Visualizing the Local Twist Angle Variation within and between Domains of Twisted Bilayer Graphene

Moirésuperlattices in twisted two-dimensional materials have emerged as ideal platforms for engineering quantum phenomena,which are highly sensitive to twist angles,including both the global value and the spatial inhomogeneity.However,only a few methods provide spatial-resolved information for characterizing local twist angle distribution.

Emergence of Chern Insulating States in Non-Magic Angle Twisted Bilayer Graphene

Twisting two layers into a magic angle(MA) of ~1.1°is found essential to create low energy flat bands and the resulting correlated insulating,superconducting,and magnetic phases in twisted bilayer graphene(TBG).While most of previous works focus on revealing these emergent states in MA-TBG,a study of the twist angle dependence,which helps to map an evolution of these phases,is yet less explored.Here,we report a magnetotransport study on one non-magic angle TBG device,whose twist angle θ changes from 1.25° at one end to 1.43°at the other.For θ=1.25° we observe an emergence of topological insulating states at hole side with a sequence of Chern number |C|=4-|v|,where v is the number of electrons(holes) in moire unite cell.When θ> 1.25°,the Chern insulator from flat band disappears and evolves into fractal Hofstadter butterfly quantum Hall insulator where magnetic flux in one moire unite cell matters.Our observations will stimulate further theoretical and experimental investigations on the relationship between electron interactions and non-trivial band topology.

Twist-angle two-dimensional superlattices and their application in(opto)electronics

Twist-angle two-dimensional systems,such as twisted bilayer graphene,twisted bilayer transition metal dichalcogenides,twisted bilayer phosphorene and their multilayer van der Waals heterostructures,exhibit novel and tunable properties due to the formation of Moirésuperlattice and modulated Moirébands.The review presents a brief venation on the development of"twistronics"and subsequent applications based on band engineering by twisting.Theoretical predictions followed by experimental realization of magic-angle bilayer graphene ignited the flame of investigation on the new freedom degree,twistangle,to adjust(opto)electrical behaviors.Then,the merging of Dirac cones and the presence of flat bands gave rise to enhanced light-matter interaction and gate-dependent electrical phases,respectively,leading to applications in photodetectors and superconductor electronic devices.At the same time,the increasing amount of theoretical simulation on extended twisted 2D materials like TMDs and BPs called for further experimental verification.Finally,recently discovered properties in twisted bilayer h-BN evidenced h-BN could be an ideal candidate for dielectric and ferroelectric devices.Hence,both the predictions and confirmed properties imply twist-angle two-dimensional superlattice is a group of promising candidates for next-generation(opto)electronics.

Theoretical Modeling for Predicting the Optimum Twist Angle of Cotton Fiber Movement on OE Yarn Made by Rotor Spinning Machine

This paper presents theoretical modeling for predicting the optimum twist angle on yarn made by open end rotor spinning machine in textile industry. Fiber movement on yarn can be used for predicting the optimum twist angle which can be used to reduce yarn breaking in spinning process. In this research the twist angle has been found and the result of this research shows the twist angle around 45°;and the theoretical result of the ratio of rotor diameter to fiber length is .

Precisely controlling the twist angle of epitaxial MoS_(2)/graphene heterostructure by AFM tip manipulation

Two-dimensional(2D)moirématerials have attracted a lot of attention and opened a new research frontier of twistronics due to their novel physical properties.Although great progress has been achieved,the inability to precisely and reproducibly manipulate the twist angle hinders the further development of twistronics.Here,we demonstrated an atomic force microscope(AFM)tip manipulation method to control the interlayer twist angle of epitaxial MoS_(2)/graphene heterostructure with an ultra-high accuracy better than 0.1°.Furthermore,conductive AFM and spectroscopic characterizations were conducted to show the effects of the twist angle on moirépattern wavelength,phonons and excitons.Our work provides a technique to precisely control the twist angle of 2D moirématerials,enabling the possibility to establish the phase diagrams of moiréphysics with twist angle.

Twisted Integration of Complex Oxide Magnetoelectric Heterostructures via Water‑Etching and Transfer Process

Manipulating strain mode and degree that can be applied to epitaxial complex oxide thin films have been a cornerstone of strain engineering.In recent years,lift-off and transfer technology of the epitaxial oxide thin films have been developed that enabled the integration of heterostructures without the limitation of material types and crystal orientations.Moreover,twisted integration would provide a more interesting strategy in artificial magnetoelectric heterostructures.A specific twist angle between the ferroelectric and ferromagnetic oxide layers corresponds to the distinct strain regulation modes in the magnetoelectric coupling process,which could provide some insight in to the physical phenomena.In this work,the La_(0.67)Sr_(0.33)MnO_(3)(001)/0.7Pb(Mg_(1/3)Nb_(2/3))O_(3)-0.3PbTiO_(3)(011)(LSMO/PMN-PT)heterostructures with 45.and 0.twist angles were assembled via water-etching and transfer process.The transferred LSMO films exhibit a fourfold magnetic anisotropy with easy axis along LSMO<110>.A coexistence of uniaxial and fourfold magnetic anisotropy with LSMO[110]easy axis is observed for the 45°Sample by applying a 7.2 kV cm^(−1)electrical field,significantly different from a uniaxial anisotropy with LSMO[100]easy axis for the 0°Sample.The fitting of the ferromagnetic resonance field reveals that the strain coupling generated by the 45°twist angle causes different lattice distortion of LSMO,thereby enhancing both the fourfold and uniaxial anisotropy.This work confirms the twisting degrees of freedom for magnetoelectric coupling and opens opportunities for fabricating artificial magnetoelectric heterostructures.

Analysis on the Characterization of the Twist Insertion Level and Its Effect of the Compact and Ring Spun Yarns

In this paper, the difference of the twist insertion levels of the compact and ring spun yarns of the same count is analyzed based on the relationship of the yarn twist factor to the twist angle and the bulk density. It is proposed that the relationship of the twist angle, twist level, and the yarn diameter should be synthetically considered when evaluating the twist insertion level of the compact yarn. The twist angle of the compact yarn is smaller than that of the ring yarn with the same twist insertion level. This results from the ordered fiber arrangement and compact structure of the compact yarn. Experiment was conducted to verify the conclusions. It is also discovered that the selection of the twist factor of the compact yarn, which can be usually lower than that of the ring yarn by 10%-15%, can be determined based on the yarn tensile strength.

Vibration and wave propagation analysis of twisted micro-beam using strain gradient theory

In this research, vibration and wave propagation analysis of a twisted micro- beam on Pasternak foundation is investigated. The strain-displacement relations （kine-matic equations） are calculated by the displacement fields of the twisted micro-beam. The strain gradient theory （SGT） is used to implement the size dependent effect at micro-scale. Finally, using an energy method and Hamilton＇s principle, the governing equations of motion for the twisted micro-beam are derived. Natural frequencies and the wave prop- agation speed of the twisted micro-beam are calculated with an analytical method. Also, the natural frequency, the phase speed, the cut-off frequency, and the wave number of the twisted micro-beam are obtained by considering three material length scale parameters, the rate of twist angle, the thickness, the length of twisted micro-beam, and the elastic medium. The results of this work indicate that the phase speed in a twisted micro-beam increases with an increase in the rate of twist angle. Moreover, the wave number is in- versely related with the thickness of micro-beam. Meanwhile, it is directly related to the wave propagation frequency. Increasing the rate of twist angle causes the increase in the natural frequency especially with higher thickness. The effect of the twist angle rate on the group velocity is observed at a lower wave propagation frequency.

Emerging properties of two-dimensional twisted bilayer materials

Recent studies in van der Waals coupled two-dimensional(2D) bilayer materials have demonstrated a new freedom for material engineering by the formation of moiré pattern. By tuning the twist angle between two layers, one can modulate their electronic band structures and therefore the associated electrical transport and optical properties, which are distinct from the original ones of each individual layer. These new properties excite great passion in the exploration of new quantum states and possible applications of 2D bilayers. In this article, we will mainly review the prevailing fabrication methods and emerging physical properties of twisted bilayer materials and lastly give out a perspective of this topic.

Geometric effects of cross sections on equilibrium of helical and twisted ribbon

In the framework of elastic rod model, the Euler-Lagrange equations characterizing the equilibrium configuration of the polymer chain are derived from a free energy functional associated with the curvature, torsion, twisting angle, and its derivative with respect to the arc-length. The configurations of the helical ribbons with different cross-sectional shapes are given. The effects of the elastic properties, the cross-sectional shapes, and the intrinsic twisting on the helical ribbons are discussed. The results show that the pitch angle of the helical ribbon decreases with the increase in the ratio of the twisting rigidity to the bending rigidity and approaches the intrinsic twisting. If the bending rigidity is much greater than the twisting rigidity, the bending and twisting of the helical ribbon always appear simultaneously.

Opportunities and Challenges in Twisted Bilayer Graphene:A Review

Two-dimensional(2D)materials exhibit enhanced physical,chemical,electronic,and optical properties when compared to those of bulk materials.Graphene demands significant attention due to its superior physical and electronic characteristics among different types of 2D materials.The bilayer graphene is fabricated by the stacking of the two monolayers of graphene.The twisted bilayer graphene(tBLG)superlattice is formed when these layers are twisted at a small angle.The presence of disorders and interlayer interactions in tBLG enhances several characteristics,including the optical and electrical properties.The studies on twisted bilayer graphene have been exciting and challenging thus far,especially after superconductivity was reported in tBLG at the magic angle.This article reviews the current progress in the fabrication techniques of twisted bilayer graphene and its twisting angle-dependent properties.

Novel cell parameter determination of a twisted-nematic liquid crystal display

In this paper a novel method is proposed to determine the cell parameters including the twist angle, optic retardation and rubbing direction of twisted-nematic liquid crystal displays （TNLCD） by rotating the TNLCD. It is a single-wavelength method. Because using subtraction equation of transmittance as curve fitting equation, the influence of the light from environment and the absorption by polarizer, the sample of TNLCD and analyser on the transmittance is eliminated. Accurate results can also be obtained in imperfect darkness. By large numbers of experiments, we found that not only the experimental setup is quite simple and can be easily adopted to be carried out, but also the results are accurate.

Depth-dependent mosaic tilt and twist in GaN epilayer:An approximate evaluation

An approach based on depth-sensitive skew-angle x-ray diffraction （SAXRD） is presented for approximately evalu- ating the depth-dependent mosaic tilt and twist in wurtzite c-plane GaN epilayers. It is found that （103） plane and （101） plane, among the lattice planes not perpendicular to the sample surface, are the best choices to measure the depth profiles of tilt and twist for a GaN epilayer with a thickness of less than 2 μm according to the diffraction geometry of SAXRD. As an illustration, the depth-sensitive （103）/（101） ω-scans of a 1.4-μm GaN film grown by metal-organic chemical vapor deposition on sapphire substrate are measured and analyzed to show the feasibility of this approach.

Thermal transport in twisted few-layer graphene

Twisted graphene possesses unique electronic properties and applications, which have been studied extensively. Recently, the phonon properties of twisted graphene have received a great deal of attention. To the best of our knowledge,thermal transports in twisted graphene have been investigated little to date. Here, we study perpendicular and parallel transports in twisted few-layer graphene（T-FLG）. It is found that perpendicular and parallel transports are both sensitive to the rotation angle θ between layers. When θ increases from 0° to 60°, perpendicular thermal conductivity κ（｜｜） first decreases and then increases, and the transition angle is θ = 30°. For the parallel transport, the relation between thermal conductivity κand θ is complicated, because intra-layer thermal transport is more sensitive to the edge of layer than their stacking forms. However, the dependence of interlayer scattering on θ is similar to that of κ⊥. In addition, the effect of layer number on the thermal transport is discussed. Our results may provide references for designing the devices of thermal insulation and thermal management based on graphene.

PMSM正切趋近律无位置传感器角度补偿方法研究

在负载转矩突变的动态过程中,基于PI调节器的角度补偿方法作用时PMSM超螺旋滑模观测器(STSMO)转子电角度估算误差波动剧烈,因此依据角度估算误差的定义,建立了转子电角度补偿算法的数学模型。根据滑模控制和趋近律理论,提出了基于正切趋近律的变步长闭环角度补偿方法,选择角度估算误差的半角正切值作为角度调节步长,并通过前馈解耦得到的角度估算误差正余弦信号计算该步长,实现了对无位置传感器控制系统动态性能和抗扰动能力的改善。根据归一化灵敏度的定义,分析调节步长随角度估算误差变化的灵敏度,提出了基于归一化补偿灵敏度的系统动态性能分析方法,衡量两种补偿算法作用下系统的动态性能。计算和仿真结果表明,正切趋近律补偿方法具有更高的归一化补偿灵敏度,在负载转矩突变等角度估算误差变化剧烈的工况下能够实现更好的补偿效果,抑制估算角度误差的波动。实验结果表明,相比传统的PI补偿方法,正切趋近律补偿方法能够将突加额定转矩动态过程中角度估算误差的波动幅度降低61.9%,动态过程持续时间缩短23%,有效提升了系统的动态性能和抗扰动能力。

地聚物混凝土构件的纯扭性能试验

目的研究地聚物混凝土构件的受扭性能,并探讨现有普通混凝土结构设计规范对地聚物混凝土构件的适用性。方法研究参数为配箍率(0.42%和0.99%)与纵筋配筋率(0.65%和1.17%),共设计3种试验工况,通过试验观察构件的破坏形态、裂缝发展情况;并记录扭矩-扭转角曲线、扭矩-钢筋应变曲线、扭矩-混凝土应变曲线,分析各参数对构件抗扭性能的影响。结果地聚物混凝土构件的破坏模式与普通混凝土构件相似,有少筋破坏、适筋破坏以及部分超筋破坏三种破坏模式;构件的极限扭矩基本随配箍率与纵筋配筋率的增大而增大;构件的主裂缝倾角均随配筋强度比的增加而减少。结论增加构件的配筋率可以显著提高地聚物构件的抗扭承载力;《混凝土结构设计规范》(GB50010—2010)对地聚物构件的极限扭矩预测效果较好,预测值与试验值比值的均值为0.92、变异系数为13.6%。

基于扭矩平衡的分时锚定能测量方法

光控取向技术一直是液晶光学器件制备中的关键环节,精确测量光控取向技术中的锚定强度与曝光条件的关系对于提高器件性能至关重要。然而,目前锚定强度的测量方法还存在一定的局限性,比如无法实现对同一取向膜的单点前后多状态的测量等,这导致实际应用中的诸多不便。鉴于此,本文提出了一种基于相位检测原理的新型方位锚定能测量方法,通过采用微分归一化拟合的方法同时结合全方位透射特性校正有效消除了起始放置误差,不仅极大地优化了测量过程,使测量误差控制在8%以内,而且大幅提高了测量时效性。理论与实验结果表明,该方法能够快速有效地测量光控取向膜锚定能随曝光参数的变化,具有较高的准确性和稳定性,能够为液晶光学元件的精密制造过程提供实时在线监测手段,对掌握光控取向物理过程和提高制造质量至关重要。

自旋扭曲光晶格中的玻色-爱因斯坦凝聚体

研究了自旋依赖双层方形光晶格中玻色-爱因斯坦凝聚体(Bose-Einstein condensates,BEC)的基态特征.双层晶格间的相对扭曲角度和层间耦合强度是影响超冷原子密度分布的重要可调参数.当光晶格的最低能带呈单阱色散时,超冷原子在莫尔晶格(Moire lattice)中的局域化受扭曲角度、层间耦合强度、原子数和晶格深度等的影响;当光晶格的最低能带呈双阱色散时,光晶格的扭曲可导致2个自旋态的反向扭曲,随着层间耦合强度的增加,2个扭曲的自旋态逐渐重合.该研究工作有助于深入探索扭曲光晶格超冷原子中的新奇量子效应.

取石网篮扭转传递性能测试装置的研发及应用

为精准评估取石网篮在体内取石时的扭转传递性能,模拟其在人体屈曲腔道内的取石效能,设计并研制了专门的扭转传递性能测试装置。测试过程中,为确保结果准确性,通过试验优选20圈、转速为50 r/min的参数和重型夹具进行试验,并结合三种弯曲通路,对取石网篮扭转传递性能进行体外模拟和定量测试。测试结果表明:当取石网篮扭转速度从50 r/min提高到100 r/min时,扭矩下降了37.6%;使用重型夹具进行测试时,扭转角传递系数的测试结果更加稳定。对比1145 mm和650 mm长度的取石网篮,前者在的扭矩与扭转角传递系数上分别降低15%和10%。360°弯曲通路下的扭矩和扭转角传递系数相较于伸直状态分别降低38%和14%。该测试装置能精确测量和表征取石网篮在不同条件下的扭转传递性能,可为网篮产品的进一步研发和临床应用提供定量化评价参考。

基于范德华材料的偏振探测器及性能增强方法(特邀)

随着高分辨率成像、信息通信、量子计算等先进技术对光电探测精度和效率的需求日益增长,偏振光探测作为光电技术的一个重要分支,正面临着前所未有的挑战和机遇。本文全面综述了基于范德华二维材料的偏振光电探测技术的最新进展,尤其聚焦于通过表面等离激元、铁电场调控以及转角材料等三种先进策略来显著提高偏振探测性能的创新方法。表面等离激元技术通过局域和传播等离激元共振增强光与材料的相互作用,显著提升了偏振探测的灵敏度和选择性。铁电场调控利用铁电材料的自发极化和局域电场,有效调节探测器件的载流子浓度和光电性能,为实现高效率和低能耗偏振探测提供了新的途径。转角材料通过调整双层二维材料之间的相对角度,引入电子结构和光学性质的可调控性,开辟了偏振探测新领域。最后展望了这些策略在未来偏振光电探测技术发展中的应用前景,强调了材料科学和微纳加工技术的综合应用将为实现高性能偏振探测器件提供强大的支持。