Unveiling the hybridization process in a quantum critical ferromagnet by ultrafast optical spectroscopy

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 $\ensuremath{\sim}85\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (${T}^*$) to $\ensuremath{\sim}140\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (${T}^{\ifmmode\dagger\else\textdagger\fi{}}$), and (2) the emergence of collective hybridization below the coherence temperature, ${T}^*$, marked by the opening of an indirect gap of $2\mathrm{\ensuremath{\Delta}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}12\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. 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.