Edmund Soji Otabe

Critical Current Characteristics in Superconducting Y-Ba-Cu-O Prepared by the Melt Process

Critical current densities in superconducting Y-Ba-Cu-O specimens with different sizes of 211 particles prepared by the melt process were measured under various magnetic fields and temperatures. Discussion is given on the flux pinning properties by 211 particles and background pinning centers. At temperatures above 60 K, 211 particles are expected to be dominant pinning centers. A rapid degradation of the critical current density with elevating temperature at low temperatures below 25 K seems to be attributed to weak links in specimens.

DC susceptibility of type-II superconductors in field-cooled processes

Abstract The DC susceptibility of oxide superconducting specimens in the field-cooled process has been experimentally found to depend not only on the applied DC magnetic field but also on the size of the specimens. The DC susceptibility is calculated using the critical state model in which the diamagnetism and the flux-pinning effect of superconductors are taken into account. It is shown that the saturated value of the DC susceptibility at sufficiently low temperatures, i.e., the so-called Meissner fraction, decreases with increasing DC field and/or increasing specimen size.

Effect of reversible fluxoid motion on AC susceptibility of high temperature superconductors

Abstract It has been found in measurements of AC susceptibility versus temperature for high temperature superconductors that, with decreasing AC magnetic field amplitude, the peak value of the imaginary part, χ decreases remarkably accompanied by a shift of its position to higher temperatures, while the height of the transition of the real part, χ does not change appreciably. This behavior contradicts the prediction by the critical state model in which the flux pinning phenomenon is assumed to be completely irreversible. It is shown that this anomalous magnetic behavior originates from reversible fluxoid motion in pinning potentials. The AC susceptibility is calculated on the basis of the Campbell model for the reversible fluxoid motion. The result obtainained warns us against incorrect use of the critical state model for analysis of experimental data.

Flux Creep in a Sinusoidal Washboard Potential in Superconductors

Flux creep is theoretically investigated for a model of a sinusoidal washboard potential in superconductors. An apparent pinning potential depth, U0*, defined by the relaxation rate of the shielding current density, is found to be significantly smaller than a given pinning potential depth. In particular, U0* reduces to zero at zero temperature, while the given pinning potential depth takes a finite value. The effect of the form of the potential barrier on such a significant flux creep at low temperatures is discussed.

History effect of critical current density and weak links in superconducting BiPbSrCaCuO tape wires

Abstract Critical current density and its history effect in Bi-based high Tc superconducting tapes were measured using an a.c. inductive method with various magnetic fields and temperatures. A rapid decrease in the critical current density with increasing temperature and the history effect were observed in the same low temperature region between 4.2 and 30 K. This suggests that the two phenomena are caused by low temperature weak links in the specimens. A weak link region is considered to lose superconductivity at temperatures higher than 30 K. Above 30 K, a strongly bonded area retains superconductivity and the Jc changes slowly with increasing temperature, showing no history effect.

Irreversibility line in superconducting Bi-2212 single grain with fine normal particles

Abstract The irreversibility line was obtained in terms of the imaginary a.c. susceptibility measurement for a melt-processed superconducting Bi-2212 single grain with fine non-superconducting particles. This line showed different temperature dependences in high and low temperature regions. The one in the low temperature region agrees approximately with the prediction from the flux creep theory with experimentally observed pinning parameters. On the other hand, the one in the high temperature region is close to the well known characteristics in Y-based superconductors, suggesting flux pinning by boundaries such as the surfaces of non-superconducting particles. A quantitative discussion is also given on flux bundle size.

Critical current density in superconducting Nb-Ti under proximity effect

Abstract It has been found that the critical current density Jc in a multifilamentary superconducting Nb-Ti wire first increases and then reaches a peak, while the critical temperature decreases monotonically as the superconducting filament diameter is decreased by drawing to the submicrometre range. This behaviour originates from a variation in the morphology of the normal α-Ti phase precipitated during heat treatment, coupled with the proximity effect between the superconducting matrix and normal α-Ti. In this paper the elementary pinning force fp of α-Ti ribbons is theoretically estimated for idealized multilayers of superconducting and normal regions, and Jc is calculated from a statistical summation of the elementary pinning interactions. The theoretical result predicts that Jc increases monotonically with drawing, owing to the increase in density of pin points, in spite of the decrease in fp. The reason for the observed degradation of Jc is discussed.

Innovative superconducting force-free cable concept

An innovative superconducting DC power cable using high-temperature superconducting wires is introduced. The new cable design is proposed to utilize the longitudinal magnetic field effect that was discovered about a half-century ago. It was shown that the critical current density of a superconducting wire could be significantly increased when the superconductor is in a parallel field configuration. It is empirically known that a force-free structure is attained for the current flowing in the superconductor in this case. In the new cable, the outer layer is twisted in such a way that the shielding current flowing in this region generates a magnetic field that is parallel to the inner conductor. As a result, a force-free structure is realized for the wiring in the inner conductor. It is shown that the current carrying capacity of the proposed cable is dramatically improved in comparison with other cables and that the improvement becomes more pronounced with the increase of the current level.

The pinning property of Bi-2212 single crystals with columnar defects

It is qualitatively understood that the condensation energy density in oxide superconductors, which is one of the essential parameters for determining their pinning strength, becomes large with increasing dimensionality of the superconductor. However, the condensation energy density has not yet been evaluated quantitatively. Its value can be estimated from the elementary pinning force of a known defect. Columnar defects created by heavy ion irradiation are candidates for being such defects. That is, the size and number density of columnar defects can be given. In addition, it is known that two-dimensional vortices like those in Bi-2212 are forced into three-dimensional states by these defects in a magnetic field parallel to the defects. Thus, the condensation energy density can be estimated from the pinning property of the columnar defects even for two-dimensional superconductors. A similar analysis was performed also for three-dimensional Y-123. A discussion is given of the relationship between the condensation energy density and the anisotropy parameter estimated from measurements of anisotropic resistivity and peak field.

Size dependencies of the peak effect and irreversibility field in superconducting Sm-123 powders

Size dependencies of the peak effect and irreversibility field in superconducting Sm-123 powder specimens were investigated. The peak effect was found to disappear in the two-dimensional pinning regime in which powder size is smaller than pinning correlation length. This shows that the peak effect does not originate directly from elementary pinning mechanisms, but from a cooperative phenomenon of flux lines, as proposed by the pinning-induced disorder transition model. In addition, when powder size is smaller than virtual pinning correlation length in the ideal creep-free case, irreversibility field decreases with powder size. This supports the hypothesis that longitudinal flux bundle size is given by this length, as predicted by flux creep theory. These findings indicate that pinning correlation length is a general parameter that determines various properties associated with flux pinning phenomena.