Self-organized critically in a rice-pile model

We present a model for relaxations in piles of granular material. The relaxations are determined by a stochastic rule which models the effect of friction between the grains. We find power-law distributions for avalanche sizes and lifetimes characterized by the exponents tau = 1.53 +/- 0.05 and y = 1.84 +/- 0.05, respectively. For the discharge events, we find a characteristic size that scales with the system size as L(mu), with mu = 1.20 +/- 0.05. We also find that the frequency of the discharge events decreases with the system size as L(-mu') with mu' = 1.20 +/- 0.05.