Recently, the kagome lattice metal $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\text{K}$, Rb, Cs) family has received wide attention due to its coexistence of superconductivity, charge density wave (CDW), and peculiar properties arising from a topological nontrivial electronic structure. With time-resolved pump-probe spectroscopy, we show that the excited quasiparticle relaxation dynamics can be explained by the formation of a temperature-dependent energy gap below the phase transition that is similar to a usual second-order CDW condensate. By contrast, the structural change shows a predominantly first-order phase transition. Furthermore, no CDW amplitude mode is identified in the ordered phase. The results suggest that the CDW order is very different from a conventional CDW condensate. We also find that a weak pump pulse can nonthermally melt the CDW order and drive the sample into its high-temperature phase, revealing a small difference in lattice potential between those phases.

• Received 30 May 2021
• Revised 16 August 2021
• Accepted 24 September 2021

DOI:https://doi.org/10.1103/PhysRevB.104.165110