Observation of soft Leggett mode in superconducting CaKFe4As4

We study the ultrafast dynamics of the multiband iron-based superconductor ${\mathrm{CaKFe}}₄{\mathrm{As}}₄$ 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 ${\mathrm{CaKFe}}₄{\mathrm{As}}₄$.