On the Effect of Boundary Conditions in the Ginzburg–Landau Theory on the Results of Calculations of the Critical State of Layered Structures

The effect of boundary conditions in the Ginzburg–Landau theory on the critical state of superconducting layered structures is studied. The method is based on the numerical solution of the Ginzburg–Landau nonlinear equations describing the behavior of a superconducting plate carrying a transport current in a magnetic field, provided the absence of vortices in it. The use of the general boundary condition for the Ginzburg–Landau system of equations leads to a change in the order parameter over the thickness of thin superconducting plates. The calculated dependences of the critical current of plates on the magnetic field applied in parallel to layers are used to determine the critical current of multilayered structures. It is assumed that the mutual influence of superconducting layers occurs only through the magnetic field induced by them.     

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.     

Theory for the upper critical field in superconducting multilayers with interface irregularities

The upper critical field in superconducting multilayers, which are composed of two kinds of superconducting layers with different diffusion constants and contain interface irregularities, investigated in the direction parallel to the layers. The interface irregularities enhance the upper critical field. The crossover temperature at which the superconducting nucleation position moves from one layer to another is increased as the effect of the irregularities becomes strong. As a result, the two-dimensional region where the nucleation position of the superconducting order is inside a layer with the larger diffusion constant is greatly reduced by the irregularities.     

Inelastic neutron scattering from superconducting rings

For the first time the differential cross section for the inelastic magnetic neutron scattering by superconducting rings is derived taking account of the interaction of the neutron magnetic moment with the magnetic field generated by the superconducting current. Calculations of the scattering cross section are carried out for cold neutrons and thin film rings from type-II superconductors with the magnetic fields not exceeding the first critical field.     

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.     

Performance improvement of a high-temperature superconducting coil by separating and grading the coil edge

In this paper, we establish a model to analyze the transport current performance of a high-temperature superconducting (HTS) coil, considering the dependencies of critical current and n-value of an HTS tape on magnetic field and magnetic field angles. This analysis shows that relatively large electric fields appear at the coil’s edges, preventing improvement in the transport current performance of the coil. To solve this problem, in this paper, we propose a graded coil in which several coil edges of different heights are separated and graded. Analysis of its performance shows that the coil’s critical current increases, thus confirming that there exists an optimum coil cross section at which the stored energy and central magnetic field improve 2.1 times and 45%, respectively, compared with a typical rectangular coil that employs the same total length of the HTS tape. It is recommended that these results of the coil should be applied to SMES.     

Josephson effect in superconducting hybrid S/F structures with parallel and antiparallel magnetizations of the ferromagnetic layers

The Josephson effect in S-MB-S, S/F-MB-S, and S/F-MB-F′/S superconducting hybrid structures with the parallel or antiparallel orientation of exchange fields in the layers of metal ferromagnets (F and F′) and in the magnetic barrier (MB) with arbitrary transparency has been investigated. It has been shown that the properties of the studied structures depend strongly on the orientation of the exchange fields: the exchange-field-induced stimulation of the critical current is possible for one orientation, whereas the critical current is suppressed at the opposite orientation of the exchange fields. A change in the orientation of the exchange fields can lead to the switching of the Josephson structures between 0 and π states. The effect of the pair-breaking mechanisms on the Josephson effect and on the spectrum of the Andreev levels in the S-MB-S structures has been analyzed. The results indicate the possibility of stimulating the critical current by the external magnetic field in the S-MB-S structures with thin-film superconducting banks.     

一种实验用交流超导磁体的研制

该文介绍了用于探测线圈法测量超导材料交流损耗的一种交流超导磁体的设计、加工与绕制。超导磁体设计采用成熟的螺线管磁体技术,磁体骨架为低温环氧树脂加工而成,所用超导线为NbTi/Cu复合超导绞线。为了满足磁体的稳定性,每两层超导线间留出了一条冷却通道。所绕磁体的磁场强度和均匀度都满足设计要求。     

超导开关研究和应用综述

超导开关是实现超导磁体闭环运行的关键部件,也称为持续电流开关(PCS),使超导磁体具有极高的磁场稳定度,广泛应用于核磁共振成像(MRI)、核磁共振谱仪(NMR)、磁浮列车、超导储能系统等技术领域. 文中介绍了超导开关的工作原理和结构组成,综述了用于超导磁体的三类主要超导开关的研究发展现状,比较分析了各自的特点和优势,为...     

Numerical modelings of superconducting wires for AC loss calculations

Superconducting properties of superconducting wires as well as the influence of their composite structure and twisting should be taken into account for their numerical modeling for AC loss calculations. Furthermore, complicated electromagnetic conditions in electrical apparatuses under which superconducting wires are used influence their AC loss properties; superconducting wires carry their transport current and are exposed to the external magnetic field whose direction and magnitude vary spatially. A series of numerical models of superconducting tapes based on the finite element method has been developed. In each model, some of the above-mentioned factors that could influence the AC loss properties are taken into account. The models are formulated with the current vector potential and the scalar magnetic potential (T–Ω method). Superconducting property is given by the E–J characteristic represented by a power law. The current distributions in non-twisted and twisted superconducting tapes carrying their transport current and/or exposed to the external magnetic field are calculated with these models to estimate their AC loss. The current distribution in a short piece of superconducting tape exposed to AC magnetic field is also calculated.     

Field polarity dependent superconducting properties in a superconductor/ferromagnet hybrid with in-plane magnetic moment

The transport properties of an Al type-II superconducting thin film covering a Py plain film with a rectangular array of triangular holes are investigated. We show that, although the magnetization of the Py lies in the plane of the structure, both the critical temperature and the critical current are asymmetric with respect to the polarity of the external field, which is applied perpendicularly to the structure. The asymmetric nucleation can be explained in terms of field compensation effects between internal and external magnetic fields, whereas the presence of vortex–antivortex pairs are responsible for the observed features in the critical current.     

Multistable current states in high-temperature superconducting composites

Conditions for current instabilities that arise in high-temperature superconducting composites with essentially nonlinear dependences of the critical current densities and resistivity on the temperature and magnetic induction have been studied. The analysis has been conducted in terms of zero-dimensional models, which has made it possible to formulate general physical mechanisms behind the formation of currents states in superconducting composites according to the external magnetic field induction, cooling conditions, and the properties of the superconductor and cladding. The possible existence of current and temperature stable steps, as well as stable steps of the electric field strength, in the absence of the superconducting–normal transition, has been demonstrated. Reasons for instabilities under multistable current states have been discussed.     

Strong magnetic fluctuations in a superconducting state of CeCoIn5

We show results on the vortex core dissipation through current-voltage measurements under applied pressure and magnetic field in the superconducting phase of CeCoIn{5}. We find that as soon as the system becomes superconducting, the vortex core resistivity increases sharply as the temperature and magnetic field decrease. The sharp increase in flux-flow resistivity is due to quasiparticle scattering on critical antiferromagnetic fluctuations. The strength of magnetic fluctuations below the superconducting transition suggests that magnetism is complementary to superconductivity and therefore must be considered in order to fully account for the low-temperature properties of CeCoIn{5}.     

Effective nonlinear susceptibility of a grained superconducting film

The effective nonlinearity of a thin grained superconducting film is calculated theoretically. It is assumed that the film has the form of a continuous superconducting matrix with superconducting grained inclusions of cylindrical structure with a generatrix perpendicular to the film surface. The current and field distributions are determined to calculate the effective nonlinearity; both the matrix and the grains are considered in the framework of the Ginzburg-Landau theory. The spacing between the grains is assumed to be much larger than their diameter so that the Maxwell-Garnet approximation can be used for calculating fields. The analytic formula derived for effective nonlinear susceptibility is analyzed for various values of physical parameters. It is shown that the effective nonlinearity exceeds the nonlinearity of the matrix and grains by a factor of several units when the grain size is much larger than the magnetic field penetration depth in the substance of the matrix and the conductivity of the grains is lower than that of the matrix.     

Anisotropy and interlayer interaction in Mo/Si multilayers

The interlayer interaction in superconducting Mo/Si multilayers is determined by the measurements of the upper critical magnetic field and the magnetoconductivity in magnetic fields, parallel and perpendicular to the layers. From these measurements we determined the anisotropy parameter, γ, and the effective thickness, L_eff, which characterize the mutual electronic interaction between the Mo layers across the Si layer. Both quantities oscillate in dependence of the Mo layer thickness for Mo/Si series with constant Si layer thickness.     

Stability of ferrimagnetic multilayers

We study the field induced instability of the ground state of ferrimagnetic multilayers consisting of a stacking alternating two different uniaxial ferromagnetic layers. For multilayers with even number of layers N, we obtain analytical expressions for the critical fields in terms of the magnetic parameters (anisotropies, and interlayer exchange coupling), for any value of N. The critical fields are calculated from the energy fluctuations for small variations in the equilibrium magnetic profile. The form of the hysteresis curves is discussed, using the expressions of the critical fields.     

A simple electromagnetic method of cyclic loss measurement for superconducting wires in a combined alternating transverse magnetic field and transport current

We measured cyclic losses in a superconducting wire, carrying alternating transport current, simultaneously exposed to an alternating transverse magnetic field. Samples of Bi-2223 Ag-sheathed tapes have configuration of a double-layer non-inductive coil, which itself is a pickup coil to measure the AC losses. Potential taps were attached to both terminals of the sample coil. The external field was applied along the axis of the sample coil. In this procedure, we can estimate an averaged Poynting's vector on a cylindrical surface between the two layers by means of signals from a pair of the potential taps and from pickup coils for the external magnetic field and the transport current. We can also measure a magnetization and an extended transport-current components of AC losses in addition to a total cyclic loss for a combined alternating external field and transport current. Obtained results are compared with numerical predictions of the critical state model taking into account the magnetic field dependence of critical current density.     

Surface induced mixed state of a superconducting film

Presented in this paper is a theoretical analysis of a planar surface induced mixed state for a superconducting film in parallel applied field. An analytical solution of the internal magnetic field is obtained based on Saint-James and de Gennes' order parameter in a film. An expression of Gibbs free energy per unit volume without restriction of a geometry is derived from non-linear Ginzburg-Landau (GL) equation in terms of a renormalized GL parameter and a modified geometric factor. Based on the Gibbs free energy, a phase diagram of distinguishing a first and second order phase transition for a type I superconducting film is calculated. The numerical results for exact solutions of spatial variation of order parameter, current density and internal magnetic field in the film geometry in parallel applied field case are presented. Near the upper critical field, the first entry of an applied field in the film exhibits a laminar structure.     

三维介观超导环的涡旋结构

在Ginzburg-Landan理论的框架下, 运用有限差分法研究了在圆环电流产生磁场下的介观超导圆环内的涡旋结构, 讨论了超导圆环尺寸和不同空间分布的磁场对涡旋形成的影响, 得到在一般超导圆环体内的基态多是巨涡旋态、而多涡旋态多以激发态形式存在的结论, 说明磁场一般从超导圆环的环孔穿过, 而很难穿过超导圆环体.     

Vortex dynamics in mesoscopic superconducting square of variable surface

In this paper we apply the time dependent Ginzburg–Landau (TDGL) equations to the study of thin superconducting films of variable surfaces in the presence of an external applied magnetic field. We have made a systematic investigation of both the lower and upper critical fields as a function of the topological parameters which control the shape of the film surfaces. We also make a detailed investigation of the vortex dynamics as they nucleate into the sample.