Unveiling the Hybridization Process in a Quantum Critical Ferromagnet by Ultrafast Optical Spectroscopy

Abstract

We report the ultrafast optical pump-probe spectroscopy measurements on the recently discovered quantum critical ferromagnet CeRh${}_6$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$85 K ($T^{\ast }$) to $\sim$140 K ($T^{†}$), and (2) the emergence of collective hybridization below the coherence temperature, $T^{\ast }$, marked by the opening of an indirect gap of 2$\hat{I}”$ $\approx$12 meV. We also observe three coherent phonon modes being softened anomalously below $T^{\ast }$, reflecting directly their coupling with the emergent coherent heavy electrons. Our findings establish the universal nature of the hybridization process in different heavy fermion systems.