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Author: Run Xiao
Publisher:
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Languages : en
Pages : 0
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This dissertation focuses on the synthesis, characterization, fabrication, and electrical transport measurements of topological materials, including magnetically doped topological insulators and Dirac semimetal Cd3As2. Bismuth-chalcogenide topological insulators have time-reversal-symmetry-protected surface states due to the strong spin-orbit coupling. Breaking the time-reversal symmetry by magnetic dopants can lead to fascinating exotic phenomena, such as the quantum anomalous Hall effect. On the other hand, Dirac semimetals host three-dimensional Dirac fermions and can be identified as a parent phase of other topological phases, such as Weyl semimetals. In this dissertation, quantum transport measurements are performed on thin films of topological materials to investigate and understand the unusual electronic states that host these topological phases. These studies can motivate and facilitate the development of potential applications of topological materials, especially in spintronics and quantum computing. The first topological material studied in this dissertation is a magnetically doped topological insulator system: Cr doped (Bi,Sb)2Te3 - (Bi,Sb)2Te3 - Cr doped (Bi,Sb)2Te3 sandwich heterostructure. By tuning the chemical and asymmetric potentials using dual gates, both the quantum anomalous hall effect, due to the topology in the momentum space, and the topological Hall effect, due to the topology in real space, can be observed in this heterostructure system. We also mapped out a phase diagram of the topological Hall and quantum anomalous Hall effects as a function of the chemical and asymmetry potentials, paving a way to understand and manipulate the chiral magnetic spin textures in real space. The second topological material is Dirac semimetal Cd3As2. We investigated the integer quantum Hall effect in Cd3As2 thin films under strong to moderate quantum confinement (thicknesses of 10 nm, 12 nm, and 15 nm). In all the films, we observed the integer quantum Hall effect in the spin-polarized lowest Landau level (filling factor [nu]=1) and at spin-degenerate higher index Landau levels with even filling factors ([nu]=2,4,6). We also observed the lifting of the Landau level spin degeneracy at v=3 with strong quantum confinement. A tight-binding calculation suggests that the enhanced g-factor due to the quantum confinement and corrections from nearby subbands can be the reason for the emergence of v=3 quantum Hall plateau. Last, we explored the introduction of the transition metal Mn into Cd3As2 thin films to break the time-reversal symmetry. Scanning transmission electron microscopy of these films shows a formation of an Mn-rich layer on top of a pure Cd3As2 layer using both uniform and delta doping methods. The low solubility of Mn in Cd3As2 can be the reason for the phase separation. The Mn-rich region shows out-of-plane magnetic anisotropy in superconducting quantum interference device magnetometry measurements. Moreover, the presence of the Mn surfactant lowers the carrier density in the Cd3As2 layer, and an incipient quantum Hall effect can be observed in low-temperature transport measurements.
Author: Run Xiao
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Languages : en
Pages : 0
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Book Description
This dissertation focuses on the synthesis, characterization, fabrication, and electrical transport measurements of topological materials, including magnetically doped topological insulators and Dirac semimetal Cd3As2. Bismuth-chalcogenide topological insulators have time-reversal-symmetry-protected surface states due to the strong spin-orbit coupling. Breaking the time-reversal symmetry by magnetic dopants can lead to fascinating exotic phenomena, such as the quantum anomalous Hall effect. On the other hand, Dirac semimetals host three-dimensional Dirac fermions and can be identified as a parent phase of other topological phases, such as Weyl semimetals. In this dissertation, quantum transport measurements are performed on thin films of topological materials to investigate and understand the unusual electronic states that host these topological phases. These studies can motivate and facilitate the development of potential applications of topological materials, especially in spintronics and quantum computing. The first topological material studied in this dissertation is a magnetically doped topological insulator system: Cr doped (Bi,Sb)2Te3 - (Bi,Sb)2Te3 - Cr doped (Bi,Sb)2Te3 sandwich heterostructure. By tuning the chemical and asymmetric potentials using dual gates, both the quantum anomalous hall effect, due to the topology in the momentum space, and the topological Hall effect, due to the topology in real space, can be observed in this heterostructure system. We also mapped out a phase diagram of the topological Hall and quantum anomalous Hall effects as a function of the chemical and asymmetry potentials, paving a way to understand and manipulate the chiral magnetic spin textures in real space. The second topological material is Dirac semimetal Cd3As2. We investigated the integer quantum Hall effect in Cd3As2 thin films under strong to moderate quantum confinement (thicknesses of 10 nm, 12 nm, and 15 nm). In all the films, we observed the integer quantum Hall effect in the spin-polarized lowest Landau level (filling factor [nu]=1) and at spin-degenerate higher index Landau levels with even filling factors ([nu]=2,4,6). We also observed the lifting of the Landau level spin degeneracy at v=3 with strong quantum confinement. A tight-binding calculation suggests that the enhanced g-factor due to the quantum confinement and corrections from nearby subbands can be the reason for the emergence of v=3 quantum Hall plateau. Last, we explored the introduction of the transition metal Mn into Cd3As2 thin films to break the time-reversal symmetry. Scanning transmission electron microscopy of these films shows a formation of an Mn-rich layer on top of a pure Cd3As2 layer using both uniform and delta doping methods. The low solubility of Mn in Cd3As2 can be the reason for the phase separation. The Mn-rich region shows out-of-plane magnetic anisotropy in superconducting quantum interference device magnetometry measurements. Moreover, the presence of the Mn surfactant lowers the carrier density in the Cd3As2 layer, and an incipient quantum Hall effect can be observed in low-temperature transport measurements.
Author: Gregory Tkachov
Publisher: CRC Press
ISBN: 9814613266
Category : Science
Languages : en
Pages : 180
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This book is the result of dynamic developments that have occurred in condensed matter physics after the recent discovery of a new class of electronic materials: 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 car
Author: Di Xiao
Publisher:
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Category :
Languages : en
Pages :
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Topological insulators are materials that are insulating in the bulk but that conduct via topologically protected states on the boundary. The concept of topology in condensed matter physics was first introduced to explain the integer quantum Hall (IQH) effect. The perfect quantization of these topologically protected edge states, insensitive to sample geometry and disorder, stimulated an extensive search for many exciting new topological materials. One of the milestones along the journey was the theoretical prediction and experimental discovery of Z2 topological insulators.The first class of Z2 topological insulators discovered was the 2-dimensional topological insulator (2D TI), also known as the quantum spin Hall (QSH) insulator. The 2D TI can be viewed as a variation of the IQH system but with time-reversal-symmetry (TRS). The topological invariant for a 2D TI is the Z2 number, defined by its nontrivial band structure instead of the Chern number in the IQH case. Generalizing this idea to 3 dimensions led to the discovery of the 3D TI with four Z2 invariants. Both the 2D and 3D TIs are of interest as model platforms for testing theoretical problems of fundamental interest. For instance, they allow us to realize artificial condensed matter analogs of fundamental particles such as Majorana fermions and axions that have yet to be observed in nature. They are also of interest for potential technological applications, principally spintronics and quantum computing.This dissertation focuses on the synthesis, characterization, and transport properties of both 2D and 3D TIs. We first discuss the 2D TI candidate material system, type II InAs/GaSb quantum wells, which exhibits a rich topological phase diagram that can be tuned by several parameters such as sample geometry or electrostatic gating. By changing the thicknesses of relevant layers, we are able to enter a new insulating regime where unexpected high-density quantum oscillations are observed. We elucidate this phenomenon through theoretical calculation and through control experiments. The seemingly controversial coexistence of high density states and the insulating regime can be explained by the effect of the attractive Coulomb interaction, which was not considered in earlier theories.The second topic we address is quantum transport in 3D TI systems. Breaking the TRS of the 3D TI surface states leads to many exotic phenomena, including the quantum anomalous Hall (QAH) effect and the axion insulator state. By constructing a sandwich heterostructure that has different magnetic coercive fields in the top and bottom magnetic layers, while keeping the center layer free from magnetic impurities, both the QAH and the axion insulator state can be observed in low-temperature transport measurements, when the magnetization alignment of the top and bottom layers is parallel and antiparallel, respectively. We also discuss the scaling behavior of the topological quantum phase transition between these two states.
Author: Michal Papaj
Publisher:
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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: Emily Elizabeth
Publisher:
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Category : Technology & Engineering
Languages : en
Pages : 0
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Author: Yukako Fujishiro
Publisher: Springer
ISBN: 9789811672958
Category : Science
Languages : en
Pages : 0
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This book addresses novel electronic and thermoelectronic properties arising from topological spin textures as well as topologically non-trivial electronic structures. In particular, it focuses on a unique topological spin texture, i.e., spin hedgehog lattice, emerging in a chiral magnet and explore its novel properties which are distinct from the conventional skyrmion lattice, and discusses the possibility of realizing high-temperature quantum anomalous Hall effect through quantum confinement effect in topological semimetal. This book benefits students and researchers working in the field of condensed matter physics, through providing comprehensive understanding of the current status and the outlook in the field of topological magnets.
Author: Jue Jiang
Publisher:
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Category :
Languages : en
Pages :
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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: Jianxiao Zhang
Publisher:
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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: Dario Bercioux
Publisher: Springer
ISBN: 3319763881
Category : Technology & Engineering
Languages : en
Pages : 261
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Book Description
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.
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