Acoustic Phonon Dynamics in Co-Doped BaFe${}_2$As${}_2$ Thin Film

Pump-probe reflectivity reveals coherent acoustic oscillations at 33 and 8.2 GHz in a Ba(Fe${}_{0.92}$Co${}_{0.08}$)${}_2$As${}_2$ thin film. The acoustic response was analyzed using a modified logistic-function model, suggesting a temporal redistribution of coherent acoustic energy consistent with anharmonic decay of the higher-frequency mode into the lower-frequency mode.