Title: Multiple charge-density-wave gaps in LaSbTe and CeSbTe as revealed by ultrafast spectroscopy

Abstract: Utilizing ultrafast time-resolved pump-probe spectroscopy measurements, we investigate the photoinduced quasiparticle dynamics of the topological materials LaSbTe and CeSbTe. In LaSbTe, the relaxation of quasiparticles is dominated by two different mechanisms: electron-phonon coupling, and phonon-assisted electron-hole recombination. Significantly, the amplitude of photoinduced reflectivity related to the former one shows two pronounced peaks at 156 K and 263 K, indicating the occurrence of two charge density wave (CDW) phase transitions. The ultrafast responses of CeSbTe share a lot of similarities with LaSbTe, and an additional CDW phase transition at 154 K is revealed in CeSbTe. However, the slower relaxation of CeSbTe exhibits an exotic behavior that deviates from the typical phonon-assisted electron-hole recombination process, probably due to the imbalance between the electron- and hole-type carriers. Unlike LaSbTe, the relaxation times of CeSbTe vary slightly with the pump power, inferring the possible participation of 4$f$ electron in the decay process. In addition, two oscillation modes around 1 THz and 3 THz are identified in both LaSbTe and CeSbTe, which are mostly likely to be coherent phonon modes. These findings unravel the existence of multiple CDW orders in LaSbTe and CeSbTe, offering insights into the underlying physics of these systems.