UNIVERSALITY CLASSES FOR RICE-PILE MODELS

We investigate sandpile models where the updating of unstable columns is done according to a stochastic rule. We examine the effect of intro ducing nonlocal relaxation mechanisms. We find that the models self-or ganize into critical states that belong to three different universalit y classes. The models with local relaxation rules belong to a known un iversality class that is characterized by an avalanche exponent tau ap proximate to 1.55, whereas the models with nonlocal relaxation rules b elong to new universality classes characterized by exponents tau appro ximate to 1.35 and tau approximate to 1.63. We discuss the values of t he exponents in terms of scaling relations and a mapping of the sandpi le models to interface models.