Self-organized criticality induced by quenched disorder: Experiments on flux avalanches in NbHx films

We present an experimental study of the influence of quenched disorder on the distribution of flux avalanches in type-II superconductors. In the presence of much quenched disorder, the avalanche sizes are powerlaw distributed and show finite-size scaling, as expected from self-organized criticality (SOC). Furthermore, the shape of the avalanches is observed to be fractal. In the absence of quenched disorder, a preferred size of avalanches is observed and avalanches are smooth. These observations indicate that a certain minimum amount of disorder is necessary for SOC behavior.We relate these findings to the appearance or nonappearance of SOC in other experimental systems, particularly piles of sand. ©2005 The American Physical Society.