Ultrafast quasiparticle relaxation dynamics in the topological materials LaSbTe and CeSbTe

We utilize ultrafast time-resolved pump-probe spectroscopy to investigate photoinduced quasiparticle dynamics in the topological materials LaSbTe and CeSbTe. Our results reveal that the relaxation dynamics in both materials are characterized by two distinct decay channels. In LaSbTe, the amplitude of the photoinduced reflectivity related to the first decay channel exhibits two pronounced peaks at 163 and 280 K, whereas in CeSbTe, a single peak emerges at 165 K. These observations are strongly indicative of charge-density-wave phase transition(s). Notably, the temperature dependence of the relaxation time for this decay process in CeSbTe deviates from that in LaSbTe, probably due to the influence of 4f electrons. For the second decay process, LaSbTe exhibits behaviors dominated by phonon-assisted electron–hole recombination, while CeSbTe demonstrates a slower relaxation at low temperatures, possibly due to the imbalance between electron- and hole-type carriers. Furthermore, two coherent phonon modes around 1 and 3 THz are identified in both LaSbTe and CeSbTe. Remarkably, the phonon amplitudes in CeSbTe exhibit an unusual decrease with decreasing temperature until a very low temperature, which can potentially be ascribed to spin–phonon interactions. These findings reveal nonequilibrium charge dynamics and collective excitation in LaSbTe and CeSbTe, offering insights into the underlying physics of topological materials.