  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 power-law 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 non-appearance of SOC in other experimental systems,
particularly piles of sand.
