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We study the ultrafast dynamics of the multiband iron-based superconductor CaKFe4As4 via time-resolved optical reflectivity. We observe a long-lived 5-GHz oscillation of the reflection changes within the superconducting regime. We assign this low-frequency oscillation to a soft Leggett mode associated with the collective fluctuation of multiple phase differences between Cooper pair condensates. Our observation evidences the presence of the time-reversal-symmetry breaking state in superconducting CaKFe4As4.
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