Ultrafast quasiparticle dynamics and collective excitations in CsV3Sb5

CsV₃Sb₅ is a recently discovered Kagome metal that exhibits a combination of superconductivity with a critical temperature (TC) of 2.5 K and charge density wave (CDW) order at TCDW = 94 K. In this study, we investigate the collective excitations and quasiparticle dynamics in CsV₃Sb₅ using ultrafast optical pump-probe spectroscopy. According to our results, in addition to the CDW phase related electron and coherent phonon dynamics, we found another electronic symmetry breaking by nematic phase transition at T* ( 20 K), which can mediate the relaxation of carriers. At temperature below T*, a gap about 4 meV related to the nematic order is open, which determines the low-temperature dynamics of carriers in the rst 2 picosecond (ps). Moreover, we also observed the abrupt change of electron dynamics caused by the CDW phase transition at TCDW. Furthermore, we identify the presence of a CDW induced mode at 1.3 THz below TCDW, as well as two collective modes at 3.1 THz and 3.8 THz below 80 K. It is noteworthy that these two collective modes appear to be associated with the breaking of C₆ rotational symmetry. These observations suggest the rich phase order below TCDW and emphasize the need for a deeper understanding of CDW and nematicity order in the Kagome metals.