Analysis of the resistive transition of a polycristalline YBCO specimen produced by an a.c. magnetic field, reveals the presence of a large conductance noise signal which is repetitive over subsequent magnetization cycles. It is shown that the noise arises from avalanche effects produced by the simultaneous resistive transition of large groups of weak links. Owing to its repeatability, the noise signal may be considered a sort of signature of the weak links critical current distribution, making it an interesting new tool for the study of high Tc ceramic superconductors, as reported measurements of noise hysteresis show. PACS: 72.70.+m, 74.40.+k The resistive transition of polycrystalline high Tc superconductors is, in general, dominated by the intergrain layers, which behave as shunted Josephson junctions (weak links). The weak link ensemble, on the other side, is characterized by a distribution of critical currents which depends on the local value of the magnetic field. When the specimen is crossed by a d.c. current and immersed in a slowly increasing magnetic field, there are gradual transitions to the resistive state of the weak links and, above a given threshold, the whole specimen becomes increasingly resistive. As shown by the reported experiments, this process is not smooth, but is characterized by a large noise. It is very interesting to notice that the noise signal represents a sort of signature of the local distribution [1] of the weak-link critical currents, since it is repetitive over different magnetic field cycles, until a magnetic perturbation, strong enough to change such a distribution by effect of flux trapping within the superconducting grains, is applied. Another important aspect of this noise, which has a completely different origin of the noise observed in stationary conditions [2 − 6], concerns the fact that it is produced by strong correlations during the transition of the weaklinks, giving rise to avalanche effects, similar to those observed in the Barkhausen noise of ferromagnets. As computer simulations show, these avalanches correspond to abrupt rearrangements of the internal current distribution, and thus of the distribution of superconducting and resistive weak-links, due to the need of satisfying Kirchoff equations in a network of highly non linear circuit elements. The step-like variation of the specimen conductance in correspondence to each weak-link transition avalanche gives rise, as reported in the following, to a 1/f 2 power spectrum, as opposed to the 1/f spectrum observed in the stationary case [7]. Also the amplitude of the noise, which is orders of magnitude larger than expected in the case of uncorrelated transition of the individual weak links, confirms the presence of these avalanche effects. Another important aspect, also confirmed by computer simulations, concerns the fact that the position along the time axis and the amplitude of these steps, when the magnetic field is varied, is strongly dependent on the local distribution of the weak-link critical currents. Thu...
