UNIVERSALITY IN SANDPILE MODELS

A classification of sandpile models into universality classes is prese nted. On the basis of extensive numerical simulations, in which we mea sure an extended set of exponents, the Mama two-state model [S. S. Mam a, J. Phys. A. 24, L363 (1991)] is found to belong to a universality c lass of random neighbor models which is distinct from the universality class of the original model of Bak, Tang, and Wiesenfeld [P. Bak, C. Tang, and K. Wiesenfeld, Phys. Rev. Lett. 59, 381 (1987)]. Directed mo dels are found to belong to a universality class which includes the di rected model introduced and solved by Dhar and Ramaswamy [D. Dhar and R. Ramaswamy, Phys. Rev. Lett. 63, 1659 (1989)].