Doping dependence of irreversibility line in Y1−xCaxBa2Cu3O7−δ

The irreversibility lines (IL) for series of Y1−xCaxBa2Cu3O7−δ (x = 0.025; 0.10 and 0.20) polycrystalline samples with different overdoping were investigated. The irreversibility fields were determined from measurements of third harmonics AC susceptibility as a function of DC field at constant temperature. For the weakly overdoped sample (with x = 0.025) Hirr(77 K) is about 7 T, which is higher than the previously reported for the non-substituted one. The irreversibility line behavior is typical for glass–liquid phase transition and this is confirmed by transport measurements. On increasing the overdoping the irreversibility fields were shifted towards lower temperatures. The behavior of Hirr(T) for the highly overdoped sample (with x = 0.20) is influenced by the surface barrier effect. It is supposed that in highly overdoped specimen the process of phase separation is enhanced and the Fermi clusters grow in size. This leads to a suppression of the bulk pinning and to a domination of the surface barrier effects and flux creep as well. As a confirmation, the obtained quadratic Jc(T) dependences were presented demonstrating the existence of S–N–S type inter-grain joints in the highly overdoped samples.