The Critical State in Type-II Superconductors with Cross-Flow Effects

A theoretical framework is presented, which allows to explain many experimental facts related to pinning and cross-flow effects between flux tubes in type-II superconductors. It is shown that critical state principles, in the manner introduced by C. P. Bean for parallel vortex lattices, may be used to describe the observed behavior. We formulate a least action principle, giving place to a variational interpretation of the critical state. The coarse-grained electrodynamic response of the superconductor is solved by minimizing the magnetic field changes, for a current density vector constrained to belong to some bounded set . It is shown that the selection of determines the specific critical state model in use. Meaningful choices of are discussed in view of the related physical mechanisms.     

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.     

Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors

No Heading We discuss the analysis of mixed-state magnetization data of type-II superconductors using a recently developed scaling procedure. It is based on the fact that, if the Ginzburg-Landau parameter does not depend on temperature, the magnetic susceptibility (H, T) is a universal function of H/H_c2(T), leading to a simple relation between magnetizations at different temperatures. Although this scaling procedure does not provide absolute values of the upper critical field H_c2(T), its temperature variation can be established rather accurately. This provides an opportunity to validate theoretical models that are usually employed for the evaluation of H_c2(T) from equilibrium magnetization data. In the second part of the paper we apply this scaling procedure for a discussion of the notorious first order phase transition in the mixed state of high-Tc superconductors. Our analysis, based on experimental magnetization data available in the literature, shows that the shift of the magnetization accross the transition may adopt either sign, depending on the particular chosen sample. We argue that this observation is inconsistent with the interpretation that this transition always represents the melting transition of the vortex lattice.PACS numbers: 74.60.-w, 74.60.Ec, 74.60.Ge, 74.72.-h     

Self-Organization of the Critical State in Granular Superconductors

We study the critical state of a one-dimensional multijunction SQUID with a random arrangement of junctions in an increasing magnetic field. Using two mathematical models (system of differential equations for gauge-invariant phase differences and a simplified algorithm), we show that the system demonstrates a self-organized behavior. An intrinsic spatial randomness introduced into the model allows to obtain self-organization in one-dimensional case under fully deterministic perturbation. We also show that our simplified algorithm represents a new model of a self-organized criticality.     

Critical Currents of High-Temperature Superconductors in Homogeneous Magnetic Fields

In this paper we report on the measurements of the critical current of a tape-formed high-temperature superconductor in a magnetic field corresponding to the field at different positions in a (transformer, reactor or magnet) winding. An applied magnetic field reduced the perpendicular component of the self-field of the sample. The measurements were performed in liquid nitrogen on a silver-sheathed Bi-2223 conductor. The results show a significant increase of the critical current with reduction of the perpendicular component of the self-field.     

Retardation of the Magnetic Relaxation in High-Temperature Superconductors Near a Ferromagnet

The creep of a magnetic flux trapped in a bulk high-temperature superconductor has been studied. It has been found that the magnetic relaxation is retarded when the superconductor is placed near a ferromagnet. The value of the retardation effect depends on the sequence of magnetization and the approach of the superconductor to a ferromagnet. The magnetic relaxation is fully suppressed when a superconducting sample first is magnetized and then is brought close to a ferromagnet. An interpretation of this effect has been discussed. Being magnetized, a ferromagnet produces its own magnetic field. While penetrating into a disk sample through its planes, the ferromagnet field induces screening currents, which circulate oppositely to the current that arises upon trapping of the magnetic flux. As a result, the stability of the magnetic structure is sharply improved since opposite driving forces can act on different sections of the vortices.     

Creep of Driven Flux Lines in Type-II Superconductors

The creep motions of flux lines in both Bragg glass (BrG) and vortex glass (VG) are studied. The power-law dependence of the creep activation barrier on the driving force is obtained. Two universal classes of creep motion are found with the exponent μ=0.5±0.02 for the BrG and μ=0.28±0.02 for the VG. The former is in good agreement with the prediction by the scaling theory and the functional-renormalization-group theory on creep, while the latter is a new estimate. The different dynamics of flux lines in the VG and in the BrG are investigated accordingly.     

Secondary Peak on Asymmetric Magnetization Loop of Type-II Superconductors

Asymmetric magnetization loops with a second peak effect were parameterized by the extended critical-state model. The magnetic field distribution in a sample is considered. An expression is suggested for a peak of the critical current density and corresponding depression on field dependence of the depth of surface layer with equilibrium magnetization. These functions determine the width and the asymmetry of a magnetization loop. The asymmetry of the secondary peak height on magnetization branches for increasing and decreasing field is reproduced on the computed magnetization curves.     

A Statistical Mechanical Model of Critical Currents in Superconductors

We present a statistical mechanical model for critical currents which is successful in describing both the in-plane and out-of-plane magnetic field angle dependence of J _c . This model is constructed using the principle of maximum entropy, that is, by maximizing the information entropy of a distribution, subject to constraints. We show the same approach gives commonly assumed forms for J _c (B) and J _c (T). An expression for two or more variables, e.g. J _c (B,T), therefore, follows the laws of probability for a joint distribution. This gives a useful way to generate predictions for J _c (B,T) for the DC critical current in wires, cables or coils.     

Topological Electronic Transitions in Vortex Cores in Type-II Superconductors

On the basis of microscopic theory, we study topological transitions in quasiparticle spectra of vortex systems, governed by an external magnetic field and transport current. We analyze two generic examples of such transitions: (i) opening of Fermi surface segments corresponding to the creation of a vortex-antivortex pairs; (ii) merging of different Fermi surface segments via the Landau–Zener tunneling. The basic properties of vortex matter such as pinning and transport characteristics, heat transport in the vortex state and peculiarities of the local density of states are strongly affected by these changes in the Fermi surface topology.     

Vortex-Antivortex Annihilation and Flux Turbulence in Layered Type-II Superconductors

New mechanism of turbulence instability of standing flux-antiflux front in layered superconductors is presented. We describe two assisting mechanisms destabilizing the standing vortex-antivortex front. There are anisotropy of the layered superconductors and the heat, released by the vortex-antivortex annihilation. We present the conditions of the front stability for various anisotropy and heating parameters. We predict that heat released by the vortex-antivortex annihilations enhances the turbulence instability in superconductors with small anisotropy. The characteristic size of the unstable pattern is estimated. The results are in a good agreement with recent experiments.     

Time Decay of the Nonlinear Microwave Response of Superconductors in the Critical State

We report experimental results on the second-order response of superconductors in the critical state exposed to intense pulsed microwave (mw) fields. The second-harmonic power radiated by the samples is investigated during the time interval in which the mw pulse endures, for mw pulses 1 ms long. The second-harmonic signal exhibits a time decay whose features depend on the investigated sample, the input power level, and the way in which the value of the static magnetic field has been reached. The transient is ascribed to processes of magnetic flux redistribution, which come into play when fluxons in the critical state are driven by electromagnetic (em) fields of high frequency.     

Spontaneous Generation of Magnetic Flux in Superconductors: Testing the Model

The Kibble-Zurek scenario of the generation of topological defects, applied to superconductors, predicts the spontaneous formation of magnetic flux during a rapid quench through T^c. The predicted amount of net flux scales as the 1/8 power of the cooling rate, and as the square root of the circumference of the sample. Our experimental data is broadly consistent with the first prediction, while the second one is yet to be tested. We describe a proposed experiment to test this prediction. In addition, we describe additional observations of spontaneous flux generated through a different mechanism. PACS numbers: 05.70.Ln, 74.40.+k, 11.27.+d.     

Effect of Magnetic Inclusions on the Effective Magnetostriction of Bulk Superconductors

A simple model is presented based on the Kim–Anderson model to further investigate the dependence of the effective magnetostriction of magnetic inclusion-superconducting matrix system on both the elastic and magnetic parameters including the elastic modulus, permeability, and volume fraction. The effect of the permeability on the magnetostriction is also obtained by implementing the continuity conditions of displacement and strain at the interface between the inclusion and the matrix through the magnetostriction loop. The results indicate that a stiffer inclusion can decrease the effective magnetostriction no matter whether the inclusion is magnetic or not and a larger effective magnetostriction can be obtained by choosing the matrix with a higher permeability, which gives an explanation about why the composite made from a matrix with a high permeability but a negligibly small magnetostriction yields unexpectedly low magnetostriction. Of particular interest is that in a certain range the effective magnetostriction of composites can be enhanced until it is saturated by increasing the permeability of matrix.     

Effect of the External Magnetic Field on the Resonance Scattering in Cuprate Superconductors

Within the kinetic energy driven superconducting mechanism, the effect of a uniform external magnetic field on the resonance scattering in cuprate superconductors in the superconducting state is studied. It is shown that the commensurate resonance scattering at zero external magnetic field is induced into the incommensurate resonance scattering by applying a large enough external magnetic field. The part of the spin excitation dispersion seems to be an hourglass-like dispersion, which breaks down at the heavily low energy regime.     

The Effect of Dy Doping on the Magnetic Behavior of YBCO Superconductors

The effect of dysprosium (Dy) doping on yttrium barium copper oxide (YBCO) prepared by conventional solid-state reaction method has been investigated by means of XRD, AC susceptibility, and DC magnetization measurements. AC susceptibility measurements for sintered YBCO pellets have been performed as a function of temperatures at constant frequency and AC field amplitude in the absence of a DC bias field. DC magnetization measurements were done at 5, 20, and 77 K upon zero field cooling (ZFC) process. The magnetization measurements showed a paramagnetic behavior existing at high magnetic fields. The magnetic field dependence of critical current density of the samples has been estimated from DC magnetization data. The partial Dy substitution for Y on YBCO superconductors improves the bulk critical current density at high magnetic fields and at high-temperature regions (higher than 20 K).