Splayed Defects

Theory shows that it is better to introduce columnar defects that are not parallel to each other, but splayed randomly in many directions [86, 103-107]. The most efficient way

Figure 5. Irreversibility line for YBajC^O, crystals before (open symbols) and after (solid symbols) irradiation with 5.3 GeV Pb ions. (a) The irreversibility line in a wide temperature range, down to 0.5 Tc. (b) Irreversibility line close to Tc. Reprinted with permission from [98], M. Kon-czykowski et al., Phys. Rev. B 44, 7167 (1991). © 1991, American Physical Society.

Figure 5. Irreversibility line for YBajC^O, crystals before (open symbols) and after (solid symbols) irradiation with 5.3 GeV Pb ions. (a) The irreversibility line in a wide temperature range, down to 0.5 Tc. (b) Irreversibility line close to Tc. Reprinted with permission from [98], M. Kon-czykowski et al., Phys. Rev. B 44, 7167 (1991). © 1991, American Physical Society.

to introduce the splayed columnar defects is to instigate nuclear fission of small number of atoms in superconducting crystal. This can be achieved by doping an HTS with small amount of 235U and then irradiating it with thermal neutrons [84, 108, 109]. 235U is split into two fission products with energies of about 100 MeV each [110]. Because of the relatively low energy of the fission products, discontinuous columnar defects are obtained [111]. These defects are randomly splayed in the crystal. They produce an increase of Jc of 10-20 times for Bi2Sr2Ca2Cu3O10 and shift of the irreversibility line to higher fields and temperatures.

Another way to introduce the splayed defects by fission fragments is irradiating Bi2Sr2CaCu2O8 that contains 209Bi with 0.8 GeV protons [87, 112]. Irradiating 209Bi with neutrons triggered nuclear fission. Typical fission products are 80 MeV Xe and 100 MeV Kr, which produce splayed discontinuous columnar defects of diameter up to 7 nm. A strong improvement of irreversibility line and Jc was observed, which was better than for the parallel columnar defects. The improvement was observed at all temperatures and fields. Similar results were obtained with neutron irradiation of lithium-doped Bi2Sr2CaCu2O8 [113]. There, neutron irradiation caused nuclear fission of Li atoms.

The effect of the splayed columnar defects on vortex pinning can be most effectively studied by rocking the HTS crystal upon high-energy heavy ion irradiation [114, 115]. As opposed to the defects introduced by fission products, the direction of the columnar defects can be easily controlled in this way. Increase of Jc by 14 times was obtained when irradiating DyBa2Cu3O7 crystal at 45° and -45° to its surface, as compared to the parallel ion tracks with the ion beam perpendicular to the crystal surface [114]. The optimum splay angle for YBa2Cu3O7 crystal was found to be ±5° with respect to the axis perpendicular to the afo-plane, with magnetic field oriented along this axis (i.e., at 0°) [116]. After irradiation with splay angle of ±5°, Jc increased respectively 10 and 104 times at temperatures of 5 and 77 K. With different splay angles, the increase was up to several times lower. However, splaying of the columnar defects seems to be much less effective for more anisotropic HTSs, like Bi2Sr2CaCu2O8 [100, 107]. This was attributed to the more pronounced pancake structure of magnetic vortices in highly anisotropic HTS, and increase of the diameter of columnar tracks by tilting the track away from the crystalline c-axis [100] [Eqs. (7)-(9)].

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