Dissipation in current-carrying high-T c superconducting ceramics in applied magnetic field

The dissipation mechanism in high-T _csuperconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.     

Intergranular fluxons in high-Tcsuperconductors

The granular high-T _c superconductors can carry very low transport current, in comparison to that found in the bulk of the material. Magnetization and critical current measurements at very low field indicate that this low transport current behaves as expected from a critical state model. The presence of weak links between the grains in granular aggregates is firmly established, together with the Josephson character of such links. The existence of some kind of magnetic particles, and of a mechanism of pinning for them, is required to explain the critical state regime. In this paper we examine the flux structures which can be present in the granular systems, which can be described by an array of SQUIDs, and we show their similarities to fluxons in a continuous medium and to fluxons in uniform Josephson junctions. A simple model, based on a two-dimensional network of pointlike Josepson junctions, is adequate to demonstrate the existance of the IF (intergranular fluxon). Its characteristics depend on a single parameter, which gives the coupling strength between grains. The discreteness of the system is the cause of an intrinsic pinning of the intergranular fluxons.     

Comparative Study on Transport and Magnetic <Emphasis Type="Italic">J</Emphasis><Subscript>c</Subscript> of Textured MgB<Subscript>2</Subscript> Thin Film

Critical current densities of c-axis-oriented MgB₂ thin film were measured comparatively by transport and magnetization measurements. Experimental results manifest that transport critical current densities J _ct are quite different from magnetic critical current densities J _cm in both quantity and field dependence. The scaling behaviors of the flux pinning force are also quite different. To explain the inequality of J _cm and J _ct, a microstructure examination has been performed. Based on our results, we believe that J _cm and J _ct correspond to different current paths and different pinning centers. More than two kinds of pinning center are effective in MgB₂ superconductors.     

Angular dependence of the current-voltage characteristics and voltage fluctuation spectrum in ceramic superconductors

Measurements were made of the current-voltage characteristics and voltage fluctuation spectrum as functions of the angle between the magnetic field applied in the plane of the sample and the direction of the transport current in a YBa₂Cu₃O_7−δ ceramic superconductor. The angular dependence of the fluctuation spectrum was measured first. It is shown that the current-voltage characteristics are not described using the vortex line cutting model and the voltage fluctuations are not caused by independent motion of vortices in the bulk of the sample. The results are consistent with the model of a self-organized critical state. Pis’ma Zh. Tekh. Fiz. 25, 8–13 (February 12, 1999)     

On the limiting factors of the critical current density in high-Tc superconducting ceramics

The temperature dependence of the critical current density at high temperatures and in weak applied magnetic field for YBa₂Cu₃O_7–y ceramic samples with a pronounced granular character is analyzed. The experimental results can be explained in terms of thermally activated motion of the intergrain Josephson vortices at grain boundaries, which may be an indication that the actual limiting factor for the critical current density in ceramic samples results from a weak pinning force density for the intergrain vortices rather than from the weak-link quenching.     

Field and temperature dependence of the current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes

The current-voltage characteristics of Bi₂Sr₂Ca₂Cu₃O₁₀/Ag multifilamentary tapes were measured at different temperatures close to the critical temperature and in different applied magnetic fields up to 1000 Oe. The data were interpreted in terms of thermally activated flux creep by using an intermediate phenomenological model that takes into account the collective pinning. Temperature and field dependences of the pinning potential and of the collective pinning exponent were determined through numerical analysis.     

Dynamic pinning model of electrical dissipation in the superconducting surface sheath

The dynamic pinning model of Yamafuji and Irie is adapted to the problem of electrical dissipation in the superconductive surface sheath. The free energy of the surface flux-spot lattice and its shear modulus—quantities required by the model—are calculated. The dc current-voltage characteristics of thick, Pb-In alloy films are analyzed in terms of an excess currentI _p due to pinning. The observed dependence ofI _p upon magnetic field is in good accord with the predictions of the model; the variations with temperature and alloy composition are made plausible as well.Work supported in part by Research Corporation.     

Asymmetry of the Pinning Force in Thin Nb Films in Parallel Magnetic Field

The pinning force, F _p, is studied in Nb films of different thickness in parallel magnetic field H. The asymmetry in the magnetic field dependence of F _p has been observed for two opposite directions of the transport current. The effect is less pronounced for thin and thick films where, respectively, single vortex pinning and pinning of the internal vortices, is relevant. At intermediate thickness, where the pinning mechanism is mostly caused by surface effects, an asymmetry in the F _p(H) dependence is clearly visible. The different surface barriers that vortices should overcome to enter the sample from opposite sides of the film explain the effect, as confirmed by numerical calculations. These have been obtained by solving the Ginzburg?CLandau equations with asymmetric boundary conditions which take into account the different superconducting properties of the film?Csubstrate and film?Cvacuum interface. Such difference can also explain the reduction of the critical current usually observed in thin films as a function of their thickness.     

“Peak effect” in the magnetic field dependence of the transport critical-current density in high-T c superconducting ceramics

The maximum observed in the magnetic field dependence of the transport critical-current density of high-T _c superconducting ceramics was analyzed. Transport critical current and magnetization measurements performed on Bi(Pb)-Sr-Ca-Cu-O bulk sintered samples allowed us to conclude that such a maximum may result from the influence of demagnetization effects at low applied field values and an increase of the intergrannular pinning at higher fields.     

Magnetic field orientation dependence of critical current in industrial Nb3Sn strands

In usual superconducting devices such as magnets for NMR, the magnetic field is perpendicular to the superconducting strand axis. But in some special devices, such as magnets for the toroidal field system of fusion machines, the strands can experience any field orientation. For NbTi strands, the pinning force is dependent on the field orientation because of the drawing process (Takacs, S., Polak, M. and Krempasky, L., Critical currents of NbTi tapes with differently oriented anisotropic defects, Cryogenics, 1983, 23, 153–159). In the case of Nb₃Sn strands, the draw and react process suggests that the pinning force is isotropic. In fact, preliminary experiments have shown the contrary, which is why the magnetic field orientation dependence of the critical current for two types of industrial Nb₃Sn strands has been measured. These measurements have been performed for seven field orientations at field strengths up to 20 T. A clear anisotropic effect has been observed, which cannot be explained by Kramer's pinning law. The results are in very good agreement with an empirical law proposed in a recent study by Takayasu et al. (Takayasu, M., Montgomery, D.B. and Minervini, J.V., Effect of magnetic field direction on the critical current of twisted multifilamentary superconducting wires, Inst. of Phys. Conf. Ser., 1997, 158, 917–920). The parameters to be used in this law could be specific to the manufacturing process.     

Pinning in superconductors with anisotropic defects

By calculating the volume pinning force in superconductors with anisotropic pinning centers we have shown that the critical current densitiesj _c in NbTi by both current and magnetization measurements can be explained. The critical current density in fields parallel toj _c can also be included. We conclude therefore that in superconductors with strong pinning centers the critical state is always determined by the equilibrium condition between the Lorentz force and the volume pinning force and not by the instability of the force-free configuration. By detailed investigation of thej _c dependence on field direction, useful information can be obtained with respect to the volume and surface contributions to the pinning force.     

Ion irradiation effects in EuBa2Cu3Oy thin film

The effect of He ion irradiation on the pinning potential in EuBa₂Cu₃O _y , thin film was investigated by measuring the temperature dependence of resistivity in magnetic fields. The pinning potential decreased as the ion fluence increased. A slower decrease of pinning potential was observed in higher magnetic field in the fluence region <3.5×10¹⁵ cm^–2.     

Vortex-induced rectification in type II superconductors

We report a large rectification effect in superconducting films in a parallel magnetic field. This rectification effect is manifested in two features in current-voltage characteristics: The critical current, I_c,is found to differ by as much as 40% for negative and positive currents, and beyond I_c,the magnitude of the voltage is different for positive and negative currents, ¦V(+I)¦ ¦V(–I)¦.Furthermore, the critical current difference ¦I_c+¦ – ¦I_c–¦shows complicated behavior, changing sign as temperature and magnetic field are varied. We discuss a model based on the Bean-Livingston surface barrier and inhomogeneous bulk pinning that accounts for all observed behavior.     

On the coupling current losses in a multifilamentary superconducting composite

The coupling current losses in a superconducting multifilamentary composite exposed to trapezoidally varying external magnetic field and carrying a small transport current are investigated for the volume pinning force density described with the dependence: F_v = α B^1?γ. Such a model allows estimation of the deviations from the solution based on Bean's (γ = 0) critical state model. Results indicate that there exists a region of small magnetic field amplitudes for which discrepancies are largest. They are also very sensitive to the rate of magnetic field change.     

Field, temperature, and angle dependence of the critical current density in Bi2Sr2CaCu2O10/Ag ribbons

Current-voltage characteristics of high-critical-current Bi₂Sr₂CaCu₂O₁₀/Ag ribbons were measured using both transport and magnetization techniques. The slope of these curves changes with magnetic field and temperature in a way very similar to the observedj _c (H, T) behavior. This correspondence between the critical current and the slope of theI–V characteristics can be explained within the thermally activated flux creep framework. The dependence ofj _c on the angle between field and ribbon is compared to the existing intrinsic anisotropy models.     

Effect of Micro-Phase Separation on Magnetic Response in Pr-Doping YBa2Cu3O7−δ Single Crystals

Pr-doping Y_0.928Pr_0.072Ba₂Cu₃O_7−δ superconducting single crystal was studied systemically by the magnetic response measurement below T _c . DC magnetization shows a possible existence of micro-phase separation caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The magnetic field dependence of critical current density J _c and pinning force density F _p had been obtained from the magnetization hysteresis loop. Flux pinning mechanisms in different magnetic fields had been discussed using the scaling function as well as the ratio of the pinning force field at H _on and H _peak. We find that the peak effect is influenced by the micro-phase separation which is caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The interface between the different T _c superconductivity areas can be the effective pinning center which is useful to the appearance of peak effect. The value of the ratio of the maximum pinning force field to the irreversibility field is discussed. The conclusion is consistent with the model constructed by Wen et al.; we finally draw out the thermomagnetic phase diagram for our samples.      

Anisotropy of flux flow in layered superconductor 2H-NbSe2−x S x

Magnetic-field dependence of the resistivity for various currents in the layered superconductor 2H-NbSe_1.8S_0.2 was investigated. The flux flow was observed for various angles between the magnetic-field direction and the crystal layers near zero-field critical temperature. Anisotropy of the pinning force and the viscosity coefficient was found. The peak effect was observed under a magnetic field perpendicular to the layers, while it was not observed under a magnetic field parallel to the layers. A strong pinning force was found acting only on flux lines parallel to the layers. These results can be explained by the pinning mechanism due to the stacking faults.     

Magnetic Field Dependence of Intergrain Pinning Potential in Bulk Granular Composites YBCO + CuO Demonstrating Large Magneto-Resistive Effect

The broadening of the resistive transition in magnetic field and isotherms of magnetoresistance of bulk composites Y–Ba–Cu–O + CuO have been studied. These composites exhibit large magneto-resistive effect in a wide temperature range below T _C due to weakening of Josephson coupling in this system. The broadening of the resistive transition and magnetoresistance are explained well by the Ambegaokar–Halperin (AH) model for phase slip in Josephson junctions. The magnetic field dependence of pinning potential in the intergrain boundaries deduced from AH model found out to be similar to that of critical current of an array of Josephson junctions. The values of pinning energy point out that the large magneto-resistive effect observed in the composites results from flux flow-like processes at the intergrain boundaries.