwich structure, rigorous quantum anomalous Hall (QAH) effect could be realized along with axion insulator state or topological Hall effect, depending on the sample structure. In Cr-doped/non-doped/V-doped TI heterostructures, QAH effect emerges when the magnetizations of the Cr-doped and V-doped magnetic layers are parallel, while an axion insulator state with zero Hall resistance and insulating longitudinal resistance appears when magnetization alignment is anti-parallel; In an Cr-doped/non-doped/Cr-doped TI structure, by tuning the chemical potential, QAH effect crossovers to topological Hall effect, where the electron spins form topologically non-trivial spin textures.Superconductivity, on the other hand, would be destroyed in a ferromagnet due to the decoupling of a Cooper pair by the exchange coupling. Therefore, a spin-singlet Cooper pair is not able to survive in a ferromagnet more than a few nanometers. In this dissertation, however, we show that in a ferromagnetic Ni nanowire (500 nm wide and 40 nm thick), by simply adding a thin Cu buffer layer with natural oxidation between superconducting/ferromagnetic interface, an unusual long-range superconducting proximity effect (up to 136 nm) emerges. Strong evidence points to the Cu oxides for providing a noncollinear magnetic profile that is crucial to the induction of spin-triplet. The spin-triplet pairing can have two electrons with the same spin direction, and thus immune to the exchange coupling of the ferromagnetic nanowire. The experiments of magnetic TI heterostructures and spin-triplet superconductivity presented in this dissertation would inspire more relating studies and pave the way for next-generation energy-efficient spintronic and electronic applications.
Author: Emily Elizabeth
Publisher:
ISBN:
Category : Technology & Engineering
Languages : en
Pages : 0
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Author: Jianxiao Zhang
Publisher:
ISBN:
Category :
Languages : en
Pages :
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The discovery of topological states of matters has sparked intense interests amongresearchers in the past decade. Topologically non-trivial band structure in thesequantum states can give rise to a variety of topological phenomena, the experimentaldemonstration of which can have a huge impact on our understandingof fundamental states of matter. Transport measurement is one of the majorexperimental techniques to probe these topological phenomena. This dissertationis devoted to theoretical and numerical studies of quantum transport phenomenain a variety of topological materials, including magnetic topological insulator films,the quantum anomalous Hall insulator/superconductor hetero-structures, the kinkstates in bilayer graphene and the photonic crystal of topological mirror insulatorphase in the optical regime. The numerical simulations of transport phenomenaand the analytical understanding of the underlying physical mechanism in thisdissertation will provide guidance for the future transport measurements.The numerical methods to simulate quantum transport in this dissertation arebased on Landauer-Bttiker formalism and Greens function method, which willbe introduced in Chapter 2. The transmission through certain sample regionscan be extracted from the Greens function method and serves as the input forthe Landauer-Bttiker formalism to compute conductance tensor that is directlymeasured in transport experiments. Physical understanding of the transportmechanism can be provided by analyzing different components of the transmissionmatrix, in combination with other analytical methods for transport phenomena.Defects and impurities can be incorporated in numerical simulations by includingrandom potentials into the model Hamiltonian, and thus this method can be appliedin different transport regimes, from ballistic to diffusive transport.Chapter 3 to 5 of the dissertation is to apply the above numerical methodsto three different topological mesoscopic systems: magnetic topological insulator(MTI) films, quantum anomalous Hall insulator (QAHI) - superconductor (SC)junctions, and bilayer graphene devices.Chapter 3 is dedicated to the study of quantum transport through magnetictextures in a thin film of MTI. We focus on both the longitudinal and Hall transports,which reveal complicated features due to the coexistence of strong spin-orbit couplingfrom TI materials and magnetic non-colinearity from magnetic textures in thissystem. The manifested Hall transport can be induced by different topologicalmechanisms, including the intrinsic anomalous Hall effect from strong SOC and thetopological Hall effect (or known as geometric Hall effect) from magnetic textures.Thus, this system provides a nice platform to understand the interplay betweenspin-orbit coupling and real-space magnetic texture, as well as disorder scatterings.Our numerical simulations have shown different roles of spin-orbit coupling in theclean and disordered limits for this system. In the clean limit when SOC strengthis increased, the topological Hall conductance (THC) almost remains constant butthe topological Hall resistance (THR) can increase by an order of magnitude dueto the reduction of longitudinal conductance, caused by SOC-induced spin flips.However, in the disordered limit, both the THC and THR increase with increasingSOC, while longitudinal conductance is not influenced much by SOC.In Chapter 4, we study the transport of chiral edge channels in a QAHI/superconductorjunction. This type of hetero-junction has been recently fabricated andmeasured in experiments, in pursue of topological superconductivity and Majoranafermions. We focus on the disorder effect in the weak superconductor proximitylimit. Our results show that the quantized valued of conductance remains robustfor a single chiral edge channel even in the presence of disorder in the zero-biaslimit. However, such quantization is broken down for a finite bias, or for multiplechiral edge modes, or for the coexistence of a single chiral edge mode with othertrivial metallic modes, when disorders are present. Our theory provides guidanceto understand transport phenomena in these systems for future experiments.Chapter 5 is a simulation of transport behaviors through the so-called kinkstates in a bilayer graphene device under external electric and magnetic fields. Thedevice, known as a valley valve and electron beam splitter, has been fabricatedby our experimental collaborators and its unusual transport properties have beenmeasured experimentally. Our numerical simulations provide a justification of theguiding center physical picture for topological transport through this device.Chapter 6 goes beyond electronic systems and concerns topological phase inphotonic systems. We utilize a method of dynamic evolution of states to studya topological crystalline insulator phase in a photonic system. The crystallineprotection, achieved by the fine manufacturing of emulated atoms in a photoniclattice, selectively pumps incident states with a certain parity while reflects theother.The studies in the dissertation are in close collaboration with experimentalgroups, including Prof. Moses Chans and Prof. Cui-zu Changs group for the transportmeasurements in MTI films and QAHI/SC junctions, Prof. Jun Zhus groupfor the experiments on the bilayer graphene device, and Prof. Mikael Rechtsmansgroup for the photonic topological systems.
Author: Michal Papaj
Publisher:
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Category :
Languages : en
Pages : 0
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Topological phases of matter attract constant attention in the condensed matter physics community, both due to the fundamental yet simple principles that govern them, and a multitude of experimental observations with the potential for technological applications. Among the ways of studying such materials, quantum transport methods prove to be of particular importance. In this thesis, I touch upon many aspects of quantum transport in topological materials. First, I introduce a novel type of Hall effect, called Magnus Hall effect, that allows one to probe Berry curvature in ballistic, time-reversal invariant systems that break inversion symmetry. Next, I present a detailed characterization of extrinsic Nernst effect in Dirac and Weyl semimetals, providing interpretation of existing experimental results and predictions for new enhanced responses in materials such as Fe3Sn2. In the following section, I demonstrate that a strong disorder can lead to a novel behavior of Dirac fermions in surface states of topological crystalline insulators, resulting in appearance of nodal arcs in place of Dirac points and in tilting of the Dirac cone. In the second part of the thesis, I focus on topological superconductors, starting by presenting a new method for creating Majorana zero modes using segmented Fermi surface. This approach, based on the Fermi surface of Bogoliubov quasiparticles allows for the reduction of the magnetic field required to induce a topological phase transition and reduces the number of spurious, low energy modes that hamper observation and utilization of Majorana zero modes. Finally, I show that the presence of multiple Majorana modes in a strongly correlated superconducting island leads to Kondo-like behavior with a topological twist.
Author: Gregory Tkachov
Publisher: Pan Stanford
ISBN: 9789814613255
Category : Science
Languages : en
Pages : 0
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This book came into being under the influence of dynamic developments that have occurred in condensed matter physics after the recent discovery of a new class of electronic materials called topological insulators. A topological insulator is a material that behaves as a band insulator in its interior, while acting as a metallic conductor at its surface. The surface current carriers in these systems have Dirac-like nature and are protected by an intrinsic topological order, which is of great interest for both fundamental research and emerging technologies, especially, in the fields of electronics, spintronics, and quantum information. The realization of the application potential of topological insulators requires a comprehensive and deep understanding of transport processes in these novel materials. The book explores the origin of the protected Dirac-like states in topological insulators and gives insight into some of their representative transport properties. These include the quantum spin–Hall effect, nonlocal edge transport, backscattering of helical edge and surface states, weak antilocalization, unconventional triplet p-wave superconductivity, topological bound states, and emergent Majorana fermions in Josephson junctions, as well as superconducting Klein tunneling.
Author: Anasua Chatterjee
Publisher:
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Languages : en
Pages : 0
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Author: Emmanouil Xypakis
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Category :
Languages : en
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Author: Xurui Zhang
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Category : Quantum Hall effect
Languages : en
Pages :
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The success of mechanical exfoliation on graphene has paved a new field of research in two-dimensional (2D) materials. As one of the transition metal dichalcogenides (TMDCs), tungsten ditelluride (WTe2) has attracted a mass of interests since a novel non-saturating positive magnetoresistance was discovered in 2014. A lot of researches in this material have been published, such as band structure studies with angle-resolved photoelectron spectroscopy (ARPES), quantum oscillations in transport measurements, superconductivity in WTe2 etc. It is worth mentioning that the topological properties of WTe2 have been verified in both bulk (type-II Weyl semi-metal) and in monolayer (2D topological insulator) forms. The topological properties make WTe2 a potential candidate for hosting Majorana bound state, which is theoretically predicted to arise from the proximity effect between a s-wave superconductor and the surface states of a topological insulator (TI). This dissertation will present quantum transport studies in multi-layer WTe2, which acts as an intermediate between the bulk and monolayer limits. Our goal is to explore the transport properties in WTe2 itself, and investigate its interaction with other quantum materials, especially superconductors. A series of different types of devices based on multi-layer WTe2, including Hall bars, FET-like devices and Josephson junctions, have been fabricated and measured in the magnetic fields up to 12 T at low temperatures down to 20 mK. In order to improve the performance of the devices, the hexagonal boron nitride (hBN) flakes are used to build sandwiched structures for thin WTe2 flakes. The main results are presented as follows. First, thickness-dependent quantum transport measurements suggest that the novel "turn-on" behavior in WTe2 take the origin of the Kohler's rule in Fermi liquid state. The "turn-on" behavior accompanied by the large magnetoresistance (MR) will be effectively suppressed by the loss of perfect carrier compensation. Strikingly, however, the trend of non-saturation is unaffected at all which indicates the possibility of other origins of the non-saturating MR. In addition, the angle-dependent MR measurements reveal that the electronic 3D nature of multi-layer WTe2 and the Fermi surface anisotropy depends on the sample thickness. Second, we observe an obvious crossover between weak anti-localization (WAL) and weak localization (WL) in an disordered ultrathin WTe2 flake. The mechanism of the crossover shows coexistence and competition among several characteristic lengths, including the dephasing length, the spin-flip length, and the mean free path. Furthermore, the interplay of quantum interference and electron-electron interaction is also observed. Third, an unconventional quasi-3D quantum Hall effect (QHE) is observed in a high quality flake with much lower carrier density and higher mobility than ordinary WTe2. The quasi-3D QHE act as a collection of several weakly-coupled 2D QHE layers, which might be resulted from a dimerization or tetramization effect. Fourth, in the Ta-WTe2-Ta Josephson junctions, supercurrent state is successfully induced into the multi-layer WTe2 by proximity effect. We observe the fast mode superconducting quantum interference pattern, which indicates the presence of edge supercurrent resulted from the intrinsic edge states of WTe2. In addition, the multiple frequencies observed in the interference pattern might be from the terrace structure along the sample edges. Finally, the presence of the multi-dips in differential resistance in the Josephson junctions with incomplete superconducting state marks the multiple Andreev reflections in WTe2, which might be due to the multiple channels formed along the Josephson junction length. Furthermore, the related experimental procedures, improvements and different data processing methods have also been presented in the main text and appendixes.
Author: Dario Bercioux
Publisher: Springer
ISBN: 3319763881
Category : Technology & Engineering
Languages : en
Pages : 261
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This book covers basic and advanced aspects in the field of Topological Matter. The chapters are based on the lectures presented during the Topological Matter School 2017. It provides graduate level content introducing the basic concepts of the field, including an introductory session on group theory and topological classification of matter. Different topological phases such as Weyls semi-metals, Majoranas fermions and topological superconductivity are also covered. A review chapter on the major experimental achievements in the field is also provided. The book is suitable not only for master, graduate and young postdoctoral researchers, but also to senior scientists who want to acquaint themselves with the subject.
Author: Sadamichi Maekawa
Publisher: Oxford University Press
ISBN: 0198787073
Category : Science
Languages : en
Pages : 541
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Book Description
In a new branch of physics and technology, called spin-electronics or spintronics, the flow of electrical charge (usual current) as well as the flow of electron spin, the so-called "spin current", are manipulated and controlled together. This book is intended to provide an introduction and guide to the new physics and applications of spin current.
