Dynamical crossover between glassy relaxation pathways in a photoexcited crystal

Anomalous relaxation is one of the hallmarks of disordered systems. Following perturbation by an external source, many glassy, jammed and amorphous systems relax as a stretched or compressed exponential as a function of time. Although these relaxation functions are usually assumed to possess distinct microscopic origins, a stretched-to-compressed exponential crossover has recently been observed in various systems including metallic glasses, sponge phases and hydrogels with added surfactants. Both the origin of and the connection between these phenomenological relaxation functions remain to be understood. Here, by photo-exciting Ca₃Ru₂O₇ across a symmetry-preserving structural transition, we observe a crossover from stretched to compressed exponential relaxation as a function of excitation fluence. Owing to the system's single crystallinity, we present a simple lattice model that shows how spatial inhomogeneity and local, strain-mediated interactions cooperate to produce the dynamical crossover. Our work uncovers a stretched-to-compressed exponential crossover in an idealized single crystal material and establishes photoexcited solids as promising platforms for revealing the underlying mechanisms behind anomalous relaxation.