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Author: Kenji Yasuda
Publisher: Springer Nature
ISBN: 981157183X
Category : Computers
Languages : en
Pages : 109
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Book Description
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.
Author: Kenji Yasuda
Publisher: Springer Nature
ISBN: 981157183X
Category : Computers
Languages : en
Pages : 109
Get Book Here
Book Description
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.
Author: Shang-Rong Yang
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Author: Masataka Mogi
Publisher: Springer Nature
ISBN: 9811921377
Category : Computers
Languages : en
Pages : 120
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Book Description
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.
Author: Abhinav Kandala
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Topological Insulators (TI) are a novel class of materials that are ideally insulating in the bulk, but have gapless, metallic states at the surface. These surface states have very exciting properties such as suppressed backscattering and spin-momentum locking, which are of great interest for research efforts towards dissipation-less electronics and spintronics. The popular thermo-electrics from the Bi chalcogenide family -- Bi2Se3 and Bi2Te3 -- have been experimentally demonstrated to be promising candidate TI materials, and form the chosen material system for this dissertation research. The first part of this dissertation research focuses on low temperature magneto-transport measurements of mesoscopic topological insulator devices (Chapter 3). The top-down patterning of epitaxial thin films of Bi2Se3 and Bi2Te3 (that are plagued with bulk conduction) is motivated, in part, by an effort to enhance the surface-to-volume ratio in mesoscopic channels. At cryogenic temperatures, transport measurements of these devices reveal periodic conductance fluctuations in straight channel devices, despite the lack of any explicit patterning of the TI film into a ring or a loop. A careful analysis of the surface morphology and comparison with the transport data then demonstrate that scattering off the edges of triangular plateaus at the surface leads to the creation of Aharonov-Bohm electronic orbits responsible for the periodicity. Another major focus of this dissertation work is on combining topological insulators with magnetism. This has been shown to open a gap in the surface states leading to possibilities of magnetic "gating" and the realization of dissipation-less transport at zero-field, amongst several other exotic quantum phenomena. In this dissertation, I present two different schemes for probing these effects in electrical transport devices -- interfacing with insulating ferromagnets (Chapter 4) and bulk magnetic doping (Chapter 5). In Chapter 4, I shall present the integration of GdN with Bi2Se3 thin films. Careful structural, magnetic and electrical characterization of the heterostructures is employed to confirm that the magnetic species is solely restricted to the surface, and that the ferromagnetic GdN layer to be insulating, ensuring current flow solely through the TI layer. We also devise a novel device geometry that enables direct comparison of the magneto-transport properties of TI films with and without proximate magnetism, all, in a single device. A comparative study of weak anti-localization suggested that the overlying GdN suppressed quantum interference in the top surface state. In our second generation hetero-structure devices, GdN is interfaced with low-carrier density, gate-tunable thin films of (Bi,Sb)2Te3 grown on SrTiO3 substrates. These devices enable us to map out the comparison of magneto-transport, as the chemical potential is tuned from the bulk conduction band into the bulk valence band.In a second approach to study the effects of magnetism on TI's, I shall present, in Chapter 5, our results from magnetic doping of (Bi,Sb)2Te3 thin films with Cr -- a system that was recently demonstrated to be a Quantum Anomalous Hall (QAH) insulator. In a Cr-rich regime, a highly insulating, high Curie temperature ferromagnetic phase is achieved. However, a careful, iterative process of tuning the composition of this complex alloy enabled access to the QAHE regime, with the observation of near dissipation-less transport and perfect Hall quantization at zero external field. Furthermore, we demonstrate a field tilt driven crossover between a quantum anomalous Hall phase and a gapless, ferromagnetic TI phase. This crossover manifests itself in an electrically tunable, giant anisotropic magneto-resistance effect that we employ as a quantitative probe of edge transport in this system.
Author: Wei-Jhih Zou
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Author: Jinsong Zhang
Publisher: Springer
ISBN: 3662499274
Category : Science
Languages : en
Pages : 128
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Book Description
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.
Author: Yukako Fujishiro
Publisher: Springer Nature
ISBN: 9811672938
Category : Science
Languages : en
Pages : 101
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Book Description
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: Murong Lang
Publisher:
ISBN:
Category :
Languages : en
Pages : 144
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Book Description
The recently discovered time-reversal-invariant topological insulator (TI) has led to the flourishing of unique physics along with promises for innovative electronic and spintronic applications. However, the as-grown TI materials are not truly insulating but with a non-trivial bulk carrier density, which makes difficulties to the transport methods. In our work, we study the fundamental transport properties of TI and its heterostructure, in which various approaches are utilized to better reveal the surface state properties. In particular, in Chapter 2, in-situ Al surface passivation of Bi2Se3 inside MBE is investigated to inhibit the degradation process, reduce carrier density and reveal the pristine topological surface states. In contrast, we show the degradation of surface states for the unpassivated control samples, in which the 2D carrier density is increased by 39.2% due to ambient n-doping, the Shubnikov-de Hass oscillations are completely absent, and a deviation from WAL weak antilocalization is observed. In Chapter 3, through optimizing the material composition to achieve bulk insulating state, we present the ambipolar effect in 4-9 quintuple layers (Bi0.57Sb0.43)2Te3 thin films. We also demonstrate the evidence of a hybridized surface gap opening in (Bi0.57Sb0.43)2Te3 sample with thickness below six quintuple layers through transport and scanning tunneling spectroscopy measurements. By effective tuning the Fermi level via gate-voltage control, we unveil a striking competition between weak antilocalization and weak localization at low magnetic fields in nonmagnetic ultrathin films. In Chapter 4, we study the magnetic properties of Bi2Se3 surface states in the proximity of a high Tc ferrimagnetic insulator YIG. Proximity-induced magnetoresistance loops are observed by transport measurements with out-of-plane and in-plane magnetic fields applied. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, which is an important step toward room-temperature application of TI-based spintronic devices. The engineering of a TI and FMI heterostructure will open up numerous opportunities to study high temperature TI-based spintronic devices, in which the TI is controlled by breaking the TRS using a FMI with perpendicular magnetization component. A YIG film with out-of-plane anisotropy at> 300 K could potentially manipulate the magnetic properties of a TI may even above room temperature.
Author: Chaowei Hu
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Topological materials are materials whose electronic band structures are described by certain non-trivial topological invariants. Forty years ago the importance of band topology in condensed matter physics was first recognized when the quantum Hall effect (QHE) was found to be related with the integer Chern number in two-dimensional (2D) electron gas. Since 2008, the discovery of three-dimensional (3D) topological insulators (TI) with a non-trivial topological invariant and gapless surface state has taken the field into a new era. Various new topological phases were proposed and band topology has become a new way to classify the state of matter. The design, synthesis and characterization of new topological materials pave essential basis to uncovering novel physics arising from non-trivial band topology and its interplay with various degrees of freedom such as spin, orbital and charge. Today, with more sought-after novel topological phases, emergent phenomena such as surface Fermi arcs, chiral anomaly, quantum anomalous Hall effect were discovered and enable future technological advances including topological quantum computation. A new topological phase can be created when additional symmetry breaking is introduced into an existing topological phase. For example, by breaking the time reversal symmetry of a 3D TI through ferromagnetism (FM), one can get a Chern insulator in its 2D limit, where QHE can be realized without external magnetic field and gives topologically-protected dissipationless chiral edge states. This phenomenon, the so-called quantum anomalous Hall effect (QAHE), has been long sought since its early proposal in the yet-to-be-realized Haldane model for graphene lattice with opposite magnetic field at neighboring atoms in 1988. Therefore, the realization of QAHE in magnetically-doped TI Cr0.15(Bi0.1Sb0.9)1.85Te3 thin films in 2013 was revolutionary. However, the unavoidable sample inhomogeneity in doped materials restrains the investigation of associated emergent phenomena in mK-regime. Ideally, magnetism from intrinsic magnetic atoms in a crystal can provide more homogeneous electronic and magnetic properties than the magnetism from dopants. To realize QAHE at higher temperatures, the intrinsic magnetic TIs with only clean topological bands but no other bands at the Fermi level are strongly desired. In 2018, MnBi2Te4 was discovered to be the first of such kinds, as an antiferromagnetic (AFM) TI with intrinsic magnetic Mn site. It is a layered van der Waals (vdW) material. When the magnetism orders below 24 K, the spins are FM aligned in the ab plane but AFM coupled along the c axis. In 2D limit, MnBi2Te4 films can have a net magnetization either in odd-layer devices, or when the even-layer devices are in the spin-flop state above ~ 3.5 T and the forced FM state above ~ 8 T. These time-reversal-symmetry breaking states are ideal for realizing the Chern insulator state. Indeed, QAHE was experimentally observed at 0 T and 1.6 K in a 5-layer device and quantized Hall conductance was realized when the even-layer devices enter the forced FM state above the saturation field of 8 T. Following this line, for QAHE to be realized at zero field and higher temperature, it is strongly desirable if the FM alignment of Mn spins can be accessed at a lower or even zero field. To do so, one must weaken the interlayer AFM interactions between [MnBi2Te4] layers. We thus propose to introduce n-1 nonmagnetic TI [Bi2Te3] layers between [MnBi2Te4] layers to get natural heterostructures of MnBi2nTe3n+1. By this rational design, we can increase the distance between the neighboring [MnBi2Te4] layers and thus reduce the interlayer AFM interaction. Under such a design principle we successfully grew single crystals of MnBi4Te7 (n=2), MnBi6Te10 (n=3) and MnBi8Te13 (n=4). Then with the physical property characterization, first-principles calculations and angle-resolved photoemission spectroscopy measurements, for the first time, we demonstrated that MnBi4Te7 is an intrinsic AFM TI with saturation field 40 times smaller than that of MnBi2Te4, and that MnBi8Te13 is the first realization of an intrinsic FM axion insulator, proving the success of our material design principle. The manipulation of magnetism is crucial to access different magnetic topological phase and novel physics. In MnBi2nTe3n+1, the control of the magnetism from AFM to FM by n is only discrete. To achieve a fine and continuous control of the magnetic transition, we doped Sb to MnBi4Te7 where the interlayer AFM coupling is weak and more tunable. Through single crystal growth, transport, thermodynamic, neutron diffraction measurements, we show that under Sb doping, MnBi4Te7 evolves from AFM to FM and then ferrimagnetic. We attribute this to the formation of Mn_(Bi, Sb) antisites upon doping, which results in additional Mn sublattices that modify the delicate interlayer magnetic interactions and changes the overall magnetism. We further investigate the effect of antisites on the band topology using the first-principles calculations. Without considering antisites, the series evolves from AFM topological insulator (x = 0) to FM axion insulators. In the exaggerated case of 16.7\% of periodic antisites, the band topology is modified and type-I magnetic Weyl semimetal phase can be realized at intermediate doping. Therefore, this doping series provides a fruitful platform with rich and continuously tunable magnetism and topology. After we achieve FM in MnBi2nTe3n+1, for practical applications especially in the pursuit of high temperature QAHE when fluctuations become important, the study on magnetic dynamics is indispensable too. We investigated the magnetic dynamics in FM MnBi8Te13 and Sb doped MnBi4Te7 and MnBi6Te10 using AC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all three compounds, suggesting its universality among FM MnBi2nTe3n+1. The origin of the relaxation behavior is attributed to the irreversible domain movements since they only appear below the saturation fields when FM domains form and evolve. These FM domains are very soft, as revealed by the low-field fine-structured domains and high-field sea-urchin-shaped remnant-state domains imaged via the magneto-optical measurements. Finally, we attribute the rare "double-peak" behavior observed in the AC susceptibility under small DC bias fields to the very soft FM domain formations. This study provides a thorough understanding of the soft FM in highly anisotropic magnets. As the first intrinsic antiferromagnetic topological insulator, MnBi2Te4 is still the major material platform to search for QAHE, so its material optimization is very urged. We develop the chemical-vapor-transport (CVT) growth for of MnBi2Te4, which has a higher success rate in observation of the field-induced quantized Hall conductance in 6-layer devices. Through comparative studies between our CVT-grown and flux-grown MnBi2Te4, we find that CVT-grown MnBi2Te4 is marked with higher Mn occupancy on the Mn site, slightly higher Mn_Bi antisites and smaller carrier concentration. On the device end, thin film from CVT-grown sample shows by far the highest mobility of 2500 cm2 V s in MnBi2Te4 devices with the quantized Hall conductance appearing at 1.8 K and 8 T. This study provides a route to obtain high-quality single crystals of MnBi2Te4 that are promising to make superior devices and realize emergent phenomena. In summary, we have discovered and established MnBi4Te7 and MnBi8Te13 as new intrinsic magnetic topological insulators. In particular, we provide deep understanding of the importance of material design, synthesis and chemical doping to the magnetism and topology in the series. The growths of high-quality single crystals and the study of magnetic dynamics provide essential basis for the search of QAHE in MnBi2nTe3n+1. Our works will shed light on future endeavors in finding novel magnetic topological materials as well as searching for QAHE and the associated emergent phenomena in the condensed matter field
Author: Joon Lee
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
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.
