Unveiling the hybridization process in a weakly hybridized ferromagnet by ultrafast optical spectroscopy

Utilizing ultrafast optical pump-probe spectroscopy, we present a comprehensive investigation on the photoexcited quasiparticle dynamics in single-crystal ${\mathrm{CeAgSb}}_2$. We elucidate the emergence of collective $c\text{−}f$ hybridization between conduction electrons and localized $f$ moments below a critical temperature $T^{*}$ ($\approx 65$ K), as indicated by the onset of an indirect gap of $\sim 9$ meV via the fluence-dependent quasiparticle relaxation and phonon renormalization processes. Our experimental data also suggest that the $c\text{−}f$ hybridization fluctuation should appear at a much higher-temperature value, i.e., $T^{†}$ ($\approx 180$ K). Below $\sim 50$ K, we further detect an additional quasiparticle relaxation channel with sub-ps timescale, which can be attributed to the crystalline electric field (CEF) excitation. These observations provide important insights into the Kondo lattice coherence and CEF effect in ${\mathrm{CeAgSb}}_2$, shedding light on its unique properties and behaviors.