Calculation of critical states of superconducting multilayers based on numerical solution of the Ginzburg-Landau equations for superconducting plates

Numerical methods are used to analyze the Ginzburg-Landau equations for a superconducting plate carrying transport current in a magnetic field. Critical current is calculated as a function of the applied magnetic field strength for superconducting plates with different thicknesses. The relations between the field dependence of critical current and the distributions of order parameter, magnetic field, and supercurrent in a plate are analyzed. The field-dependent critical currents computed for plates are used to determine the critical current as a function of the applied magnetic field strength and local magnetic field and current distributions for multilayers in parallel magnetic fields. The constituent superconducting layers are assumed to interact only via magnetic field. A simple method is proposed for analyzing the critical states of multilayers in magnetic fields of arbitrary strength, based on elementary transformations of the critical current-density distribution over individual layers in zero applied magnetic field. The method can be used to analyze experimental results.     

背景磁场下Y系超导带材临界电流的测量

高温超导电力装置一般工作于低磁场情况下,测量超导带材在低场下的临界电流特性非常重要.文中通过自行设计的实验平台,采用四引线法对Y系超导带材低场下的临界电流特性进行了测量,得到了带材临界电流随外加磁场变化的曲线,并与Bi系带材低场下的临界电流特性进行比较.     

优化磁共振纳米医学中磁性纳米粒子的热疗效率

磁共振热疗(magnetic resonance hyperthermia)是近年来新兴的一种纳米医学治疗方法,由磁共振的硬件架构产生特定交变磁场,有效地加热磁性纳米粒子,以直接或间接地杀死癌细胞,体现诊疗一体化。提高磁性纳米粒子的加热效率是当前磁共振热疗领域亟待解决的难题之一。磁性纳米粒子的加热效率不仅与粒子本身的大小、性质以及尺寸分布有关,还和聚集状态有关。该研究利用3D Metropolis蒙特卡罗模拟方法,模拟了不同温度下磁性纳米粒子的磁共振热动力学行为及其团聚与分离现象;并通过修正过的郎之万方程,建立了相变临界温度与外加磁场频率的函数关系。模拟结果显示,磁性纳米粒子悬浮液中多聚体的相对含量随着温度的升高而降低,达到临界温度后,多聚体完全分离成单体;而提高交变磁场频率可以显著降低临界温度,且存在临界频率,高于此临界频率后临界温度不再受外加磁场频率影响,达到稳定。因而在临界频率下预热磁性纳米粒子悬浮液,使得多聚体分离成单体,可优化磁性纳米粒子的热疗效率。     

外场中超导结的直流约瑟夫逊电流的计算

外加磁场对超导体/半导体/超导体结的临界电流有一定影响。超导结的临界电流与磁场的曲线非常类似于单色光在单狭缝衍射的夫琅和费图样。该文在未考虑外加磁场的超导体/半导体/超导体结的临界电流的基础上,进一步对外加磁场的情形进行了研究,并对其结果借助M athem atica软件做了进一步的讨论。     

Effect of longitudinal applied magnetic field on the self-pinched critical current in intense electron beam diode

The effect of applied longitudinal magnetic field on the self-pinched critical current in the intense electron beam diode is discussed. The self-pinched critical current is derived and its validity is tested by numerical simulations. The results shows that an applied longitudinal magnetic field tends to increase the self-pinched critical current. Without the effect of anode plasma, the maximal diode current approximately equals the self-pinched critical current with the longitudinal magnetic field applied; when self-pinched occurs, the diode current approaches the self-pinched critical current.     

Magnetic fluids in an external field

Within mean field approximation we investigate the phase diagrams of magnetic fluids in presence of a magnetic field. In a finite field the magnetic phase transition is absent, but instead a line of first order liquid-liquid transitions ending in a critical point occurs for a magnetic interaction, which is sufficiently strong. Varying the magnetic field these critical points extend from the tricritical point at H=0 to a critical endpoint. For a fluid with Ising spins we calculate the critical lines and several tricritical exponents analytically. For Heisenberg fluids we obtain the phase diagrams from a numerical solution of the mean field equations of state. Received 20 March 1998     

高温超导变压器中带材临界电流随磁场变化的研究

通过自行设计的小型磁体产生稳态磁场,测量了高温超导带材在不同磁场下的临界电流,得到带材临界电流随外加磁场变化的曲线．并提出了高温超导变压器漏磁场优化设计的目标．     

Ginzberg-Landau理论对超导薄膜的适用性

本文指出虽然三十年来Ginzberg-Landau理论(以下简称G-理论)被人们广泛地用于描述超导薄膜的临界场,而且它还是描述强耦合超导薄膜临界场的唯一理论,但G-L关于薄膜临界场的理论不仅一直未得到实验证明,而且它既不能用于描述结晶态的弱耦合超导薄膜的临界场,也不适用于描述强耦合超导薄膜的临界场。本文还提出了一个在薄膜极限下定域的新判据:ξ<<λ,ξ<关键词：     

Evaluation of critical current based on a new transition concept

The critical current of high-temperature superconductor (HTS) tapes depends on temperature, magnetic field, and flux angle against the tape. Analyzing the performance of an HTS coil relies on the critical current equation, which has these dependencies. To date, the critical current equation has been obtained by fitting the measured data. However, this fitting method requires the time-consuming and laborious work of measuring the tapes. In this study, based on a new transition concept in applying a magnetic field, we show how to obtain the critical current equation, while keeping the temperature constant. From the wave function of the Cooper pair which distributes along a magnetic flux quantum, a wave function at the transition state is obtained; this yields the wave functions with perpendicular and parallel magnetic components against the tape. Combining these wave functions yields the conclusive critical current density equation. To confirm the accuracy of this equation, used Bi-2223/Ag tapes, it was compared with the measured critical current under various magnetic fields and flux angles; it was found that they agree well. The parameters of this equation can be obtained by measuring the perpendicular/parallel critical magnetic field component. This equation does not require fitting the measured data, therefore, it can be employed with any Bi-2223/Ag tape.     

Thermal entanglement in a mixed-spin Heisenberg XXZ model under a nonuniform external magnetic field

The thermal entanglement in (1/2,1) mixed-spin Heisenberg XXZ model is investigated under an external nonuniform magnetic field. In the uniform magnetic field system, the critical magnetic field B _c and critical temperature T _c are increased by increasing the anisotropic parameter k. The degree of magnetic field b plays an important role in improving the critical temperature and enlarging the region of entanglement in the nonuniform magnetic field system.     

Dependence of current and field driven depinning of domain walls on their structure and chirality in permalloy nanowires

A magnetic domain wall (DW) injected and pinned at a notch in a permalloy nanowire is shown to exhibit four well-defined magnetic states, vortex and transverse, each with two chiralities. These states, imaged using magnetic force microscopy, are readily detected from their different resistance values arising from the anisotropic magnetoresistance effect. Whereas distinct depinning fields and critical depinning currents in the presence of magnetic fields are found, the critical depinning currents are surprisingly similar for all four DW states in low magnetic fields. We observe current-induced transformations between these DW states below the critical depinning current which may account for the similar depinning currents.     

Pattern of the induced magnetic field in high-temperature superconducting ceramics

Our experiments performed earlier have shown that, when an external magnetic field is absent, the transport critical current in 3-d superconducting ceramics is a homogeneous function of the sample transverse sizes. The transport critical current density and magnetic field induced by the current are homogeneous functions of a point on the sample cross-section. Using these experimental results equations describing the induced magnetic field pattern in ceramic sample have been derived. The distributions of the transport critical current density and induced magnetic field in the samples having polygonal, diamond-shaped cross-sections illustrate the results.     

A phenomenological study of superconductivity in rare earth compounds

The effect of critical magnetic scattering is studied in paramagnetic phase of rare earth compounds showing superconductivity and magnetic order. If the exchange coupling between the conduction electrons and the magnetic ions is small the system becomes superconducting between two transition temperatures. The lower one is slightly higher than the magnetic ordering temperature and this is due to the critical magnetic scattering of the conduction electrons. The influence of the lifetime of one electron on the phase transition is briefly discussed.     

斯格明子相关的螺旋磁有序体系的临界行为

介绍了与斯格明子相关的螺旋磁有序体系的临界行为.首先阐述了连续相变中的临界现象、临界指数、标度律、普适性等概念;随后介绍了磁相变体系中几种临界指数的获得方法,包括直流磁性迭代法、磁熵变法;进而,分析了几类与斯格明子相关的螺旋磁有序体系的临界行为.MnSi是典型的斯格明子材料,临界指数显示其磁性行为符合三重临界行为.MnSi的临界行为揭示:外磁场可以抑制这一体系在零场下的一级相变,使其转变为二级相变,从而在螺旋磁有序、锥形磁有序、顺磁相的三相交汇点形成三重临界点.斯格明子体系FeGe和Cu_2OSeO_3的临界行为符合三维海森伯相互作用,表明它们的磁性行为主要是由近邻的各向同性的自旋耦合作用所决定;而Fe_(1-x)Co_xSi和新发现的斯格明子体系Fe_(1.5-x)Co_xRh_(0.5)MoN的临界行为显示Co掺杂可以有效地调制其中的磁性耦合.对螺旋磁有序体系的临界行为研究表明,尽管这些体系都表现出类似的斯格明子态,但是它们的磁性耦合机制却大不相同,并且其耦合机制可以受到外界手段的调制.最后,根据普适性原理和标度方程,阐述了一种构建磁场诱导相变体系在临界温度附近H-T相图的方法.     

Effect of carbon substitution on low magnetic field AC losses in MgB2 single crystals

The DC magnetization and AC magnetic susceptibilities were measured for MgB₂ single crystals, unsubstituted and carbon substituted with the composition of Mg(B_0.94C_0.06)₂. AC magnetic losses were derived from the AC susceptibility data as a function of the AC amplitude and the DC bias magnetic field. From the DC magnetization loops critical current densities were derived as a function of temperature and DC field. Results show that the substitution with carbon decreases critical current densities at low external magnetic fields, in contrast to the well known effect of an increase of the critical current densities at higher magnetic fields.     

Effect of electromagnetic boundary condition on dynamo actions

In this paper,based on the mean field dynamo theory,the influence of the electromagnetic boundary condition on the dynamo actions driven by the small scale turbulent flows in a cylindrical vessel is investigated by the integral equation approach.The numerical results show that the increase of the electrical conductivity or magnetic permeability of the walls of the cylindrical vessel can reduce the critical magnetic Reynolds number.Furthermore,the critical magnetic Reynolds number is more sensitive to the varying electrical conductivity of the end wall or magnetic permeability of the side wall.For the anisotropic dynamo which is the mean field model of the Karlsruhe experiment,when the relative electrical conductivity of the side wall or the relative magnetic permeability of the end wall is less than some critical value,the m=1(m is the azimuthal wave number)magnetic mode is the dominant mode,otherwise the m=0 mode predominates the excited magnetic field.Therefore,by changing the material of the walls of the cylindrical vessel,one can select the magnetic mode excited by the anisotropic dynamo.     

The behavior of the critical current density below and above the first matching field in superconductors with periodic square arrays of pinning sites

We have studied the effect of the applied magnetic field on critical depinning force at zero and finite temperatures and for several values of pinning strength. This was achieved by conducting extensive series of molecular dynamic simulations on driven vortex lattices interacting with periodic square arrays of pinning sites. We have found that the critical depinning force decreases as the applied magnetic field is increased. We have also observed two distinct behaviors of dependence of the critical depinning force on the applied magnetic field below and above the first matching filed.     

Response of a superconducting cylinder to a longitudinal magnetic field at the field dependence of critical current

The distributions of the magnetic induction and current density in a superconducting circular cylinder with a longitudinal external magnetic field, which take into account the field dependence of the critical current, are obtained within the nonlocal model of critical state.     

Change of the effective fluctuation dimensionality in vanadium thin films in a magnetic field

Lowering of the effective fluctuation dimensionality in the critical region has been found for thin superconducting vanadium films placed in strong perpensicular magnetic fields. The effect shows itself as a change of the critical index characterizing the temperature dependence of the excess film conductivity. By varying the magnetic field, one can observe both two-dimensional and zero-dimensional critical fluctuations on the same specimens.     

Magnetic transition phase diagram of cobalt clusters electrodeposited on HOPG: Experimental and micromagnetic modelling study

The magnetic transition from mono- to multidomain magnetic states of cobalt clusters electrodeposited on highly oriented pyrolytic graphite electrodes was studied experimentally using Magnetic Force Microscopy. From these images, it was found that the critical size of the magnetic transition is dominated by the height rather than the diameter of the aggregate. This experimental behavior was found to be consistent with a theoretical single-domain ferromagnetic model that states that a critical height limits the monodomain state. By analyzing the clusters magnetic states as a function of their dimensions, magnetic exchange constant and anisotropy value were obtained and used to calculate other magnetic properties such as the exchange length, magnetic wall thickness, etc. Finally, a micromagnetic simulation study correctly predicted the experimental magnetic transition phase diagram.