Point Defects

Point defects are introduced by low-energy particles (ions, protons, neutrons, electrons), which have low stopping power in HTS crystals. A common property of the pointlike pinning centers is that they generally improve Jc at temperatures lower than 0.5 Tc [88, 90, 91]. At higher temperatures, the improvement is insignificant. The pinning energy of these defects differs very little from the energy of the defects already in the crystal before the irradiation [91, 102]. Therefore, Jc increases because of the increased density of the pinning centers with the irradiation.

As opposed to the columnar pinning centers, the irre-versibility line does not change with introduction of pointlike pinning centers. The irreversibility line for 3 MeV proton irradiated YBa2Cu3O7 crystals was found to be independent of the proton fluence, even though Jc was improved by more than 10 times at 77 K [83]. On the other hand, irradiation of Bi2Sr2CaCu2O8 with 400 MeV oxygen ions resulted in only a slight shift of irreversibility line to higher values of fields and temperatures [117]. This is consistent with the dependence of the type of damage track on the energy loss [68]. Protons of 3 MeV produce point defects, which do not affect the irreversibility line. On the other hand, light oxygen ions produce a small number of columnar tracks, together with point defects, affecting the irreversibility line slightly.

High-energy neutrons and protons can produce cascade defects, which have vortex pinning properties somewhere between the point and columnar defects. Irradiation of YBa2Cu3O7 crystals with 3 MeV protons resulted in defects, 30% of which were cascade defects [83]. Jc increased by an order of magnitude at both 5 and 77 K, whereas the pinning energy increased by only 25%. The irreversibility line, however, did not change. Irradiation of YBa2Cu3O7 crystals with fast neutrons (En > 0.1 MeV) resulted in point and cascade defects. Value of Jc increased by an order of magnitude, at both low and high temperatures [77, 78, 80]. As opposed to the point defects, the cascade defects could not be annealed out at 300 °C [78]. The irreversibility line was also shifted to higher values of field and temperatures. Irradiation of HgBa2CuO4+s [118] and HgBa2Ca2Cu3O8+s [119-121] with fast neutrons (En > 0.1 MeV) also resulted in an increase of Jc, Hirr, and Tirr at low and high temperatures.

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