EXPERIMENTAL EVIDENCE OF FLUX JUMPS IN Y-Ba-Cu-O SUPERCONDUCTORS

The time dependence of the penetration of a weak ac field into Y-Ba-Cu-O superconductors at various dc fields,0-1500G,was measured at liquid nitrogen temperature.It is found that the signals are very unstable at certain field regions for some of the samples.We believe that this is a direct evidence of the existence of flux jumps which are characteristic of granular superconductors.