Stimulated flux dynamics during the superconducting-normal zone sweep along a Y-Ba-Cu-O sample

To compare superconducting transitions under slow and fast altered temperat ure gradients the ac susceptibility was measured simultaneously in three di fferent regions of Y-Ba-Cu-O ceramic samples. The flux dynamics of a Y-Ba-C u-O sample either exposed or not to a slowly varying temperature gradient r emain the same (conventional behaviour). The flux dynamics of the same samp le exposed to a quickly varying temperature gradient (v > 0.32 Ks(-1)) are qualitatively different. During fast cooling (or heating) the start (or the end) of the flux expulsion (or admission) in the first region of the incre asing state stimulates the beginning (or the end) of the flux expulsion (or admission) in the regions of the following decreasing state. Consequently in some regions the transition of the decreasing state begins (terminates) prematurely well over (under) the conventional onset temperature of the sam ple. Thus, during cooling, overheated superconducting regions and, during h eating, undercooled normal macroscopic regions appear in the sample. The me aningful explanation of this behaviour needs further investigations.