Current Density In High-temperature Superconductor Research Articles

Influence of the physical parameters on the limits of magnetic stiffness of high-Tc superconducting levitation systems

Magnetic stiffness is one of the important stability parameters of high temperature superconducting levitation systems. Till to now, great efforts have been made to understand levitation properties including flux penetration, magnetization curves, levitation force, ac susceptibilities, etc. In this paper we present a quadratic approximation method for the limit of magnetic stiffness in a high temperature superconducting levitation system based on Kim’s critical state model and Ampère law. The system is composed with a cylindrical permanent magnet (PM) and a coaxial high temperature superconductor (HTS). It is found that the limit of magnetic stiffness depends upon both the penetration history of shielding currents distribution in HTS and applied magnetic field gradients. Furthermore, the influence of the physical parameters, such as critical current density in HTS and applied magnetic field, on the limits of magnetic stiffness is investigated in detail. The obtained results display that magnetic stiffness decreases with the increasing of critical current density, since shielding currents have not penetrated into the large portion of the HTS. With the increase of applied magnetic field, the magnetic stiffness obtain a larger magnification factor. It is related to the increase of the shielding current penetration volume and the internal magnetic field in HTS.

Depairing current density through a low-angle grain boundary in a superconducting film

In this paper, the effect of a grain boundary (GB) on the depairing current density of a high-temperature superconducting film is investigated. The modified effective free energy is proposed by considering the interaction of the superconducting condensate with the deformation of the superconductor due to the dislocations which constitute a grain boundary. After the elastic strain field of the dislocation is obtained, we analyzed the depress effect of the GB on the depairing current density of a superconducting film. The results are qualitatively agreement with the classic exponential relationship with the misorientation angles of the critical current density of high-temperature superconductors.

Scaling rules for critical current density in anisotropic biaxial superconductors

Recent researches highlight the additional anisotropic crystallographic axis within the superconducting plane of high temperature superconductors (HTS), demonstrating the superconducting anisotropy of HTS is better understood in the biaxial frame than the previous uniaxial coordinates within the superconducting layer. To quantitatively evaluate the anisotropy of flux pinning and critical current density in HTS, we extend the scaling rule for single-vortex collective pinning in uniaxial superconductors to account for flux-bundle collective pinning in biaxial superconductors. The scaling results show that in a system of random uncorrected point defects, the field dependence of the critical current density is described by a unified function with the scaled magnetic field of the isotropic superconductor. The obtained angular dependence of the critical current density depicts the main features of experimental observations, considering possible corrections due to the strong-pinning interaction.

Numerical Studies of Axial and Radial Magnetic Forces Between High Temperature Superconductors and a Magnetic Rotor

The magnetic bearing is considered as one of the most prospective applications of high temperature superconductors (HTSs) which can realize stable levitation in a magnetic field generated by permanent magnet devices or coils. The exploration of this kind of HTS bearing through numerical investigation is usually made by assuming the induced current circulates only within the ab-plane and thus simplifying the actual three-dimensional problem to a two-dimensional one. In this paper, on the basis of the three-dimensional model of the HTS bulk established before, we further introduce the developed finite-element software to calculate the magnetic field generated by a magnetic rotor which is composed of permanent magnet (PM) rings and ferromagnet (FM) shims, and in this way, we can investigate the magnetic forces (radial force and axial force) of a simplified HTS bearing model, i.e., two symmetric HTS bulks and a magnetic rotor, at a three-dimensional level. The investigations performed in this paper lead to the observations: the favorable configuration to construct the HTS bearing is that the axial height of each HTS element should be equivalent to the axial heights of PM ring plus FM shim; the increase of the radial thickness of PM ring will improve both the radial force and the axial force considerably, but its margin decreases; the enhancement of critical current density of HTSs due to the decrease of operating temperature can result in a higher increase of both the radial and axial force with a lower nominal gap between the HTSs and the magnetic rotor.

On the Relation Between the Nonlinear Surface Impedance and the Superfluid Current Density in High-Temperature Superconductors

The nonlinear surface impedance is related to a local description of the nonlinear superfluid current density that depends on the condensate velocity, by use of the definition of surface impedance as the ratio between the electric field and magnetic field at the conductor surface. To obtain this relation, we follow a rigorous approach based on the time-domain nonlinear London equations and the two-fluid model. The formulation is compared with harmonic balance simulations of an equivalent transmission line circuit that models a plane wave propagating across a vacuum-superconductor boundary, and shows very good agreement for the resulting surface impedance as a function of current density at the surface.

High-Speed Method for Analyzing Shielding Current Density in High-Temperature Superconductor

A fast method is proposed for calculating the shielding current density in a high-temperature superconductor. When the power law is assumed as the constitutive relation linking the shielding current density and the electric field, the initial-boundary-value problem for the shielding current density cannot be always solved with the Runge-Kutta method. This is the case even when an adaptive step-size control algorithm is incorporated. In order to overcome this difficulty, the constitutive relation is modified for a large value of the shielding current density. By using this method, a numerical code is developed for calculating the shielding current density. The results of computations show that the proposed method is much faster than the conventional implicit one for a large-scale problem.

Numerical simulation of inductive measurement of critical current density in HTS: high-performance method for solving shielding current equations

A numerical code has been developed for analyzing the time evolution of the shielding current density in the high-temperature superconductor (HTS) bulk. In the code, the shielding current equations are discretized by means of the element-free Galerkin method and the resulting nonlinear system is solved by use of the pseudo Newton method with a variable relaxation factor. By means of the code, the shielding current density is calculated in cases where the ac current flows in a coil placed just above the HTS. The voltage induced in the coil is evaluated as a function of time and its spectral analysis is performed. The results of computations show a qualitative agreement with Mawatari's experimental results.

Time-domain analysis of shielding current density in high-temperature Superconductor

The numerical code has been developed for the time-domain analysis of the shielding current density in the high-temperature superconductor (HTS). By means of the code, the shielding current density is calculated in cases where the ac current flows in a 1-turn coil placed just above the HTS. The voltage induced in the coil is evaluated as a function of time and its spectral analysis is performed. The results of computations show that, above a certain limit of the coil current, the third-harmonic voltage is suddenly excited and it grows more rapidly than any other harmonic voltage. It is also found that, if the dc magnetic field as well as the coil field is applied, the square root of the third-harmonic voltage is proportional to the coil current.

Transport properties and flux pinning by self-organization in YBa2Cu3O7−δfilms on vicinal SrTiO3(001)

We have observed anisotropic flux pinning and resistivity in the substrate plane of YBa${}_{2}$Cu${}_{3}$O${}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) thin films grown on vicinal SrTiO${}_{3}$(001) with the $c$ axis oriented along the SrTiO${}_{3}$ [001] direction. Using UHV scanning tunneling microscopy and transmission electron microscopy we demonstrate the influence of a periodic nanoscale step structure of the clean substrate surface on the morphology and defect microstructure of subsequently deposited YBCO films. A linear array of dislocations generated via self-organization of the YBCO leads to an exceptionally large critical current density up to $8\ifmmode\times\else\texttimes\fi{}{10}^{11}$A/m${}^{2}$ at 4.2 K. The findings indicate that the critical current density of high-temperature superconductors can be enhanced in a controlled way growing thin films on tailored substrate surfaces.

Nonlinear effective activation energy and field dependence of the critical current density in HgBa2Ca2Cu3O8+ delta.

The effective activation energy of HgBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+{delta}} superconductor was extracted from the magnetic relaxation measurements below 30 K, and a power-law dependence on current density was found. We also obtained the effective activation energy from the whole magnetic hysteresis ({ital M}-{ital H}) loops plotted at constant field-sweep rate. The data sets for the {ital M}-{ital H} loops at different temperatures overlap each other, and all fall on the smooth {ital U}{sub eff}({ital J},0) curve based solely on the magnetization relaxation measurements. This indicates that the field dependence of magnetic critical current density in high-temperature superconductors is determined by the flux creep via nonlinear effective activation energy at low temperatures.

Critical current densities and activation energy of BiPbSrCaCuO tapes

The critical current density and the resistive transition curves of Ag-sheathed, textured (Bi/sub 1-x/Pb/sub x/)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ tapes have been studied in magnetic fields up to 12 T. The results can be described by the modified flux creep model proposed by E. Zeldov et al. (1990). It is shown that this model yields a simple explanation for the exponential field dependence of the critical current density in high-temperature superconductors. >

Interfaces in high-temperature superconductors

After the discovery of high-temperature superconductors (HTS) five years ago, it soon became apparent that their interfacial characteristics would play an extremely important role in any foreseeable applications of these materials. In recent commercial devices, the weak-link characteristics of grain boundaries (GBs) have in fact been exploited to manufacture Josephson junction SQUIDS. On the other hand, the low critical current density of HTS is a considerable limitation for practical applications of the zero-resistance property, particularly in bulk materials. The weak-link behavior of GBs is largely responsible for this, but other types of interfaces such as those formed by metallic contacts or the interfaces between the substrate and a HTS thin film are also critical to the application of these materials. We shall review here some of the important interface issues that have been addressed by TEM techniques, but shall focus largely on the connection between the critical current (Jc) that can be transported across a grain boundary and its atomic-scale structure and composition.