Repulsive Flux Pinning Force in NbTi/Nb Superconductor/Superconductor Multilayers

Flux pinning characteristics have been investigated for the Nb _x Ti_100-x /Nb(x = 65, 50 and 28) and Nb ₂₈ Ti ₇₂/Nb ₆₅ Ti ₃₅ superconductor(S)/superconductor(S^′) multilayers. The maximum of the pinning force F _{p⊥ max} perpendicular to the layer plane as a function of the structure modulation length λ has a peak in the quasi-two-dimensional region (λ～20 nm). The maximum. values of the F _{p⊥ max} versus λ curve are proportional to the difference of the GL coherence length (ξ _GL ) between the superconductive sublayers S and S^′. The results suggest that the large F _{p⊥ max} in the S/S^′ multilayer is caused by the repulsive pinning force due to Nb layers with larger ξ _GL .     

Effect of Thermal Dilation Stress and Flux Pinning Induced Stress on Superconductor/Tube System

The use of an alloy tube to impose pressure on a superconducting cylinder during magnetizing reduces pinning-induced tensile stress in high temperature superconductors as has been well established. In this paper, the contact problem between superconducting cylinder and metal tube around it is analyzed. Exact solutions are obtained for the problem of the isotropic magnetoelasticity with the superconductor behaving magnetically and for the problem of the thermal mismatch between the metal tube and the superconducting cylinder. The expressions of the stresses and displacement under thermal and flux pinning effect are given. The results show that the stresses induced by thermal mismatch can significantly reduce the tensile stress resulting from the magnetizing process of superconducting cylinder; and there is a linear relationship between the radial displacement of the tube metal and the magnetostriction of free superconducting cylinder.     

Flux Pinning with Addition of Gold Nanoparticles in CuTl-1223 Superconductor

We investigated the effects of gold (Au) nanoparticles on flux pinning in Cu_0.5Tl_0.5Ba₂Ca₂Cu₃O_10?δ (CuTl-1223) superconductor by infield temperature-dependent dc-resistivity measurements. The values of \(\phantom {\dot {i}\!}T_{\mathrm {c}}^{\text {onset}}\) (K) remained almost unaffected by applying external magnetic field on (Au) _x /CuTl-1223; (x = 0～1.5 wt.%) composites samples. But a decreasing trend in T _c(0) and increasing trend in resistive broadening (ΔT) by increasing external applied magnetic field were reduced after addition of Au nanoparticles in CuTl-1223 superconducting matrix. The activation energy (U _o) was calculated according to thermally activated flux flow (TAFF) model by using the Arrhenius Law. The increase in T _c(0), U _o, and upper critical field (H _c2) indicates a strong flux pinning after the inclusion of Au nanoparticles and found optimum for x = 1.0 wt.%.     

Computer Simulation of Flux Pinning in the High-Temperature Superconductors Bi-2212

The magnitude of critical current density J _c (or critical magnetic induction B _c), is closely related to the flux pinning property of the high-temperature superconductors (HTSC) materials. So, the investigation on flux pinning mechanism is an important research field in the high-temperature superconductors. There are different pinning centers by analyzing the flux pinning force of the HTSC. In this study, based on Sun and Zou et al.’s previous works, we studied the flux pinning mechanism in Bi-2212 single crystals with an improved scaling function of pinning density, which is simplified and possesses determined physical meaning. Then, a computer simulation was conducted to determine the category of flux pinning in Bi-2212 according to Sun’s experimental data. It is revealed that the simulation result is consistent with the experiment.     

Numerical Modeling of Iron Yoke Levitation Using the Pinning Effect of High-Temperature Superconductors

A ferromagnetic material can be levitated by the pinning effect of a field-cooled superconductor. This paper presents two methods for modeling this effect: 1) an approximate calculation to determine the relationship between attractive force and air gap at both room temperature and superconductive temperature (77 K) and 2) a novel way of modeling the pinning effect by a finite-element method (FEM). A comparison of analytical and FEM results with experimental results verifies the validity of the methods. The methods can be used to estimate the system's behavior when the cylindrical yoke is replaced by a ring yoke. The stiffness of the system will increase by 70% (to 5.3 N/mm) when a ring yoke with the same surface area is used instead of a cylindrical yoke.      

Effect of Rare-Earth Oxides Doping on the Superconductivity and Flux Pinning of MgB<Subscript>2</Subscript> Superconductor

A series of rare-earth-oxide-doped MgB₂ bulks are prepared by in situ solid-state reaction with Pr₆O₁₁, CeO₂, Lu₂O₃, and Ho₂O₃ as the dopants. The superconducting properties are investigated and compared with the nano-Fe₃O₄ doped MgB₂. It is found that different from doping ferromagnetic nano-Fe₃O₄ which drastically suppresses superconductivity of MgB₂, doping the rare-earth-oxides has little effect on superconductivity of MgB₂ although most rare-earth elements have strong magnetic moment. In addition, some boride impurities formed during the reaction between rare-earth oxides and boron can work as effective pinning centers and significantly improve J _c and H _irr of MgB₂ when these fine nanoboride precipitates (<20 nm) are embedded into the MgB₂ intragrains.     

Modeling and experimental investigation of microstrip resonators and filters based on High-Temperature Superconductor films

Complex approach to the investigation of microstrip resonators and filters based on High-Temperature Superconductor (HTS) films is described, which includes modeling of the electrodynamic parameters of HTS films, designing of microstrip resonators and filters, their manufacturing, and testing. Test samples were prepared using YBCO films on 0.5-mm-thick lanthanum aluminate substrates. The resonators and filter structures were patterned using ion-beam photolithography. Experimental data were used to determine the parameters of the model of surface impedance of the YBCO film and the film thickness and permittivity. Adequacy of the model and reliability of the model parameters, which were used for the synthesis and design of a 4th-order filter, were confirmed by coincidence of the experimental and calculated characteristics.     

Analyses of the Pinning Mechanism for the Superconductor/Normal-Metal and Superconductor/Superconductor Multilayers

The vortex pinning force of superconductor(S)/normal-metal(N) multilayers, Nb/Ti and Nb_100−x Ti _x /Ti, and superconductor(S)/superconductor(S′) multilayers, Nb_100−x Ti _x /Nb and Nb₂₈Ti₇₂/Nb₆₅Ti₃₅, has been studied as a function of the layer thickness d for various compositions of x. The transverse pinning force density F _{p ⊥} acting perpendicular to the layer planes makes a peak around d∼ 2ξ_GL (ξ_GL: the GL coherence length) for both the S/N and S/S′ multilayers, where they exhibit the quasi-two-dimensional superconductivity. In contrast, the longitudinal pinning force density F _p‖ acting parallel to the layer plane hardly depends on d. The pinning mechanism inherent in the layered structure has been analyzed based on the GL free energy, which clarified that the effective pinning force due to the multilayer structure originates from the difference in the potential energy for the S/N multilayer and from the difference in the kinetic energy for the S/S′ multilayer between the constituent layers. The d dependence of the F _{p ⊥}, however, has been found to be qualitatively similar for the S/N and S/S′ multilayers.     

Flux Quantization and Aharonov-Bohm Effect in Superconducting Rings

Superconductivity is a macroscopic coherent state exhibiting various quantum phenomena such as magnetic flux quantization. When a superconducting ring is placed in a magnetic field, a current flows to expel the field from the ring and to ensure that the enclosed flux is an integer multiple of h/(2|e|). Although the quantization of magnetic flux in ring structures is extensively studied in literature, the applied magnetic field is typically assumed to be homogeneous, implicitly implying an interplay between field expulsion and flux quantization. Here, we propose to decouple these two effects by employing an Aharonov-Bohm-like structure where the superconducting ring is threaded by a magnetic core (to which the applied field is confined). Although the magnetic field vanishes inside the ring, the formation of vortices takes place, corresponding to a change in the flux state of the ring. The time evolution of the density of superconducting electrons is studied using the time-dependent Ginzburg-Landau equations.     

Modeling and Predicting the Central Magnetic Flux Density of the Superconducting Solenoid Surrounded with Iron Yoke via SVR

A novel machine learning method based on support vector regression (SVR) approach, combined with a particle swarm optimization (PSO) algorithm for its parameter optimization, was proposed to predict the magnetic field in the centre of a superconducting solenoid surrounded by a cold iron yoke in terms of the geometrical parameters of the yoke. The leave-one-out cross validation (LOOCV) test results of SVR reveal that the prediction ability of the SVR model is greater than that of the conventional multivariate nonlinear regression. The maximum absolute percentage error of 26 samples obtained by SVR did not exceed 0.50% and the statistical mean absolute percentage error was solely 0.05%, which was quite accurate and satisfactory with the requirement of ultraprecision engineering and manufacturing. This investigation provides a clue that the hybrid PSO-SVR approach elaborated in this paper is a promising and practical methodology to precisely design the physical dimension of the iron yoke surrounded around the superconducting solenoid.     

Surface Barrier and Bulk Pinning in MgB2 Superconductor

We present a modified method of preparation of the new superconductor MgB₂. The polycrystalline samples were characterized using X-ray and magnetic measurements. The surface barriers control the isothermal magnetization loops in powder samples. In bulk as prepared samples we always observed symmetric magnetization loops indicative of the presence of a bulk pinning mechanism. Magnetic relaxation measurements in the bulk sample reveal a crossover of surface barrier to bulk pinning.     

Effect of External Nonuniform Magnetic Field on Flux Creep Process in Superconductor

The effect of an external nonuniform magnetic field on the flux creep rate in a high-temperature superconductor with trapped magnetic flux was studied. The magnetic relaxation was suppressed when the superconductor was put into a field of permanent magnets or when it approached a ferromagnet. The effect arises when the field sources (being magnetized, the ferromagnet produces its own field) are placed near the superconductor surface, where the flux line ends are located. For these cases, we carried out the calculations of vortex and current density distributions, which demonstrate that reverse currents flow in the near-surface regions of the sample. This verifies the hypothesis suggested earlier about the influence of counter Lorentz forces retarding the creep of the vortices. In the interpretation of the results, we also take into consideration the magnetic force acting on the vortex ends in the external nonuniform magnetic field that allows us to explain the experimental results, in which the current structure in the sample is unipolar.     

The High-Temperature Superconductor EuBa2Cu3O6 + x: Role of Thermal History on Microstructure and Superconducting Properties

We present here combined resistivity and micro-Raman spectroscopy measurements for the superconductor EuBa₂Cu₃O_{6 + x} (Eu-123, EuBCO). We found an important dependence of microstructure and critical temperature on preparation temperature. We also show a strong orientation effect and a T _c of 93 K in samples prepared at high temperature relative to those prepared at lower temperature, which exhibit lower T _c, broad transitions, and no grain orientation. In addition, we show the influence of annealing conditions on superconducting properties by preparing samples with the same oxygen content (x = 0.38) but with various T and P(O₂). The peculiar behavior of Eu-123 with temperature by considering a different hole injection sequence for Eu-123 with respect to Y-123 is explained. Micro-Raman measurements seem to support this hypothesis.     

Measurement Method to Evaluate the Practical Levitation Force of Onboard Bulk Superconductor in the High-Temperature Superconducting Maglev Vehicle System

As the basic parameter in the design of high-temperature superconducting (HTS) magnet levitation (maglev) vehicle system, the levitation force data are usually obtained by experiments, so a reliable measurement method is needed to know the practical levitation force of onboard bulk superconductor. Considering in the actual operation the vehicle will experience some small vertical or lateral movements, especially during the passenger going on or off the vehicle, the usual measurement method for levitation force, in which force with gap and force relaxation are measured, cannot be able to predict the levitation force change due to the vertical or lateral movements. So a measurement method is presented regarding effects of force with gap, relaxation, vertical and lateral movements. Compared with a former measurement method, the presented method may support more reliable levitation force data for the HTS maglev vehicle design.     

The Effects of Pinning on Critical Currents of Superconducting Films

Using molecular dynamics simulations, we analyze the effects of artificial periodic arrays of pinning sites on the critical current of superconducting thin films as a function of vortex density. We analyze two types of periodic pinning array: hexagonal and Kagomé. For the Kagome pinning network we make calculations using two directions of transport current: along and perpendicular to the main axis of the lattice. Our results show that the hexagonal pinning array presents higher critical currents than the Kagomé and random pinning configuration for all vortex densities. In addition, the Kagomé networks show anisotropy in their transport properties.     

Peculiarities of Pinning and Microwave Absorption Hysteresis in Thin Superconducting Films

The results of experimental and theoretical studies of the hysteretic microwave absorption (MWA) in the superconducting Bi₂Sr₂CaCu₂O₈ thin films are presented. It has been found experimentally that the hysteresis loop manifests some unusual features such as the non-monotone hysteresis variation and the change of a hysteresis sign. We have shown that such unusual behavior is due to the special nature of the bulk pinning in a superconducting film with a thickness comparable with the field penetration depth. The theoretical model of the MWA hysteresis has been developed taking into account the inhomogeneous distribution of centers with different symmetry of a pinning potential and their variation with the magnetic field value.     

Thickness-Dependent Current Density and Flux Distribution in Thin Current-Carrying Superconducting Films Exposed to a Magnetic Field

With the rapid development of high-temperature superconductors, it is of great significance to get the precise current density and flux distribution within thin high-temperature superconducting films subjected to a transport current and an applied magnetic field. The transport current distribution and flux density in thin high-temperature superconducting films are calculated by a numerical method based on the Kim model and exponential model in this paper. The influences of transport current, applied magnetic field, width, and thickness of a superconducting film on the current distribution are discussed. The results reveal that the thickness has a significant effect on the critical current density of superconducting films.     

Effect of Flux Jump on Current Density Distributions in Type-II Superconductors

The effect of flux jump on the current density distributions is studied in type-II superconducting slab. The magnetothermal diffusion equations based on the Kim model in the regime of flux creep are presented to estimate the effect of flux jump on the distributions of current density inside the slab and the changes of surface current density in the flux jump process. Numerical results show that the current density distributions are dependent on the flux jump. It is intended that the flux jump analysis presented be useful to researchers interested in flux jump instability characteristics.