The kagome lattice, characterized by its corner-sharing triangle geometry, hosts distinctive electronic structures such as topological flat bands, Dirac bands, and van Hove singularities (VHS) [Fig. 1(a)]. These features enable kagome materials to support exotic electronic phases. Recently, the kagome metals $A$V₃Sb₅ ($A=$K, Rb, Cs) have attracted intense interest due to diverse phenomena [1] including superconductivity, charge-density wave (CDW), pair density wave, and giant anomalous Hall effects. The suppression of CDW by pressure or doping in $A$V₃Sb₅ enhances superconductivity, illustrating a strong interplay between these two states.

Despite significant research efforts [1], the microscopic origin of CDW remains under active debate. Angle-resolved photoemission spectroscopy (ARPES) experiments suggest an electronic instability mechanism, highlighting VHS near the Fermi level ($E_{\mathrm{F}}$) with favorable nesting conditions. This is further supported by the absence of the acoustic phonon anomalies. Conversely, density functional theory calculations indicate lattice instabilities via unstable phonon modes, corroborated by lattice distortions observed in scanning tunneling microscopy and x-ray diffraction experiments. Due to the complex interplay between electrons and phonons, static experimental methods face limitations in conclusively identifying the primary driving force behind the CDW.

To address these challenges, we conducted comprehensive time-resolved ARPES measurements on CsV₃Sb₅ [2], providing direct insights into the dynamic interplay of electrons and phonons. We demonstrate an ultrafast response of the VHS to laser excitation [Fig. 1(b)]. Remarkably, upon ultrafast optical pumping, the VHS immediately shifts closer to $E_{\mathrm{F}}$, altering the electronic density of states. Moreover, these energy shifts exhibit coherent oscillations at a frequency of approximately 1.3 THz, matching a previously reported phonon mode strongly associated with the CDW.

Another striking experimental finding is the persistence of the 1.3-THz phonon mode above the CDW transition temperature, suggesting the existence of fluctuating CDW orders [Fig. 2]. These fluctuations, normally undetectable under equilibrium conditions [3], become observable through the intense, ultrafast optical excitations utilized in time-resolved ARPES, providing new insights into transient quantum states that may be relevant for manipulating superconductivity.

These findings have important implications for understanding the CDW in CsV₃Sb₅. The observed VHS-phonon coupling indicates a synergistic effect between electron-phonon interactions and electronic correlations, together generating the CDW with exotic time-reversal symmetry-breaking behavior. Furthermore, ultrafast laser excitation can dynamically tune the VHS closer to the Fermi level, hinting at potential mechanisms for enhancing superconductivity through nonequilibrium conditions.

References

• [1] S. Wilson and B. Ortiz, Nat. Rev. Mater. 9, 420 (2024).
• [2] Y. Zhong et al., Phys. Rev. Res. 6, 043328 (2024)
• [3] S. Wu et al., Phys. Rev. B 105, 155106 (2022)

Authors

• Y. Zhong, T. Suzuki, H. Liu^a, K. Liu, Z. Nie^a, Y. Shi^a, S. Meng^a, B. Lv^b, H. Ding^b, T. Kanai, J. Itatani, S. Shin, and K. Okazaki
• ^aChinese Academy of Science
• ^bShanghai Jiao Tong University