强磁场技术及其在科学研究中的应用

强磁场的含义在技术上通常是指超过3T(1T—10～4G)的磁场. 产生强磁场的基本方法是让电流通过多匝线圈.这项技术始于十九世纪,正是这一发明导致了近代电力工业的出现. 本世纪三十年代末,由于F.Bitter在美国麻省理工学院开创了水冷磁体设计技术[1],从而使稳态强磁场的产生技术有了大的突破.五十年代,许多国家开始建造强磁场实验装置;六十年代后,大型的强磁场实验室在许多国家相继建立,并广泛开展了强磁场下的科学研究工作.目前,水冷磁体产生的场强达到23.5T.与此同时,由于六十年代初,高临界参量(Hc1,Tc,Jc)Ⅱ类超导体的出现,使相当数量的…     

我国强磁场研究与技术的最新进展

中国物理学会第二届全国强磁场学术讨论会于1989年10月19日至23日在合肥中国科学院等离子休物理研究所召开.来自中国科学院、冶金工业部、机械电子工业部、铁道部、高等教育部等所属17个单位38位代表参加了会议.会议收到论文、报告34篇。其中邀请报告6篇.下面对这次会议作一简略评述,以揭示我国强磁场研究与技术的最新进展. 一、高场磁体技术1.20T混合磁体装置 中国科学院等离子体物理研究所经过几年的努力,建成我国第一台20T混合磁体装置,并投入正常运行.该装置包括水冷系统、监控系统和液氦低温及氦气致冷系统.超导线圈已研制完成,正待试…     

42T水冷磁体容器模态及抗震分析

42 T水冷磁体是我国稳态强磁场实验室在建的设计指标最强的高场水冷磁体装置.本文聚焦于42 T水冷磁体容器在本地7级地震这一极端工况下结构强度可靠性研究.首先基于有限元分析软件对水冷磁体容器进行高压工况下的应力分析，再通过自重分析研究其20阶非零模态的固有频率；根据前20阶固有频率，获得地震水平反应谱值，再结合Response Spectrum模块模拟地震作用下42 T容器的响应谱分析.结果显示，在3 MPa水压工况下容器最大应力为115 MPa,在地震谱激励下产生的最大等效应力为1.69 MPa,最大位移为0.016 mm, 304不锈钢材料许用应力为137 MPa,故42 T水冷磁体容器设计符合7级地震工况要求.     

感应法测量脉冲强磁场及其分布

一、前 言 在现代科学技术领域中,强磁场得到了广泛的应用.通常,为了降低磁体的发热,提高供电系统的效应,大体积强磁场系统愈来愈多的采用脉冲强磁场,即在较短的作用时间内产生具有较大幅值的磁场.本文主要介绍感应法测量脉冲强磁场及其分布,并介绍我们自制的测量仪器(多道仪)的线路及实验结果. 二、原 理 许多与磁场有关的物理效应都可用于测量磁场.与其它方法比较起来,感应法测量脉冲强磁场具有使用方便、设备简单、测量范围宽等优点,它仍是目前测量脉冲强磁场的主要方法. 根据感应法原理研制成功的《多通道脉冲强磁场测量仪》(以下简称多…     

强磁场技术的新进展

强磁场是核聚变和高能物理研究的前提,因此超强磁场的设计和有关的超导材料已成为磁学领域一个重要的问题.超强磁场的建立往往成为衡量一个国家工业、技术实力的标准之一.因此,各主要工业国竞相建立超强磁场,这场竞争一直在持续进行.本文介绍强磁场技术最近获得的突破性进展. 众所周知,产生10T以上稳态强磁场通常有两种方法:一是用高功率水冷电阻式磁体(WM),这类磁体的最高纪录是法国Grenoble的场强达23.4T的磁体;另一种方法是用超导磁体(SM), 其最高纪录是日本国立金属研究所1977年建立的17.5T强磁场,笔者于1984年参观过安装在日本筑波…     

强磁场的产生及应用

 强磁场通常指的是场强为几特斯拉（T）（1T=10⁴奥斯特）的磁场.强磁场是进行物性研究和应用研究的重要条件之一.在许多领域中有广泛的应用.许多发达国家都有强磁场实验室.本文简单扼要地介绍强磁场产生的方法及其在某些方面的重要应用.一、强磁场的产生强磁场有稳态式和脉冲式两种.产生稳态强磁场的磁体通常是用导体绕成的螺旋管,它可分为三种:普通导体磁体、超导体磁体、由普通导体和超导体结合绕成的混合磁体.普通导体的磁体所能达到的场强一般为20T.     

用于激光等离子体中脉冲强磁场产生的电感耦合线圈

脉冲强磁场装置是磁化激光等离子体实验的核心设备.本文研制了一种用于优化脉冲强磁场设备的电感耦合线圈,相对于单匝磁场线圈可以进一步提高磁场强度.通过实验和模拟研究了电感耦合线圈的初级螺线管匝数和直径对磁场强度的影响,发现对于2.4μF电容的放电系统,电感耦合线圈的初级螺线管在35匝、35 mm直径时,可以在5 mm内径的次级磁场线圈中获得最高的峰值磁场强度,是相同尺寸单匝磁场线圈产生磁场强度的3.6倍.在充电电压20 kV时,峰值磁场强度达到19 T,使用铍铜材料的电感耦合线圈克服强磁场中线圈炸裂问题,在35 kV的充电电压下得到了33 T的峰值磁场强度.这种新方法产生了更强的磁场、降低了对回路电感的要求、提升了实验排布的灵活性,为研究强磁场下的激光等离子体行为创造了条件.     

中国科学院强磁场科学中心

正中国科学院强磁场科学中心于2008年成立。中心目标:发展强磁场科学技术;开展强磁场下多学科前沿研究;推动技术转移转化促进经济技术发展。其建成的稳态强磁场实验装置取得了一系列成就,磁体技术和综合性能处于国际领先地位。成功研制了创造世界纪录的系列水冷磁体、国际一流水平的混合磁体及其磁体支撑装备系统;成功研制了国际唯一的高场扫描隧道显微系统,国际独创的组合成像显微系统;国际领先的强磁场、超高压、低温综合极端实验条件。在国际上实现了强磁场实验条件从跟跑到领跑的跨越,使我国稳态强磁场科学研究条件跃升至世界一流水平,已成为国际上     

中心对称的阻挫磁体中斯格明子直径的调节

在带有垂直各向异性的二维三角晶格磁体中,当同时存在最近邻铁磁性和第三近邻反铁磁性交换作用时,垂直于膜面施加外磁场会使体系内自旋沿着非共面的方向排列,甚至出现拓扑稳定的斯格明子自旋结构.基于蒙特卡罗模拟方法,本文研究了在该二维阻挫磁体中,竞争性交换作用和外磁场对斯格明子直径的影响.与常规非中心对称的手性磁体中的斯格明子性质类似,外磁场会磁化斯格明子外围自旋而减小斯格明子直径.但是,磁体中反铁磁性交换作用的增强会整体压缩斯格明子.本文结合自旋波理论和蒙特卡罗模拟,首次量化了此类阻挫磁体中斯格明子的直径.结果表明:在弱的反铁磁性交换作用磁体中,斯格明子直径随磁场增大而快速线性减小;随着反铁磁性交换作用的增大,斯格明子直径随外磁场增大的减小变得相对平缓,但在强磁场下也会造成斯格明子直径的加速减小;随着反铁磁性交换作用的增强,斯格明子在不同外磁场下的直径的最大值和中值均从逐渐减小到渐趋稳定,而直径的最小值则从快速减小到表现出很大的涨落.这些现象都可以通过分析斯格明子在不同交换作用和外磁场下的构型和磁能变化加以解释.该项工作阐明了在中心对称的阻挫磁体中斯格明子直径的可调节性,不仅完善了我们对斯格明子本身物理机理的认识,同时也为发展基于斯格明子的新一代存储和逻辑器件提供了理论支撑.     

“闪光二号”5 T脉冲强磁场装置

为了满足闪光二号加速器材料热力学效应研究的新需求,设计了一套电容器储能型脉冲强磁场装置。装置主要由储能电容器、半导体放电开关、磁场线圈及高压恒流充电源组成。磁场线圈中心处最大磁感应强度可达5 T,并且可以通过调整磁场线圈与二极管的相对位置实现磁透镜比的调节。通过理论计算和数值模拟相结合的方法对脉冲强磁场的关键参数进行了分析,然后进行了脉冲强磁场的工程设计,最后使用该强磁场装置进行了实验研究。强磁场实验中,当储能电容器充电21 kV时,在磁场线圈中心处获得了5.3 T脉冲强磁场。     

Geometrical magnetothermopower in semiconductors

The geometry of a semiconductor sample can be designed to create a very large change of the thermoelectric power in a magnetic field, similar to the effects of the sample geometry on the magnetoresistance. In semiconductors in which the minority carriers have a higher mobility than the majority carriers, this geometrical magnetothermopower can freeze out the contribution of the former to the total thermopower. This opens a new route toward high-efficiency thermoelectric materials. We also examine the thermoelectric reciprocity relations for these macroscopic systems.     

Emerging Theory,Materials, and Screening Methods: New Opportunities for Promoting Thermoelectric Performance

High‐performance thermoelectric materials have attracted immense interest due to the capability of directly converting thermal energy into electrical energy. The correlation and inherent complexity between the thermoelectric parameters pose serious challenges to improving the materials’ thermoelectric performance. Herein, the emerging novel theories in the field of thermoelectrics are summarized, such as the coherent phonon, nanophononic metamaterial, rattling effect, topological phonon, and topological electron. The impacts of these new concepts on thermoelectric performance are then reviewed. Finally, a number of promising thermoelectric materials such as one‐dimensional nanowires, two‐dimensional layered materials, and nanomesh structures are discussed. The advanced understanding of thermal and electrical transport properties in thermoelectric materials is presented herein, providing new opportunities for improving thermoelectric performance.     

Transport phenomena in spin caloritronics

The interconversion between spin, charge, and heat currents is being actively studied from the viewpoints of both fundamental physics and thermoelectric applications in the field of spin caloritronics. This field is a branch of spintronics, which has developed rapidly since the discovery of the thermo-spin conversion phenomenon called the spin Seebeck effect. In spin caloritronics, various thermo-spin conversion phenomena and principles have subsequently been discovered and magneto-thermoelectric effects, thermoelectric effects unique to magnetic materials, have received renewed attention with the advances in physical understanding and thermal/thermoelectric measurement techniques. However, the existence of various thermo-spin and magneto-thermoelectric conversion phenomena with similar names may confuse non-specialists. Thus, in this Review, the basic behaviors, spin-charge-heat current conversion symmetries, and functionalities of spin-caloritronic phenomena are summarized, which will help new entrants to learn fundamental physics, materials science, and application studies in spin caloritronics.     

On the anomalous low-resistance state and exceptional Hall component in hard-magnetic Weyl nanoflakes

Magnetic topological materials, which combine magnetism and topology, are expected to host emerging topological states and exotic quantum phenomena. In this study, with the aid of greatly enhanced coercive fields in high-quality nanoflakes of the magnetic Weyl semimetal Co_3Sn_2S_2, we investigate anomalous electronic transport properties that are difficult to reveal in bulk Co_3Sn_2S_2 or other magnetic materials. When the magnetization is antiparallel to the applied magnetic field, the low longitudinal resistance state occurs, which is in sharp contrast to the high resistance state for the parallel case. Meanwhile, an exceptional Hall component that can be up to three times larger than conventional anomalous Hall resistivity is also observed for transverse transport. These anomalous transport behaviors can be further understood by considering nonlinear magnetic textures and the chiral magnetic field associated with Weyl fermions, extending the longitudinal and transverse transport physics and providing novel degrees of freedom in the spintronic applications of emerging topological magnets.     

Recent observations of negative longitudinal magnetoresistance in semimetal

The discovery of Dirac semimetal and Weyl semimetal has motivated a growing passion for investigating the unique magneto-transport properties in the topological materials.A Weyl semimetal can host Weyl fermions as its low-energy quasi-particle excitations,and therefore perform exotic features analogous to those in high-energy physics,such as the violation of the chiral charge conservation known as the chiral anomaly.One of the electrical transport signatures of the chiral anomaly is the Adler-Bell-Jackiw(ABJ) anomaly which presents as a negative magnetoresistance when the magnetic field and the current are parallel.Very recently,numerous experiments reported negative longitudinal magnetoresistance(NLMR) in topological materials,but the details of the measurement results are various.Here the materials and the corresponding experiment results are briefly reviewed.Besides the plausible explanation of the ABJ anomaly,some other origins of the NLMR are also discussed.     

Strategies for discovery and optimization of thermoelectric materials: Role of real objects and local fields

Thermoelectric materials provide a renewable and eco-friendly solution to mitigate energy shortages and to reduce environmental pollution via direct heat-to-electricity conversion. Discovery of the novel thermoelectric materials and optimization of the state-of-the-art material systems lie at the core of the thermoelectric society, the basic concept behind these being comprehension and manipulation of the physical principles and transport properties regarding thermoelectric materials. In this mini-review, certain examples for designing high-performance bulk thermoelectric materials are presented from the perspectives of both real objects and local fields. The highlights of this topic involve the Rashba effect, Peierls distortion, local magnetic field, and local stress field, which cover several aspects in the field of thermoelectric research. We conclude with an overview of future developments in thermoelectricity.     

Angular momentum density of a Gaussian vortex beam

The Gaussian vortex beam is assumed to be linearly polarized.The analytical expression of the electric field of a linearly polarized Gaussian vortex beam propagating in free space is derived by using the vectorial Rayleigh-Sommerfeld integral formulae.The propagating magnetic field of the linearly polarized Gaussian vortex beam is presented by taking the curl of the electric field.By employing the electromagnetic field of the linearly polarized Gaussian vortex beam beyond the paraxial approximation,the analytical expression of the angular momentum density of the linearly polarized Gaussian vortex beam is derived.The three components of the angular momentum density of a linearly polarized Gaussian vortex beam are demonstrated in the reference plane.The effects of the linearly polarized angle and the topological charge on the three components of the angular momentum density are investigated.To acquire the more longitudinal angular momentum density requires such an optimal choice that the linearly polarized angle is set to be zero and the topological charge increases.This research is useful to the optical trapping,the optical guiding,and the optical manipulation.     

Effect of electric field on the electrical conductivity of defected carbon nanotube: Multifractal properties of the wavefunctions

A theoretical analysis of controllable metal–insulator transition is performed by carrying out a quantum chaos analysis for a single-walled carbon nanotube which is affected by topological Stone–Wales defect. Nanotubes have recently attracted attention as promising materials for flexible nanoelectronic devices. Individual topological Stone–Wales defects have been identified experimentally in carbon nanotubes (CNTs) and graphene. The findings reveal that defected CNT displays a gradual crossover from metal to insulator phase in a longitudinal electric field. By determining the threshold value of the electric field for metal–insulator transition, CNT may be used as a switch in electronic devices. Our results are obtained by calculating the singularity spectrum of a nearest-neighbor tight-binding model. Also, quantum chaos theory is used for obtaining a detailed understanding of a dynamic phase transition from delocalized states (chaotic) to localized states (Poisson). More interestingly, the appearance of negative differential resistance for pure CNT suggests potential applications in nanoelectronic devices.     

Smectic islands in antiferroelectric nanofilms

Heterochiral islands, in which topological dipoles are oppositely directed, are observed in freestanding antiferroelectric (SmC _A ^*) films. The topological dipoles in films with a transverse electric polarization and a planar molecule orientation at island boundaries are coplanar with an electric field. The topological dipoles in films with a longitudinal polarization and a planar orientation at island boundaries are perpendicular to an electric field. For a radial director orientation at island boundaries, the topological dipoles in films with a longitudinal polarization are coplanar with a field. Changing the orientation of an electric field, we can control the position of a topological defect at an island boundary and the orientation of a topological dipole. Heterochiral islands can form dimers with an anomalously small interisland distance.     

Cloaking magnetic field and generating electric field with topological insulator and high permeability material

Topological insulator is a new state of quantum matter. When applied magnetic field is applied on a topological insulator, not only the magnetic field is induced, but also the electric field is induced, vice versa. We designed bi-layer magnetic cloak with topological insulator and high permeability material (HPM), derived the electric field and magnetic field inside and outside the bi-layer topological insulator and HPM. Calculation and simulation results show that the applied magnetic field is cloaked by the bi-layer topological insulator and HPM, and the uniform electric field is induced in the cloaked region.