Irreversibility from reversible plastic events

The reversibility-irreversibility transition in cyclicly sheared amorphous solids occurs when the shearing amplitude is increased to values beyond the yielding point. For sub-yield amplitudes, the system reaches a periodic state in which particle configurations repeat indefinitely, while for post-yield amplitudes, particle configurations never repeat. Since plastic transitions between configurations involve combinations of qualitatively identical local particle rearrangements, called ``soft spots'', it is unclear why some of them can be reversed by reversing the direction of straining while others cannot. Here, we discuss the role of soft spot overlap (sharing of particles) in the emergence of irreversible dynamics. We show that there are two types of overlaps, and one of these can cause persistent irreversible dynamics at post-yield drives. We then extend a model of soft-spot dynamics to include both types of overlaps and show that including the second type of overlap is essential for obtaining an irreversibility transition in the model.