Unconventional charge density wave and photoinduced lattice symmetry change in the kagome metal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>CsV</mml:mi></mml:mrow><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Sb</mml:mi></mml:mrow><mml:mn>5</mml:mn></mml:msub></mml:math> probed by time-resolved spectroscopy
Z. X. Wang, Qiong Wu, Q. W. Yin, Chunsheng Gong, Z. J. Tu, Ting Lin, Qiaomei Liu, L. Y. Shi, S. J. Zhang, Dong Wu, Hechang Lei, Tao Dong, Nanlin Wang
Abstract
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.