When many-particle assemblies with disordered distribution are subjected to a periodic shear with a small amplitude d, the particles gradually self-organize to avoid next collisions and transform into an organized configuration. For small d, the particles settle into a reversible state where all the particles return to their initial position after each shear cycle, while they reach an irreversible state for d above a threshold d_c [1]. We have studied a reversible-irreversible transition (RIT) using periodically driven vortices in amorphous films with random pinning that causes local shear, as a function of d [2]. The relaxation time to reach the reversible or irreversible state shows a power-law divergence at d_c. The critical exponent agrees with the value expected for an absorbing phase transition in the two-dimensional directed-percolation universality class.

In previous work, the experiment has been conducted at relatively high frequency, giving rise to large vortex velocity [2]. When we used lower frequencies for a given d and the vortex velocity was decreased, the pining force that the moving vortices feel would become more effective. This would increase the local shear and hinder the observation of the reversible behavior. Contrary to the expectation, however, we find a trend for the reversible regime to increase at small velocities, while the critical behavior of RIT stays essentially unchanged. To explain the results, we will propose two different models: the conventional collision model and the potential-energy-landscape one [3,4].

[1] L. Corté et al., Nat. Phys. 4, 420 (2008) : N. Mangan et al., Phys. Rev. Lett. 100, 187002 (2008).
[2] S. Maegochi, K. Ienaga, S. Kaneko, and S. Okuma, Sci. Rep. 9, 16447 (2019).
[3] Frank H. Stillinger, Science 267, 1935 (1995).
[4] I. Regev and T. Lookman, J. Phys.: CM. 31 (2019) 045101.

Keywords: reversible-irreversible transition, effective pinning, potential energy landscape