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Critical current properties in Bi-2223 superconductor were investigated in detail for silver-sheathed tapes fabricated by the usual process in a normal pressure, the previous controlled over-pressure (CT-OP) process and the recently improved CT-OP process at the final heat treatment. It was shown that the critical current density, the irreversibility field and the n-value were remarkably improved by application of the CT-OP process. The results were analysed using the flux creep-flow model to seek the reason for the improvements. It was found that the grain connectivity and the flux pinning strength were improved by the previous and recent CT-OP processes, respectively. The possibility of further improvements of the critical current properties in Bi-2223 tapes is discussed.

Effect of the over pressure processing on the distribution of the critical current density in mono-core tapes was measured in detail using Campbell's method. It was found that the magnetic flux profile is more straight in the over-pressured specimen, suggesting that the critical current density is more uniform. This seems to be partly caused by the improved uniformity of the thickness due to the disappearance of voids and the reduction in sausaging of superconducting layer by application of the over pressure. The possibility of the further improvement of the critical current density is discussed

The relationship between the condensation energy density and the anisotropy parameter, ?a, has been derived for Bi-2212 superconductors in various anisotropic states by analysing the critical current density due to columnar defects introduced by heavy ion irradiation. The critical current density depended on the size of the defects, determined by the kind and irradiation energy of the ions. A significantly large critical current density of 17.0?MA?cm?2 was obtained at 5?K and 0.1?T even for the defect density of a matching field of 1?T in a specimen irradiated with iodine ions. The dependence of the critical current density on the size of the defects agreed well with the prediction from the summation theory of pinning forces, and the condensation energy density could be obtained consistently from specimens irradiated with different ions. The condensation energy density obtained increased with decreasing ?a over the entire range of measurement temperature, and reached about 60% of the value for the most three-dimensional Y-123 observed by Civale et al at 5?K. This gives the reason for the very strong pinning in Bi-2212 superconductors at low temperatures. The thermodynamic critical field obtained decreased linearly with increasing temperature and extrapolated to zero at a certain characteristic temperature, T*, lower than the critical temperature, Tc. T*, which seems to be associated with the superconductivity in the block layers, was highest for the optimally doped specimen. This shows that the superconductivity becomes more inhomogeneous as the doped state of a superconductor deviates from the optimum condition.

The dependence of the superconducting layer thickness on the critical current properties was investigated in the range of 0.27–2.0 µm for YBCO coated tapes made by PLD processing on IBAD substrates. The critical current density at low fields in the direction of the c-axis was found to decrease with the increasing thickness d in proportion to d−1/2 up to 1 µm. This seems to agree with the prediction of the two-dimensional collective pinning of random point pins. However, this dependence did not change over a wide temperature range of 5–60 K and the critical thickness for the two-dimensional pinning was found to be much smaller than 1 µm. This result suggests that the observed thickness dependence does not come from the pinning mechanism but simply from the change of the superconducting layer structure with increasing thickness. The effect of flux creep on the critical current properties at high temperatures is also investigated, and the observed thickness dependences of irreversibility field and n-value are compared with the theoretical predictions of the flux creep-flow model.

The critical current density J c and the apparent pinning potential U 0 ∗ in a single crystal NdBa 2 Cu 3 O y superconductor which shows a broad peak effect are investigated by measuring a DC magnetization and its relaxation. The field-induced pinning mechanism does not explain the temperature dependence of peak field B p and dip field B d . The experimental results of J c and U 0 ∗ are compared with the theoretical analysis based on the flux creep-flow model, taking the distribution of the flux pinning strength into account. The number of flux lines in the flux bundle ( g 2 ), the most probable value of pinning strength ( A m ) and distribution width ( σ 2 ) are determined so that a good fit is obtained between the experimental and theoretical results. The behavior of these parameters is discussed in correspondence to the disorder transition of flux lines.

The critical current density was measured at low temperatures for Bi-2212 single crystals with small anisotropy prepared by oxygen doping and by addition of Pb. The anisotropy parameters of these specimens were about 66. The Pb-doped specimen showed the peak effect up to T / T c = 0.7. The critical current density at high fields of these specimens was significantly improved as well as the irreversibility field than specimens with larger anisotropy.

When an ac magnetic field of a suitable magnitude is applied locally normal to a wide surface of a superconducting thick film, a third harmonic voltage V3 is induced in a pick-up coil. It is predicted by the critical state model that V3 is proportional to the driving current amplitude I 0 2 when I0 is smaller than the threshold current Ith at which the shielding current in the film reaches the opposite side of the film. A numerical analysis using finite element method (FEM) is carried out for the V3–I0 characteristics for the film. It is found that V3 is proportional to I 0 2 only when I0 is smaller enough than Ith, and V3 takes a smaller value than predicted by the theory near I0 = Ith. The deviation between the theoretical prediction and the result of FEM is found to be attributed to the different distribution of the shielding current in the film.

The magnetic field angle dependence of J c was measured by four probe method and DC magnetization method. The latter measurement was used for the specimen before and after bending to break the intergrain current in order to investigate only the intragrain current. It was found that the intragrain critical current density could be increased by a factor 2.7 in a parallel magnetic field when the c -axis misorientation could be improved. From the field angle dependence of the intragrain critical current density, the anisotropy parameter was estimated as 20.4.

The effect of the thickness of Bi-2212 superconducting thin films and single crystals on the critical current property is investigated in the range of 0.1–10 µm. The peak effect of critical current density at low temperatures disappears for films thinner than the critical thickness in the range of 0.5–1.0 µm. On the other hand, the irreversibility field increases with increasing thickness over the entire range of investigation at low temperatures, suggesting the critical thickness for the irreversibility field is larger than 10 µm. This behaviour is different from that in three-dimensional RE-123 superconductors in which the two critical thicknesses are the same. While the critical thickness for the irreversibility field is given by the pinning correlation length in both types of superconductors, the critical thickness for the peak effect in Bi-2212 seems to be given by the interlayer coupling length. On the other hand, the irreversibility field in the same Bi-2212 specimens is found to decrease with increasing thickness at high temperatures, which is completely opposite to the dependence at low temperatures. This behaviour appears to contradict the theoretical prediction of the flux creep model. However, a detailed investigation reveals that this agrees with the theoretical prediction under the principle of irreversible thermodynamics which describes how the transverse order of flux lines is developed so as to reduce the energy dissipation due to the flux creep for extremely thin films.

The critical current property of a superconducting Bi-2223 tape produced by the over pressure processing at the final heat treatment was investigated. The critical current density increased by a factor of 1.6-1.8 at 77.3 K in comparison with the tape produced by the usual process. Thus, the critical current increased in spite of a reduction in the filament thickness. It was also found that the irreversibility field increased by a factor of 1.34 at 77.3 K by the new process. As a result, the enhancement of the critical current density at higher fields is remarkable. These results suggest that the over pressure process not only enriches the mass concentration of the superconducting phase but also contributes to the c-axis alignment. This is supported directly by the reduction of FWHM of rocking curve and indirectly by an enhancement the n value. The critical current property at lower temperatures is also investigated and the possibility of the further improvement of the property is discussed.

A significant overestimation of the critical current density Jc was discovered in a measuring method that utilizes induced third harmonic voltage for thin superconductors with a thickness less than 1 μm. This is speculated to be caused by the reversible motion of flux lines in the pinning potential, which reduces nonlinear properties such as hysteresis loss or third harmonic voltage. It is known that the effect of reversible flux motion is prominent for superconductors comparable to or smaller than the pinning correlation length at the sub-micrometer order called Campbell's AC penetration depth. In this paper, the effect of reversible flux motion on the estimation of Jc utilizing this method is investigated using a simplified Campbell's model for the force-displacement characteristics of flux lines, and the factor of overestimation of Jc is discussed.

Critical current density and irreversibility field were measured at various temperatures and magnetic fields for MgB 2 PIT tape specimens with different sheaths materials. The experimental results were compared with theoretical estimations using the flux creep-flow model. It is found that the hardness of sheath material indirectly affects the pinning property only through the packing density of MgB 2 . It is considered that the critical current density is mainly determined by a low value of distributed local critical current density determined by grain connectivity. On the other hand, the irreversibility field which is approximately the same among the three tapes is mainly determined by the average pinning strength.

When an AC magnetic field of a suitable magnitude by a small coil is applied locally to a surface of a superconducting coated conductor (CC) tape, a third harmonic voltage ( V 3 ) is induced in the coil. The critical current density ( J c ) of the film is evaluated from AC magnetic field dependence of V 3 . In this study, a minimum width of the film for evaluation of correct J c is numerically investigated using finite element method. It is found that the correct measurement can be done when the width is more than 1.2 times as large as the outer diameter of the coil. This shows that the third harmonic voltage method is useful for the J c measurement of narrow CC tapes.

The condensation energy which determines the pinning strength is estimated from the pinning property of the columnar defects for Bi-2212 superconductor in various doped states of oxygen. It is found that the condensation energy increases drastically with decreasing anisotropy parameter. In the low temperature region the condensation energy density of Bi-2212 increases abnormally with decreasing temperature and this suggests a drastic change in the superconductivity in the block layer.

The E – J characteristics were measured for a YBCO coated-conductor by using AC inductive method of third-harmonic voltage. The results are compared with the measurements obtained by the four probe method and DC relaxation method of magnetization. The results by three kinds of measurements meet approximately on a single curve and E – J curve in a wide range of electric field is obtained. Hence, the third-harmonic voltage method is useful for a continuous measurement of J c property of long coated tape. The obtained E – J characteristics are argued.

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.

The E–J characteristics of Bi-2223 silver-sheathed tape in a DC field normal to and parallel to the tape surface were measured in a wide range of electric field by a combination of the four probe method, Campbell's method for AC magnetic field with various frequencies and the relaxation of magnetization. These characteristics were different depending on the kind of measurements, and the difference was reasonably explained by the anisotropy of Jc with respect to the field angle and the filament sausaging. It was found that the experimental results when compensated for these effects agree with the theoretical analysis based on the flux creep-flow model.

When an AC magnetic field of a suitable magnitude is applied locally to a wide surface of a superconducting film, a third harmonic voltage is induced in a pick-up coil. This measurement is used for an evaluation of the critical current density of the film. Mawatari et al. analyzed the relationship between the third harmonic voltage amplitude (V3) and the driving current amplitude for the field (I0) by using the critical state model and a simple assumption that the magnetic field is locally parallel to a flat surface. In this study, this relationship is numerically analyzed by finite element method (FEM) for a condition similar to the experiment by Mawatari et al. This result is compared with the above theoretical and experimental results. Some deviations from theory are found. However, the scaling of V3–I0 curves holds and I0 at which V3 starts to appear is proportional to the critical current density.

It is known that the critical current density vs. field angle of an Y-123 coated tape has a broad peak around the field direction normal to the tape surface. In this work, the angular dependencies of the irreversibility field and the critical current density are analytically investigated using the flux creep-flow theory with an assumption of the angular dependence of flux pinning strength, and these results are compared with the experimental results. The angular dependence of flux pinning strength obtained so as to get a good agreement between the experimental and theoretical results suggests an existence of long pinning centers along the c-axis.

The E – J characteristics were measured for a YBCO-coated conductor by using the four probe method and the relaxation method of DC magnetization. The result on the latter measurement at extremely low electric fields was theoretically analyzed based on the flux creep-flow model. From the result on n -value the flux lines in YBCO are considered to be in the glass state even at high fields. The apparent pinning potential U 0 ∗ estimated from the relaxation of DC magnetization is much larger than that of Bi-2223. The present theoretical analysis clarifies a much stronger flux pinning and a higher dimensionality of YBCO in comparison with Bi-2223.

The critical current density and the irreversibility field were measured before and after nickel ion irradiation for Bi-2212 single crystals in an optimally doped or an overdoped state in a magnetic field parallel to the direction of irradiation. The critical current density was theoretically calculated by using the summation theory and the flux creep-flow model. From the comparison with experimental results the condensation energy density was estimated for each specimen. These results are compared with a similar analysis on three-dimensional Y-123.

Because of pressing global environmental challenges, focus has been placed on materials for efficient energy use, and this has triggered the search for nanostructural modification methods to improve performance. Achieving a high density of tunable-sized second-phase nanoparticles while ensuring the matrix remains intact is a long-sought goal. In this paper, we present an effective, scalable method to achieve this goal using metal organic deposition in a perovskite system REBa2Cu3O7 (rare earth (RE)) that enhances the superconducting properties to surpass that of previous achievements. We present two industrially compatible routes to tune the nanoparticle size by controlling diffusion during the nanoparticle formation stage by selecting the second-phase material and modulating the precursor composition spatially. Combining these routes leads to an extremely high density (8 × 1022 m−3) of small nanoparticles (7 nm) that increase critical currents and reduce detrimental effects of thermal fluctuations at all magnetic field strengths and temperatures. This method can be directly applied to other perovskite materials where nanoparticle addition is beneficial. An industrially compatible method for creating nanoparticles with controllable size has been demonstrated by a team in Japan and the USA. The thermoelectric and superconducting properties of a perovskite material can be improved by adding nanoparticles. But while a technique known as metal organic deposition enables high-performance materials to be grown, it is difficult to control nanoparticle size because it is determined by the diffusion of atoms. Now, Masashi Miura from the Seikei University/Los Alamos National Laboratory and co-workers have achieved a high density of tunable nanoparticles using metal organic deposition. They controlled diffusion by selecting the second-phase material and modulating the precursor composition. The team showed that BaHfO3 nanoparticles created in this way increased the critical current and reduced thermal fluctuations in a perovskite-composite cuprate superconductor. Nanostructural modifications, in particular nanoparticle (NP) additions, have been shown to have great success in improving energy-related material performance. We show how an economically viable method, namely metal organic deposition, can be used to obtain tunable small size (7 nm) and high number density NPs (8 × 1022 m−3) while maintaining the crystallinity of the perovskite-composite cuprate superconductor film matrix. Critical current density Jc(H) measurements demonstrate that the NPs are highly effective as pinning centers, decreasing vortex motion and fluctuation effects for all temperatures, magnetic field strengths and orientations measured. Our synthesis method can be applied to other perovskite-composite materials to improve their functionality.

The local and global critical current densities of polycrystalline bulks Sr 0.6 K 0.4 Fe 2 As 2 were investigated by both the remnant magnetic moment and Campbell’s methods. It was found that the global critical current density was improved by addition of Ag. It was speculated that the reason for the enhancement of the global critical current density was ascribed to the enlarged size of particle and the clean boundaries due to addition of Ag. It was also found that the critical current density estimated from usual magnetization measurement was a type of average value of both local and global critical current densities and can be explained by simple contributions from magnetic moments by local and global shielding currents. Therefore, the size of the grain was estimated by the present model and the agreement with the observation by SEM was obtained.

The magnetization loss density in stacked GdBa2Cu3O7−δ coated conductor was measured using a SQUID magnetometer in a magnetic field normal to the tape surface to investigate the effect of stack on the magnetization loss. The AC loss characteristics are successfully collapsed to a single curve by the normalization at all the measurement temperatures. The losses are similar at high magnetic fields regardless the number of tapes. In addition, the AC loss decreases with increasing the number of tapes in stack at low magnetic fields.

The recent discovery of superconductivity in iron arsenide compounds REFeAsO (RE = rare earth) or AFe 2 As 2 (A = alkaline earth) has attracted great attention due to the high T c that raised above 50 K. In this study, the effect of Ag addition (20 wt.%) on the critical current density J c of polycrystalline Sr 0.6 K 0.4 Fe 2 As 2 was investigated for improvement of the weak links in polycrystalline iron-based superconductors. Although effect of Ag addition on the intra-grain current density J c local was not confirmed, the connectivity among grains is speculated to be improved by filling voids among grains by Ag.

The critical current property of Bi-2223 silver-sheathed tape has been remarkably improved by application of ConTrolled Over Pressure (CT-OP) process with adjusted temperature and oxygen partial pressure. For further improvement of the property of Bi-2223 tapes, the effect of variation in the lead composition was investigated. It was found that the critical current density J c was higher and its decrease with increasing magnetic field was weaker in the tape with higher lead composition than other tapes in a parallel magnetic field. On the other hand, although J c of this tape was higher, its decrease with increasing magnetic field was stronger in a normal magnetic field. This seems to be attributed to the larger condensation energy density due to the carrier doping by lead and to the better crystal axis alignment.

The anisotropy of the critical current density was estimated for stoichiometric Bi 2 Sr 2 CaCu 2 O y (Bi-2212) and Bi 2.1 Sr 1.8 Ca 1.0 Cu 2.0 O y single crystals. For this purpose, the critical current density in the a–b plane J c ab and that along the c-axis J c c were discriminated from measurement of DC magnetic moments for specimens with different aspect ratio. The magnetic moment of specimen was measured by a SQUID magnetometer in a magnetic field parallel to the wide surface which is normal to the c-axis. Both of J c ab and J c c are larger in the stoichiometric specimen. In addition, the anisotropy, J c ab / J c c , is found to be significantly reduced in the stoichiometric specimen.

One of the most important factors which determine the critical current density in superconducting MgB 2 bulks and wires is the electrical connectivity and this can be improved by making the packing density of the superconductor high and by reducing the insulating oxide layers. In this study the flux pinning strength, which is another important factor, is investigated in detail. It is empirically known that the critical current density is proportional to the flux pinning force of grain boundaries and to the reciprocal size of MgB 2 grains. The flux pinning force of grain boundaries is estimated using the theoretical result of Yetter et al. for the electron scattering mechanism using the impurity parameter estimated from the residual resistivity of superconducting grains. The obtained result with the observed grain size approximately explains the experimental result and shows that the flux pinning by grain boundaries is dominant in MgB 2 . The potential for the high critical current density of MgB 2 is discussed by comparing with that of Nb 3 Sn.

Force-displacement characteristics of flux lines were measured using the Campbell method in the vicinity of the peak effect for an overdoped Bi-2212 single crystal. It was revealed that there are two quasi-stable states with different critical current densities, suggesting a phase transition of the first order. It was also found that the interaction distance increased discontinuously around the peak field, while the Labusch parameter showed no anomaly. This shows that the flux-pinning mechanism does not change across the peak field and that the peak effect is caused by the change in the property of flux lines. It is considered that the dimensional crossover of flux lines brings about the first-order transition and such characteristics. The elastic correlation length of flux lines along the c-axis was of the order of and agreed with the theoretical prediction. This shows that the flux lines are not in the complete two-dimensional state even above the peak field.

Broad double peaks of the critical current density were observed in an untwinned Y-123 single crystal prepared by a pulling technique. Force-displacement characteristics of flux lines were measured using the Campbell method in the vicinity of the peak fields. It was found that the interaction distance decreased monotonically in a wide range of the field, while the increasing rate of the Labusch parameter changed appreciably at the magnetic field at which the critical current density started to increase. These results are completely different from those in the case of Bi-2212. This fact seems to suggest that the peak effect is ascribed to the change in the flux-pinning mechanism but not to the change in the property of flux lines. Field-induced pinning by weakly superconducting regions seems to be a candidate for the responsible pinning mechanism. However, the characteristic field at which the critical current density starts to increase increases with elevating temperature. Hence, this temperature dependence cannot be explained by such a simple mechanism. The synchronization mechanism known in metallic superconductors may also work in Y-123.

Force-displacement characteristic of fluxoids in a superconducting Bi-2212 single crystal was measured by using the Campbell method at low temperatures and low magnetic fields where the peak effect of critical current density was observed. It is found that the interaction distance, d i , has a peak around the peak field while the Labusch parameter, α L , increases monotonously with increasing magnetic field. The peak of d i means an increment of flexibility of fluxoids to randomly distributed pins and this is considered to be brought about by some deformation of fluxoid cores along the length. Although this may be associated with the cross-over from three-dimensional flux lines to two-dimensional pancakes, the characteristic length of deformation is considered to be much longer than the distance between the CuO 2 planes.

The correlation length of flux lines responsible for the scaling of current-voltage curves in the transition region between the glass and liquid states in high temperature superconductors is argued based on experimental results on various specimens of Bi-based superconductors. From the correspondence between the temperature dependence of the pinning correlation length and the static critical index, v , it is concluded that the responsible correlation length is the pinning correlation length. This insists that the pinning interaction governs the flux lines in the glass state. This hypothesis is supported by the fact that the transition line depends on the flux pinning strength and the dynamic critical index, z , is appreciably influenced by the width of the distribution of flux pinning strength. It is concluded that the so-called vortex glas-liquid transition is a kind of transition of flux lines between the glass state governed by the pinning and the depinned melt state due to the thermal activation.

The current-voltage curves are measured for a dip-coated Bi-2212 tape wire at various temperatures under the magnetic field parallel to the c-axis. It is found that the current-voltage curves are approximately scaled on two master curves by normalizing as predicted in the vortex glass-liquid transition theory. However, the obtained dynamic critical index was about 3 and too small in spite of the two-dimensional flux line system. These results are compared with the theoretical analysis based on the flux creep-flow model taking account of the distribution of pinning strength. It is found that the theoretical result approximately explains the experimental result on the scaling behavior, the critical indices and the transition line.

The longitudinal elastic correlation length, l44, of flux lines in a superconducting Bi-2212 single crystal in the low field region near the peak effect is directly measured using the Campbell method under the magnetic field parallel to the c-axis. It is found that l44 is longer than several μm even in this field region similarly to the previous measurement at much high fields. The obtained result agrees well with the pinning correlation length estimated using the measured critical current density, Jc. The apparent pinning potential estimated from the relaxation rate of magnetization has a peak near the peak field and can be explained well by the collective flux creep theory in which the obtained Jc and l44 are used. It is concluded from these results that the flux lines are strongly coupled over a fairly long distance along the c-axis even in the vicinity of peak field. Discussion is given on the correlation with the observed l44 and the local distortions of flux lines which are speculated to be the reason for the peak effect.

The current-voltage characteristics are measured for two Bi-2223 tape specimens with different critical current densities at various temperatures under the magnetic field parallel to the c -axis. It is found that the current-voltage curves are scaled on two master curves by normalizing as predicted in the vortex glass-liquid transition theory. However, the obtained dynamic critical indices increased, and the static critical indices decreased appreciably with increasing magnetic field. These experimental results are compared with the numerical calculation based on the flux creep and flow model, taking the distribution of the flux pinning strength into account. Agreement is obtained between the experimental and theoretical results not only on the scaling curve but also on the critical indices and the transition line. The static critical index showing a divergence of the correlation length near the transition temperature can also be explained from the temperature dependence of the pinning correlation length. These results suggest that the scaling behavior is not correlated to the phase transition of fluxoids assumed in the vortex glass-liquid transition theory but seems to be correlated to a kind of transition governed by the flux pinning.

The longitudinal elastic correlation length, l 44 , of flux lines along the c -axis is measured using the Campbell method with the magnetic field parallel to the c -axis for a superconducting Bi-2212 single crystal. It is found that l 44 is of the order of 10μm and four orders of magnitude longer than the distance between the CuO 2 planes. In addition, l 44 increases with increasing magnetic field and/or temperature. These results agree well with the theoretical estimate of the pinning correlation length using the observed critical current density. The apparent pinning potential associated with the magnetic relaxation rate is calculated using these results and compared with the experimental result. Agreement is satisfactory. It is also found that the transverse flux-bundle size is as short as the flux-line spacing. It is concluded from these results that the flux lines are rather one-dimensional and strongly coupled along the c -axis.

The pinning-force density and the irreversibility field are measured for Bi-2223 tape wires at various temperatures under the magnetic fields normal and parallel to the c -axis. It is found that the scaling laws of pinning-force density in the low- and high-temperature regions and the temperature dependence of the irreversibility field are formally the same between the two field directions in spite of some anisotropy factor, suggesting that the pinning mechanism is essentially the same for the two field directions. These results are compared with the numerical calculation based on the flux-creep model. The experimental results are well explained by the theory including the anisotropic property. The pinning potential estimated from the flux-creep theory at 80 K and 1.0 T is of the order of several 10 meV and the factor of the anisotropy is about 1.3. The value of the pinning potential under the magnetic field along the c -axis suggests that the flux lines are strongly connected along this direction. The longitudinal elastic correlation length of the flux lines is of the order of several μm and this supports the hypothesis. The pinning characteristics of a Bi-2223 tape wire to which the strong pinning centers are successfully introduced are discussed using the flux-creep theory for evaluating the potential of this material for application. The irreversibility field for the direction parallel to the c -axis is estimated to be sufficiently high even at 77.3 K and it is concluded that Bi-2223 has a sufficient potential for application.

The irreversibility line was measured for three silver-sheathed Bi-2223 tape wires with different critical current densities. It was found that the irreversibility line was shifted to a higher temperature region according to an increasing critical current density. The temperature dependence of the irreversibility line was divided into two regions. In the lower temperature region the irreversibility line strongly depends on the temperature, while the dependence at higher temperatures is weaker and is similar to that observed for Y-Ba-Cu-O. Such a difference is considered to be due to different pinning mechanisms. Nonsuperconducting particles observed microscopically are expected to be responsible for a pinning interaction, contributing to the enhancement of the irreversibility line at high temperatures. This possibility is clarified by comparison of the observed irreversibility line with theoretical prediction.

The authors proposed an innovative superconducting dc power cable using the longitudinal magnetic field effect, i.e., a significant enhancement of the critical current density of a superconductor in a parallel magnetic field. It was expected that a cable with a high current-carrying capacity could be realized with REBa 2 Cu 3 O 7-σ (REBCO) coated conductors. However, the critical current density in most commercial coated conductors does not increase but slightly decreases with increasing parallel magnetic field. Nevertheless, the critical current density in the parallel magnetic field is remarkably higher than that in a normal in-plane magnetic field, and it is possible to construct a dc cable with a higher current-carrying capacity using this characteristic in comparison with conventional superconducting cables. In this paper, we propose a new design of dc power cable suitable for present commercial coated conductors. The optimal condition of the cable is discussed.

It was theoretically shown by the authors that the current-carrying capacity of superconducting dc power cable can be enhanced by choosing a force-free configuration under a parallel magnetic field produced by the current flowing back in the outer shielding conductor. This was experimentally checked for a single layer cable using Bi-2223 tapes in an applied parallel magnetic field. It was found that the current-carrying capacity took on a peak value under the force-free condition for the total magnetic field, including the self-field. This shows that the proposed structure is suitable for practical dc power transmission. The possibility of the innovative dc superconducting power cable with multi-layers with higher current-carrying capacity is discussed.

The scaling of current-voltage curves is investigated and the irreversibility field and the vortex glass-liquid transition field are obtained for two specimens of multilayered Bi-2212 superconducting tapes with different thicknesses of superconducting layer. The results were compared with the theoretical analysis using the flux creep-flow model in which the distribution of flux pinning strength was taken into account. A good agreement was obtained, although the dependence of the irreversibility field on the thickness of superconducting layer was not obtained since the longitudinal flux bundle size was still shorter than the thickness of superconducting layers, resulting in bulk pinning. This agreement shows that the present, theoretical model describes correctly the phenomena associated with the motion of flux lines under the influence of flux pinning.

The longitudinal and transverse flux bundle sizes are estimated for a three-dimensional single crystal Y-123 superconductor under a magnetic field along the c-axis and the result is compared with previous results of Bi-2212 and Bi-2223. It is considered that the longitudinal flux bundle size is determined only by the flux pinning strength independently of the dimensionality of superconductor. The number of flux lines in the flux bundle,g 2 , which is related to the transverse flux bundle size, is estimated to be larger than 8. This number is much larger than those in Bi-2212 and Bi-2223.

The irreversibility field was observed for two multilayered Bi-2212 tape wires with different thicknesses of superconducting layers. The irreversibility field does not depend on the thickness of superconducting layers but is determined only by the flux pinning strength. The result agrees well with the theoretical analysis of the flux creep-flow model. The analysis shows that the longitudinal flux bundle size is of the order of a few µm on the irreversibility line. The present experimental result is reasonable, since the flux bundle size is shorter than the thickness of superconducting layers, resulting in the three-dimensional pinning.

Flux bundle size is estimated for a three-dimensional bulk Y-123 superconductor prepared by a quench and melt growth process and the result is compared with previously estimated result for a two-dimensional single crystal Bi-2212 superconductor. The longitudinal elastic correlation length along the c-axis, l 44, which was considered to correspond to the longitudinal flux bundle size, was directly measured using the Campbell method. It was shown that this length depends only on the flux pinning strength and is independent of the dimensionality of the superconductor. On the other hand, it was found that the transverse flux bundle size estimated from the observed relaxation rate and l 44 depends on the dimensionality and is likely to take a larger value for a superconductor with higher dimensionality. That is, the typical number of flux lines in the flux bundle is 1 for Bi-2212 and 5 for Y-123. This difference causes a large difference in the pinning potential. This speculation is quite different from the image drawn in the pancake vortex model.

Force-displacement characteristic of flux lines in a superconducting Bi-2212 single crystal was measured using the Campbell method at low temperatures and low magnetic fields where the peak effect of the critical current density was observed. It was found that the interaction distance, d i, had a peak around the peak field, while the Labusch parameter, α L, increased monotonously with increasing magnetic field. The peak of d i means an increment of the reversible limit of motion of pinned flux lines. This seems to be caused by an introduction of some disorder in the flux line lattice at the so-called 3-dimensional to 2-dimensional transition field. However, the characteristic length of the deformation along the c-axis is considered to be much longer than the distance between the CuO2 planes.

It is known that the irreversibility line and the transition line between the so-called glass and liquid states of flux lines are different, although those are closely correlated to each other. This difference is speculated to be associated with the distribution width of the flux pinning strength. The two characteristic lines are analyzed as a function of the distribution width of the flux pinning strength using the flux creep-flow model for a Bi-2212 superconducting tape. Discussion is given on the difference between the two characteristic lines.

The distribution of pinning strength is estimated from measured current-voltage curves using the Weibull function for a Bi-2223 silver-sheathed tape wire under a magnetic field parallel to the c-axis. The obtained result is compared with a simple distribution function assumed in the analysis of the scaling of current-voltage curves using the flux creep-flow model. It is found that the simple distribution function describes well. The wide distribution seems to be caused by percolating current paths due to weak lines.

It has been recognized that BaHfO3 (BHO) pinning centers introduced into GdBa2Cu3O6 +y (GdBCO) coated conductors show high pinning performance at high magnetic fields. This pinning property was ascribed to the strong pinning force of the BHO nano-rods and also to the improved upper critical field, Bc2, probably due to the scattering of electrons by the interface between the nano-rods and the superconducting matrix. In this paper the critical current properties of GdBCO coated conductors with different amounts of BHO nano-rods are theoretically investigated. It is found that BHO nano-rods are promising to improve the high field performance of GdBCO coated conductors.