Superconductor flux pinning

Frozen current vortices (FCV) in superconductors can serve as essential elements of magnetic recording and as a convenient subject for investigations magnetic flux pinning at a transport current / across the superconductor, annihilation of two FCV with opposite magnetic flux, quantum behavior of frozen magnetic flux in the FCV, collective flow of FCV under the influence of the transport current I (Fig.5a). 

Johansen, T.H. (2000) Flux-pinning-induced stress and magnetostriction in bulk superconductors, Supercond. Sci. Technol. 13, R. 121-37. 

Further examples of positron study of defects in HTSC are studies carried out to understand the nature of flux-pinning defects that lead to an increase in critical-current density on neutron-irradiated Y 1 2 3. Experiments [59] on positron lifetime and critical-current density measurements on various neutron-irradiated samples of Y 1 2 3 indicate that the critical current density is correlated with the micro-void density, as obtained from the analysis of positron lifetime measurements. Investigation of defects in other HTSC superconductors, such as La-Sr-Ca-Cu-0 [60], Bi-Sr-Ca-Cu-0 [49], and Nd-Ce-Cu-O [52], have also been carried out. 

Magnetic flux pinning effect of impurities in copper oxide superconductors... 

Murakami, M., Flux pinning of melt textured processed YBCO superconductors and their applications, in Studies of High Temperature Superconductors, Vol. 9 (A. V. Narlikar, ed.). Nova Science, New York, 1991, pp. 1. 

Intern. Discussion Meeting on Flux Pinning in Superconductors, Akademie der Wissenschaften, Gottingen, West Germany (1975). 

A. DasGupta, L. Schultz, H. C. Freyhardt, and P. Haasen, in Proceedings Intern. Discussion Meeting on Flux Pinning in Superconductors, Akademie der Wissenschaften, Gottingen, Germany (1975), p. 281. 

Ovchinnikov (1974, 1979) proposed the collective pinning theory, which introduced certain collective actions of several defects over a certain correlation length or volume for the first time, a theory that represents the basis for most flux-pinning considerations in high-temperature superconductors because of their generally weak pinning potentials. 

Fig. 5. Flux pinning in superconductors (a) strong pinning through the core interaction by normal conducting precipitates of various shapes (b) schematic view of the pinning potentials without (top panel) and under the action of a Lorentz force (bottom panel) (c) schematic view of pinning of a single flux line by the collective action of many small point pinning centres (after Ullmaier 1975).

In conclusion, as pointed out in the introduction, the results to be presented in sects. 5—8 are not aimed at a detailed discussion of these fundamental aspects of flux pinning and flux dynamics, but rather at possibilities to improve 7c in the 123 superconductors. We must be aware of the fact, however, that the Jc s quoted in the literature often do not refer to the true critical current densities, since the data are affected by creep and relaxation, and should rather be quoted as shielding current densities 7s- Furthermore, we will refiain from judgements of the nature of the boundary line, where 7s goes to zero, and will refer to it as the irreversibility line. This characteristic parameter is subject to the same restrictions as mentioned above i.e., sensitive to the resolution of the experiment and its time scale. It is useful to note that the shape of this curve (// c) follows a power law (with an exponent of 1.5) for more three-dimensional HTS, as predicted by Malozemoff et al. (1988) on the basis of a depiiming argument, but an exponential law for two-dimensional systems, as shown schematically in fig. 6. 

Next, we have to address the question of whether or not these precipitates are directly responsible for flux pinning and, if so, through which mechanism. Normal-conducting precipitates are, of course, the first choice for strong pinning centres (cf sect 3), but their size should be comparable to the coherence length. From earlier work on low-temperature superconductors (Coote et al. 1972), we know, however, that sharp interfaces between... 

Fig. 43. Effects of heavy-ion irradiation under different angles on flux pinning in Y-I23 superconductors (a) hysteresis loops at (top) 5 K and (bottom) 70 K of an Y-123 single crystal irradiated with 580 MeV Sn ions under an angle of 30° with respect to the c-axis and measured with the field at +30° or -30° from the c-axis (cf. the schematic of the field and track orientations, from Civale 1997) (b) angular dependence of the transport critical current densities in Y-123 thin films irradiated under various angles with respect to the c-axis (top parallel to the c-axis, 340 MeV Xe middle 30°, 770 MeV Pb bottom 60°, 340 MeV Xe from Kraus et al. 1994b) (c) angular dependence of the transport critical current densities in an Y-123/Pr-123 multilayer system irradiated by 770 MeV Pb ions under an angle of 30° with respect to the c-axis (Kraus et al. 1994b).

It is safe to state, in conclusion, that progress with our understanding of issues related to improve flux pinning in R-123 superconductors has been breathtaking over the past decade, and will certainly remain in the focus of further research, particularly with respect to all micro- and nano-structural aspects of material properties and defect configurations. 

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