Flux pinning evolution in multilayer Pb/Ge/Pb/Ge/Pb superconducting systems

Multicomponent superconductors exhibit nontrivial vortex behaviors due to the various vortex–vortex interactions,including the competing one in the recently proposed type-1.5 superconductor.However,potential candidate that can be used to study the multicomponent superconductivity is rare.Here,we prepared an artificial superconducting multilayer to act as an alternative approach to study multicomponent superconductivity.The additional repulsive length and the coupling strength among superconducting films were regulated by changing the thickness of the insulting layer.The magnetization measurements were performed to clarify the effect of the competition between the repulsive vortex interactions on the macroscopic superconductivity.The vortex phase diagram and the optimum critical current density have been determined.Furthermore,a second magnetization effect is observed,and is attributed to the upper layer,which provides the weak pinning sites to localize the flux lines.The pinning behaviors switches to the mixed type with the increase of the insulting layer thicknesses.Our results open a new perspective to the study and related applications of the multilayer superconducting systems.

Biomass-derived carbon doping to enhance the current carrying capacity and flux pinning of an isotopic Mg^(11)B_(2)superconductor

Low activation isotopic boron(11B)based magnesium diboride(Mg^(11)B_(2))superconductors doped with biomass-derived activated carbon were synthesized using11B and magnesium powder via solid-state reaction.The effect of carbon doping on the lattice structure and superconducting properties of Mg^(11)B_(2)bulks were evaluated using X-ray powder diffraction,high resolution transmission electron microscopy,scanning electron microscopy and magnetization measurements.Precise refinement of structural parameters indicates successful substitution of carbon in Mg^(11)B_(2)bulks.The critical current density(Jc)of carbon doped Mg^(11)B_(2)synthesized at 650℃was enhanced more than two times compared with the pure Mg^(11)B_(2)bulk.Similar improvement was observed for the Mg^(11)B_(2)bulks heat-treated at 800℃.This enhancement is due to successful substitution of biomass-derived carbon with high surface area into Mg^(11)B_(2)lattice.The flux pinning mechanism of pure and doped Mg^(11)B_(2)bulks were investigated using the Dew-Hughes model.This study provides information regarding enhancement of the Jc of low activation Mg^(11)B_(2)superconductors suitable for next-generation fusion magnets.

Pinning of Vortices for a Variational Problem Related to the Superconductivity with Thermal Noise

This paper is concerned with the minimization problem related to the superconductivity with thermal noise. We study the asymptotic behavior of the minimizes of this problem as the parameters tending to zero and prove the vortices-pinning mechanism.

Improvement in Superconducting Properties of MgB_2 Superconductors by Nanoscale Carbon-Based Compound Doping

MgB2 is a relatively new superconductor; it has attracted great interest from superconductor researchers all over the world. Thorough investigations have been carried out to study the material fabrication, as well as to study the material and superconducting properties from a fundamental physics point of view. The University of Wollongong has played a very active role in this research and a leading role in the research on high critical current density and high critical magnetic fields. Our recent research on the improvement of critical current density and the upper critical magnetic field by carbon-based compound doping is reviewed in this paper.

ESTIMATES OF LOWER CRITICAL MAGNETICFIELD AND VORTEX PINNING BY INHOMO-GENEITIES IN TYPE II SUPERCONDUCTORS

The effect of an applied magnetic field on an inhomogeneous superconductor is studied and the value of the lower critical magnetic field HC1 at which superconducting vortices appear is estimated. In addition, the authors locate the vortices of local minimizers, which depends on the inhomogeneous term a(x).

Effect of Mg Addition on Inhibiting Austenite Grain Growth in Heat Affected Zones of Ti-Bearing Low Carbon Steels

To study the effect of Mg addition on inhibiting weld heat affected zones （HAZ） austenite grain growth of Ti-bearing low carbon steels, two steels with and without Mg treated were prepared using a laboratory vacuum. The welding testing was simulated by Gleeble 3500 thermomechanical simulator. The performance of HAZ was investiga ted that the toughness was improved from 3.3 to 185 J by adding 0. 005%Mg （in mass percent） to the steel, and the fracture mechanism changed from cleavage fracture to toughness fracture. Through in-situ observation by a confocal scanning laser microscope, a significant result was found that the austenite grain of the steel with Mg treated was still keeping fine-grained structure after holding at 1 400℃ and lasting for 300 s. This inhibition of austenite grain growth was mainly attributed to the formation of pinning particles after the addition of Mg. The obtained results pro pose a potential method for improving HAZ toughness of structure steels.

On Ginzburg-Landau Vortices of Superconducting Thin Films

In this paper, we discuss the vortex structure of the superconducting thin films placed in a magnetic field. We show that the global minimizer of the functional modelling the superconducting thin films has a bounded number of vortices when the applied magnetic field hex 〈 Hc1 ＋ K log ｜ logε｜ where Hc1 is the lower critical field of the film obtained by Ding and Du in SIAM J. Math. Anal., 2002. The locations of the vortices are also given.