Controlling Magnetic and Optical Properties of the van der Waals Crystal CrCl3−xBrx via Mixed Halide Chemistry

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy—a tendency of spins to align in a certain crystallographic direction known as easy‐axis. In layered materials, two limits of easy‐axis are in‐plane (XY) and out‐of‐plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin‐orbitronic functions. Here, a unique experiment is designed to control the easy‐axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl_3?xBr_x crystals between CrCl₃ with XY and CrBr₃ with Ising anisotropy. The easy‐axis is controlled between the two limits by varying spin–orbit coupling with the Br content in CrCl_3?x Br_x. The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly with x. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.     

Chemical Intercalation of Topological Insulator Grid Nanostructures for High‐Performance Transparent Electrodes

2D layered nanomaterials with strong covalent bonding within layers and weak van der Waals' interactions between layers have attracted tremendous interest in recent years. Layered Bi₂Se₃ is a representative topological insulator material in this family, which holds promise for exploration of the fundamental physics and practical applications such as transparent electrode. Here, a simultaneous enhancement of optical transmittancy and electrical conductivity in Bi₂Se₃ grid electrodes by copper‐atom intercalation is presented. These Cu‐intercalated 2D Bi₂Se₃ electrodes exhibit high uniformity over large area and excellent stabilities to environmental perturbations, such as UV light, thermal fluctuation, and mechanical distortion. Remarkably, by intercalating a high density of copper atoms, the electrical and optical performance of Bi₂Se₃ grid electrodes is greatly improved from 900 Ω sq^?1, 68% to 300 Ω sq^?1, 82% in the visible range; with better performance of 300 Ω sq^?1, 91% achieved in the near‐infrared region. These unique properties of Cu‐intercalated topological insulator grid nanostructures may boost their potential applications in high‐performance optoelectronics, especially for infrared optoelectronic devices.     

Realization of Epitaxial Thin Films of the Topological Crystalline Insulator Sr3SnO

Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr₃SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this material, however, is challenging. Guided by thermodynamic calculations, a deposition approach is designed and implemented to achieve the adsorption-controlled growth of epitaxial Sr₃SnO single-crystal films by molecular-beam epitaxy (MBE). In situ transport and angle-resolved photoemission spectroscopy measurements reveal the metallic and electronic structure of the as-grown samples. Compared with conventional MBE, the used synthesis route results in superior sample quality and is readily adapted to other topological systems with antiperovskite structures. The successful realization of thin films of Sr₃SnO opens opportunities to manipulate topological states by tuning symmetries via strain engineering and heterostructuring.     

Deep‐Subwavelength Holey Acoustic Second‐Order Topological Insulators

Higher‐order topological insulators (HOTIs) belong to a new class of materials with unusual topological phases. They have garnered considerable attention due to their capabilities in confining energy at the hinges and corners, which is entirely protected by the topology, and have thus become attractive structures for acoustic wave studies and control. However, for most practical applications at audible and low frequencies, compact and subwavelength implementations are desirable in addition to providing robust guiding of sound beyond a single‐frequency operation. Here, a holey HOTI capable of sustaining deeply confined corner states 50 times smaller than the wavelength is proposed. A remarkable resilience of these surface‐confined acoustic states against defects is experimentally observed, and topologically protected sound is demonstrated in three different frequency regimes. Concerning this matter, the findings will thus have the capability to push forward exciting applications for robust acoustic imaging way beyond the diffraction limit.     

拉盖尔-高斯光束在含拓扑绝缘体周期薄膜中的传输特性

基于平面角谱扩展法和4×4矩阵传输理论,研究了拉盖尔-高斯光束(LGB)在含拓扑绝缘体(TI)周期性层状薄膜中的反射和透射特性,对线偏振的LGB入射到周期性层状薄膜中的反射场和透射场的强度分布进行了分析和详细讨论。研究结果表明,TI的拓扑磁电极化率(TMEP)和薄膜的周期个数对强度分布有很大影响,通过改变TMEP或周期个数可以操纵涡旋光的光场。所提方法不仅可以推广到其他含TI的多层介质体系,而且对进一步研究TI光子晶体中的光子能带结构和带隙具有一定的意义。     

高压绝缘子自然污秽中阴离子组成的分析

利用抑制性离子色谱仪对全国各地户外高压绝缘子自然污秽物水溶液中的F-、Cl-、NO3-、SO42-4种阴离子含量进行了分析。定量方法采用标准曲线法。结果表明,污秽物中F-离子含量最低,大多数都小于1 mg/g;SO42-含量远远高于其它3种一价阴离子,基本都超过了100 mg/g,最高的达到了300 mg/g。表明离子色谱法能够准确地测定电缆污秽物中阴离子的种类与含量,具有检测限低,检测范围宽,快速准确的特点。     

重覆冰条件下防冰防雷绝缘子电场仿真与伞裙优化

为研究防冰防雷绝缘子在重覆冰条件下的电场分布特性,进而优化防冰防雷绝缘子的伞裙结构,缓解其在覆冰状态下的电场畸变,建立了220 k V防冰防雷绝缘子的有限元仿真模型,计算得到了不同伞裙结构的绝缘子在清洁和覆冰条件下的电场分布,根据仿真结果对伞裙结构进行了优化,并对比了伞裙优化前后防冰防雷绝缘子的电场分布特性。结果表明:清洁状态下,伞裙结构对绝缘子电场分布影响很小;覆冰状态下,超大伞裙数目为5时可以更加有效地缓解重度覆冰对空间电场产生的畸变。优化后的伞裙结构可以使绝缘子在覆冰条件下的空间电场畸变有所缓解,起到提高冰闪电压的作用。     

Charge Contrast Imaging of Nonconductive Samples in the High‐Vacuum Field Emission Scanning Electron Microscope

The charge contrast images (CCI) on insulating or poorly conducting samples were observed under steady‐state charging conditions with a thermal field emission scanning electron microscope under high vacuum by using an Everhart‐Thornley detector. The charge contrast on plumbous titanate‐nickel composite particles and patterned sapphires could be the indicators of near‐surface features, compositional variations and conductivity distributions. Optimum imaging conditions for observing the CCI include the electron energy, the electron flux density and the electron dose. Contrast characteristics associated with surface and near‐surface secondary electron emission yield enhanced above the trapped charge‐up regions, as charge trapping selectively enhanced the poorly conductive phase and lattice distorted area. SCANNING 29: 230‐237, 2007. © 2007 Wiley Periodicals, Inc.     

Dirac Cone Spin Polarization of Graphene by Magnetic Insulator Proximity Effect Probed with Outermost Surface Spin Spectroscopy

The effects of the proximity contact with magnetic insulator on the spin‐dependent electronic structure of graphene are explored for the heterostructure of single‐layer graphene (SLG) and yttrium iron garnet Y₃Fe₅O₁₂ (YIG) by means of outermost surface spin spectroscopy using a spin‐polarized metastable He atom beam. In the SLG/YIG heterostructure, the Dirac cone electrons of graphene are found to be negatively spin polarized in parallel to the minority spins of YIG with a large polarization degree, without giving rise to significant changes in the π band structure. Theoretical calculations reveal the electrostatic interfacial interactions providing a strong physical adhesion and the indirect exchange interaction causing the spin polarization of SLG at the interface with YIG. The Hall device of the SLG/YIG heterostructure exhibits a nonlinear Hall resistance attributable to the anomalous Hall effect, implying the extrinsic spin–orbit interactions as another manifestation of the proximity effect.     

基于太赫兹波的复合绝缘子界面检测研究

复合绝缘子的交界面缺陷问题严重威胁着电网的安全运行。为了有效评估复合绝缘子的界面性能,研究基于太赫兹波的无损检测方法。首先理论分析了太赫兹反射波在不同性能界面中的传播过程。测试界面气隙、蚀损缺陷情形的太赫兹反射波,分析太赫兹波在含缺陷界面中的反射传播特性。针对气隙缺陷,提出通过太赫兹波形特征比对的定量检测方法;针对蚀损缺陷,提出基于波形差值的成像检测方法。根据太赫兹波形特征信息,可对界面气隙和蚀损缺陷进行分类诊断。对实际缺陷的测试表明,太赫兹时域反射波在复合绝缘子界面缺陷检测中具有较高的应用价值。