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Electron properties of topological insulators. The structure of edge states and photogalvanic effects
摘要： Integrating the ideas of topology and topological transitions into solid-state physics has led to the theoretical prediction and subsequent experimental discovery of topological insulators, a new class of three- or quasi-two-dimensional dielectric crystalline systems exhibiting stable conducting surface states. This paper briefly reviews the electronic properties of topological insulators. The structure of edge and bulk electronic states in two- and three-dimensional HgTe-based topological insulators is described in particular detail. Recent theoretical and experimental results on the interaction of an electromagnetic field with topological insulators and on edge and surface photogalvanic effects are presented.
关键词： photogalvanic effects,topological insulators,edge and surface states
更新于2025-09-23 15:23:52
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A New Family of Two-Dimensional Topological Materials: CdX (X?=?F, Cl, Br, and I)
摘要： Two-dimensional (2D) transition-metal halides have attracted great interest owing to their versatile applications in electronics, optoelectronics, and renewable energy storage/conversions. Using first-principles calculations, it is proposed that a new series of 2D transition-metal halide CdX (X ? F, Cl, Br, and I) monolayers with honeycomb lattice structure show topological properties. When omitting the spin–orbital coupling (SOC) effect, all of them behave as Dirac semimetal whose Fermi surface is composed of two Dirac points at high symmetry K and K’ points in Brillouin zone. When considering the SOC effect, CdCl, CdBr, and CdI monolayers behave as topological insulators with global band gap, whereas CdF is converted from Dirac semimetal into topological metal with local band gap. The nontrivial topological properties are further proved by their nontrivial edge states. The fascinating properties of the CdX (X ? F, Cl, Br, and I) monolayers show their potential for future quantum computing and next generation of high-speed electronic devices.
关键词： topological insulators,topological metals,two-dimensional transition-metal halides
更新于2025-09-23 15:23:52
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Experimental Observation of Bound States of 2D Dirac Electrons at Surface Steps of the Topological Insulator Bi <sub/>2</sub> Se <sub/>3</sub>
摘要： Topologically protected surface states of three-dimensional topological insulators provide a model framework for studying massless Dirac electrons in two dimensions. Usually a step on the surface of a topological insulator is treated as a scatterer for the Dirac electrons, and the study of its effect is focused on the interference of the incident and scattered electrons. Then a major role plays the warping of the Dirac cone far from the Dirac point. The existence of another significant effect near the Dirac point brought about by the presence of steps is experimentally demonstrated here. Namely the band bending in the vicinity of steps leads to formation of 1D bound states in the corresponding potential wells. The observation of bound states in such potential wells in our scanning tunneling microscopy and spectroscopy investigation of the surface of the topological insulator Bi2Se3 is reported. Numerical simulations support our conclusion and provide a recipe for the identification of such states.
关键词： density of states,Dirac electrons,surface states,edge states,bound states,scanning tunneling microscopy,Bi2Se3,topological insulators,scanning tunneling spectroscopy
更新于2025-09-23 15:23:52
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Energy relaxation of hot carriers near the charge neutrality point in HgTe-based 2D topological insulators
摘要： We present experimental results of non-linear transport in HgTe-based 2D topological insulators, where the conductance is dominated by Dirac-like helical edge states when the Fermi level is pinned to the bulk insulating gap. We find that hot carrier’s energy relaxation is faster close to the charge neutrality point (CNP) which can be attributed to localized nature and incompressibility of charge puddles resulting from inhomogeneous charge distribution near CNP. The tunnel-coupling of these puddles (quantum dots) to 1D edge channels can randomize phase memory leading to incoherent inelastic processes. Hot edge carriers, excited by the electric field, relax to equilibrium via thermalization in multiple puddles resulting in the emission of phonons in the puddles. At relatively low temperature (T≤ 10K), the energy relaxation time shows strong temperature dependence (τ_e ∝ T^{-3.5}), which is interpreted as small angle scattering, consistent with resistance saturation at low temperatures.
关键词： Non-linear transport,electron-phonon scattering,energy relaxation mechanisms,inelastic processes,Topological insulators
更新于2025-09-23 15:22:29
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Robust midgap states in band-inverted junctions under electric and magnetic fields
摘要： Several IV–VI semiconductor compounds made of heavy atoms, such as Pb1?xSnxTe, may undergo band-inversion at the L point of the Brillouin zone upon variation of their chemical composition. This inversion gives rise to topologically distinct phases, characterized by a change in a topological invariant. In the framework of the k·p theory, band-inversion can be viewed as a change of sign of the fundamental gap. A two-band model within the envelope-function approximation predicts the appearance of midgap interface states with Dirac cone dispersions in band-inverted junctions, namely, when the gap changes sign along the growth direction. We present a thorough study of these interface electron states in the presence of crossed electric and magnetic fields, the electric field being applied along the growth direction of a band-inverted junction. We show that the Dirac cone is robust and persists even if the fields are strong. In addition, we point out that Landau levels of electron states lying in the semiconductor bands can be tailored by the electric field. Tunable devices are thus likely to be realizable, exploiting the properties studied herein.
关键词： crystalline topological insulators,Landau levels,midgap states,electric and magnetic fields
更新于2025-09-23 15:22:29
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Statistical classification of contamination in glass insulators by reading its spectrum
摘要： This paper proposes to use a statistical approach for classifying the contamination level of insulators used in high voltage transmission lines. This studied case considers the voltage applied on glass insulators used in a chain of six elements of a 69 kV power line. When submitted to high voltage, the insulators radiate radio frequency signals up to frequencies of some GHz. In this frequency range the signal can be detected using portable antennas, and then be applied to post processing. These received spectra are analyzed by statistical methods, mean and standard deviation, and the cross relation between them is used to classify the pollution level in insulators. To reduce complexity and cost a simple data processing using the mean and the standard deviation are used. Two types of glass insulators are considered: clean or polluted, and to cover the entire frequency range from 30 MHz to 1 GHz two types of antennas were needed. The results obtained with the mean and the standard deviation show that the first 100 MHz bandwidth can be used for statistical of the insulator pollution level.
关键词： radio frequency spectrum,mean value,High voltage transmission lines insulators,statistical classification methods,standard deviation
更新于2025-09-23 15:22:29
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Rare Earth Doping of Topological Insulators: A Brief Review of Thin Film and Heterostructure Systems
摘要： Magnetic topological insulators (MTIs) are a novel materials class in which a topologically nontrivial electronic band structure coexists with long-range ferromagnetic order. The ferromagnetic ground state can break time-reversal symmetry, opening a gap in the topological surface states whose size is dependent on the magnitude of the magnetic moment. Doping with rare earth ions is one way to introduce higher magnetic moments into a material, however, in Bi2Te3 bulk crystals, the solubility limit is only a few percent. Using molecular beam epitaxy for the growth of doped (Sb,Bi)2(Se,Te)3 TI thin films, high doping concentrations can be achieved while preserving their high crystalline quality. The growth, structural, electronic, and magnetic properties of Dy, Ho, and Gd doped TI thin films will be reviewed. Indeed, high magnetic moments can be introduced into the TIs, which are, however, not ferromagnetically ordered. By making use of interfacial effects, magnetic long-range order in Dy doped Bi2Te3, proximity-coupled to the MTI Cr:Sb2Te3, has been achieved. Clearly, engineered MTI heterostructures offer new possibilities that combine the advantageous properties of different layers, and thus provide an ideal materials platform enabling the observation new quantum effects at higher temperatures.
关键词： rare earth doping,MBE,topological insulators,heterostructure,bismuth telluride,antimony telluride
更新于2025-09-23 15:22:29
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Preparation of thulium iron garnet ceramics and investigation of spin transport properties in thin films
摘要： Compensated ferrimagnetic thulium iron garnet ceramics were prepared by the sol-gel method, which is significantly better than conventional solid-state reactions. Saturation magnetization yielded a maximum of 16.7 emu/g for specimens sintered at 900 °C. Subsequently, using the TmIG bulk as the target, an epitaxial film was grown by pulsed laser deposition. Furthermore, the spin transport properties of film with an attached 5 nm-thick platinum layer were studied in terms of longitudinal spin Seebeck effect (LSSE) and spin Hall magnetoresistance (SMR). The LSSE coefficient was as high as 0.38 μV/K and the SMR was 0.0072%. These results indicated that the bulk material and thin film presented excellent qualities, which are extremely important for the fundamental investigation of novel spin transport behaviour in compensated ferrimagnetic insulators. These results also provide more alternatives for spintronic materials.
关键词： Magnetic insulators,Thulium iron garnet,Spin transport properties,Compensated ferrimagnet
更新于2025-09-23 15:22:29
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Cs adsorption on Bi2Se3
摘要： Bi2Se3 is a topological insulator whose unique properties result from topological surface states (TSS) in the band gap. The adsorption of Cs onto a Bi2Se3 surface is investigated by low energy ion scattering and work function measurements. Much of the deposited Cs quickly diffuses to the step edges forming one-dimensional chains of positively charged adatoms, along with some deposition on the terraces. The work function decreases until a coverage of 0.1 ML is reached, beyond which it increases slightly. The minimum in the work function is due to depolarization of the dipoles induced when the concentration of adatoms in the chains reaches a critical value. A slow diffusion of adsorbed Cs from the terraces to the step edges is also marked by changes in the neutralization of scattered Na+ and work function over time. The spatial distribution of the conductive charges in the TSS, which are primarily positioned between the first and second atomic layers, is confirmed by comparison of the neutralization of Na+ scattered from Bi and Se.
关键词： Topological insulators,Alkali adsorption,Work function,Low energy ion scattering
更新于2025-09-23 15:22:29
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Topological Insulator-Based van der Waals Heterostructures for Effective Control of Massless and Massive Dirac Fermions
摘要： Three dimensional (3D) topological insulators (TIs) are an important class of materials with applications in electronics, spintronics and quantum computing. With the recent development of truly bulk insulating 3D TIs, it has become possible to realize surface dominated phenomena in electrical transport measurements e.g. the quantum Hall (QH) effect of massless Dirac fermions in topological surface states (TSS). However, to realize more advanced devices and phenomena, there is a need for a platform to tune the TSS or modify them e.g. gap them by proximity with magnetic insulators, in a clean manner. Here we introduce van der Waals (vdW) heterostructures in the form of topological insulator/insulator/graphite to effectively control chemical potential of the TSS. Two types of gate dielectrics, normal insulator hexagonal boron nitride (hBN) and ferromagnetic insulator Cr2Ge2Te6 (CGT) are utilized to tune charge density of TSS in the quaternary TI BiSbTeSe2. hBN/graphite gating in the QH regime shows improved quantization of TSS by suppression of magnetoconductivity of massless Dirac fermions. CGT/graphite gating of massive Dirac fermions in the QH regime yields half-quantized Hall conductance steps and a measure of the Dirac gap. Our work shows the promise of the vdW platform in creating advanced high-quality TI-based devices.
关键词： quantum Hall effect,van der Waals heterostructures,Topological insulators,ferromagnetic insulators
更新于2025-09-23 15:21:21