Creeping instability in YBCO ceramics under non-isothermal superconducting transition

A high number of normal-superconductor transitions of sintered YBCO samples have been monitored by AC susceptibility measurements simultaneously in three different regions of samples, in the presence of strong thermal gradient. With this method, different accidental mesoscopic vortex structure instabilities have not only been observed but also located. Even if local magnetothermal instabilities of oscillatory nature (f=0.1–0.02 Hz) appear in strong thermal gradient fields, their evolution remains localized. However, when such instabilities appear in superposed temperature and electric (DC) potential gradient fields, they travel together with the mixed-state zone (v=0.06–0.1 mm s−1) along the macroscopic specimen. The interaction of transport currents with shielding current instabilities of the mixed-state — modifying some percolative network paths — can be considered as a possible macroscopic scenario for the traveling instabilities.