Logarithmic aging via instability cascades in disordered systems

D Shohat and Y Friedman and Y Lahini, NATURE PHYSICS (2023).

DOI: 10.1038/s41567-023-02220-2

Many complex and disordered systems fail to reach equilibrium after they have been quenched or perturbed. Instead, they sluggishly relax toward equilibrium at an ever-slowing, history-dependent rate, a process termed physical aging. The microscopic processes underlying the dynamic slow- down during aging and the reason for its similar occurrence in different systems remain poorly understood. Here, we reveal the structural mechanism underlying logarithmic aging in disordered mechanical systems through experiments in crumpled sheets and simulations of a disordered network of bistable elastic elements. We show that under load, the system self-organizes to a metastable state poised on the verge of an instability, where it can remain for long, but finite, times. The system's relaxation is intermittent, advancing via rapid sequences of instabilities, grouped into self-similar, aging avalanches. Crucially, the quiescent dwell times between avalanches grow in proportion to the system's age, due to a slow increase of the lowest effective energy barrier, which leads to logarithmic aging. Many complex systems relax slowly towards equilibrium after a perturbation, without ever reaching it. Experiments with crumpled sheets now show that these relaxations involve intermittent avalanches of localized instabilities, whose slow- down leads to logarithmic aging.

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