  Two-component sandpile models are investigated numerically and theoretically.
Monte Calro simulations are performed to show that probability distribution
functions of avalanche size and lifetime obey power laws whose exponents are
approximately equal to 1.5 and 2.0 and the system exhibits SOC. A mean-field
theory is developed to discuss the essence of the processes. We find that
two-component models approach a steady critical state belonging to a different
universality class from that of one-component models. Conservation of two kinds
of sands at local toppling causes an infinite number of stable states which
substitute for artificial boundary dissipation. Among two control parameters
appearing in one-component models, therefore, a rate constant of dissipation is
removed in two-component models. It is concluded that the more conserved
quantities result in the less control parameters and a novel class of SOC.
