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

TL;DR

This study uses ultrafast optical spectroscopy to investigate the hybridization process in the quantum critical ferromagnet CeRh$_6$Ge$_4$, revealing a two-stage development and collective hybridization below a certain temperature.

Contribution

It uncovers the detailed temperature-dependent hybridization process and phonon coupling in CeRh$_6$Ge$_4$, providing insights into universal behaviors in heavy fermion systems.

Findings

Hybridization fluctuation from ~85 K to ~140 K

Emergence of collective hybridization below T* (~85 K)

Anomalous softening of phonon modes below T*

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^*$) to $\sim$140 K ($T^\dagger$), and (2) the emergence of collective hybridization below the coherence temperature, $T^*$, marked by the opening of an indirect gap of 2$\Delta$ $\approx$12 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 the hybridization process in different heavy fermion systems.