Persistent self-organization of sandpiles

We have studied the avalanche fluctuations resulting from the grain-by-grain quiescent perturbation of real sandpiles of various sizes. Contrary to previous reports, we find the attribute of self-organization, namely, a power-law distribution of avalanche sizes, to be the generic behavior, independent of system size. However, as the system size is increased, the power-law behavior is supplemented by uniformly large, periodic avalanches that carry away the dominant fraction of the total avalanched mass. Additionally, we find that a scale invariance in pile-mass fluctuations depends on the ratio of pile to grain diameters.