Ultrafast Dynamics of the Charge Density Wave in Kagome Metals

F. Cilento1

1Elettra – Sincrotrone Trieste ScpA, Strada Statale 14, km 163.5, Basovizza, Trieste, Italy

The Charge Density Wave (CDW) order, descending from a metallic parent state, offers an intriguing playground to study the interplay of structural and electronic degrees of freedom in complex materials. Recently, this phenomenology has been discovered also in Kagome metals. With dispersive and correlation features including topological Dirac-like itinerant states, van-Hove singularities, correlated flat bands, and magnetic transitions at low temperature, kagome metals are located in the interesting regime where both phonon and electronically mediated couplings are significant. In particular, the van-Hove singularities, which are intrinsic to the kagome tiling, have been conjectured to play a key role in mediating the CDW instability. However, to date, the origin and the main driving force behind this charge order is elusive. Here, we use the topological bilayer kagome metal ScV6Sn6 as a platform to investigate this puzzling problem, and combine time-resolved optical spectroscopy, to unveil the ultrafast dynamics of its CDW phase, with angle-resolved photoelectron spectroscopy and density functional theory [1]. We identify the structural degrees of freedom to play a fundamental role in the stabilization of charge order. In particular, we find ScV6Sn6 to feature a charge density wave order that predominantly originates from phonons, as odd with other recent findings on other kagome metals like those from the AV3Sb5 (A = K, Rb, Cs) family, where the CDW originates from an electronic instability. As we shed light on the lattice-mediated low-temperature ordered phase, our findings pave the way for a deeper understanding of ordering phenomena in CDW kagome metals.

1. M. Tuniz et al., arXiv:2302.10699