Defects flux-pinning

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. 

The nature of the defect responsible for the dramatic Tc decreases in A-15 compounds (specifically disorder vs. large static displacements) and the role of these defects in relation to other defects present in determining flux pinning changes. 

X is anisotropic. For YBCO Xab 1.5 nm and Xc 0.4 nm. A major problem in HTSC is to find a crystal defect that pins the flux vortices, but does not disrupt current flow. 

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. 

In summary, various possibilities of introducing artificial defect structures into all families of high-temperature superconducting materials have been explored during the past few years, only a few of them being effective for enhancing their flux-pinning... 

The main issue that will require close attention, also in the foreseeable future, is the optimisation of the defect structures for flux pinning. The record performance of R-123... 

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. 

Oxygen defects. Daumling et al. observed a peak in the magnetization hysteresis in oxygen-deficient single crystals at an intermediate field, which they ascribed to flux pinning. This kind of pinning center is not the most important one. 

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