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. However, despite their ubiquity, the origins of and the connection between these phenomenological relaxation functions remain to be understood. Here, we observe a tunable crossover from stretched- to compressed-exponential relaxation by photoexciting single crystal ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ across a structural phase transition. We present a simple lattice model that shows how spatial inhomogeneity and local, strain-mediated interactions cooperate to produce the dynamical crossover. Our work reveals anomalous relaxation dynamics in an idealized single crystal material and establishes photoexcited solids as promising platforms for probing the mechanisms underlying anomalous relaxation.

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• Dynamical phase transitions
• Glass transition
• Mesoscopics
• Metal-insulator transition
• Nonequilibrium statistical mechanics
• Stress propagation