We report the ultrafast optical pump-probe spectroscopy measurements on the recently discovered quantum critical ferromagnet ${\mathrm{CeRh}}_6{\mathrm{Ge}}_4$. Our experimental results reveal the two-stage development of the hybridization between localized $f$ moments and conduction electrons with lowering temperature, as evidenced by (1) the presence of hybridization fluctuation for temperatures from $\sim 85\mathrm{K}$ ($T^{*}$) to $\sim 140\mathrm{K}$ ($T^{†}$), and (2) the emergence of collective hybridization below the coherence temperature, $T^{*}$, marked by the opening of an indirect gap of $2\mathrm{\Delta }\approx 12\mathrm{meV}$. We also observe three coherent phonon modes being softened anomalously below $T^{*}$, reflecting directly their coupling with the emergent coherent heavy electrons. Our findings establish the universal nature of fluctuation and coherence during the hybridization process in heavy fermion systems.

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• Kondo effect
• Phase transitions
• Phonons
• Strongly correlated systems
• Pump-probe spectroscopy