Ultrafast optical polarimetry in magnetic phases of the Kondo semimetal CeSb

We investigated photoinduced ultrafast transient dynamics in different magnetic phases in CeSb by means of time resolved magneto-optical spectroscopy. We observe distinctive coherent oscillations in the ground-state antiferromagnetic (AF) phase and the high-magnetic-field ferromagnetic (F) phase. While the AF-phase oscillation frequencies match the recent Raman scattering findings the F-phase oscillation frequency does not correspond to the previously observed magnetic excitation. The large spectroscopic factor, $g=3.94$, and optical polarization properties suggest that it corresponds to a previously undetected Ce³⁺ coherent crystal-field state excitation. The AF-phase oscillations show no magnetic-field dependence so their lattice origin cannot be entirely excluded. The nonoscillatory part of the transients is qualitatively similar in all investigated magnetic phases with a faster subpicosecond dynamics in the ferromagnetic and ferroparamagnetic phases and is attributed to differences in the electronic structure, which affect the photoexcited quasiparticle energy relaxation kinetics.