Critical behavior of density-driven and shear-driven reversible-irreversible transitions in cyclically sheared vortices.

Random assemblies of particles subjected to cyclic shear undergo a reversible-irreversible transition (RIT) with increasing a shear amplitude d or particle density n, while the latter type of RIT has not been verified experimentally. Here, we measure the time-dependent velocity of cyclically sheared vortices and observe the critical behavior of RIT driven by vortex density B as well as d. At the critical point of each RIT, [Formula: see text] and [Formula: see text], the relaxation time [Formula: see text] to reach the steady state shows a power-law divergence. The critical exponent for B-driven RIT is in agreement with that for d-driven RIT and both types of RIT fall into the same universality class as the absorbing transition in the two-dimensional directed-percolation universality class. As d is decreased to the average intervortex spacing in the reversible regime, [Formula: see text] shows a significant drop, indicating a transition or crossover from a loop-reversible state with vortex-vortex collisions to a collisionless point-reversible state. In either regime, [Formula: see text] exhibits a power-law divergence at the same [Formula: see text] with nearly the same exponent.