Non-Thermal Pathways to The Origin of a Charge Density Wave

Highly correlated systems host in many cases a complex electronic phase diagram, with different phases competing or emerging from one-another. A new group of kagome metals AV₃Sb₅ (A = K, Rb, Cs) exhibit a variety of intertwined unconventional electronic phases, which emerge from a puzzling charge density wave phase. Understanding of this parent charge order phase is crucial for deciphering the entire phase diagram. However, the mechanism of the charge density wave is still controversial, and its primary source of fluctuations – the collective modes – have not been experimentally observed. In my talk I will show how we use ultrashort laser pulses to melt the charge order in CsV₃Sb₅ and record the resulting dynamics using femtosecond angle-resolved photoemission. We resolve the melting time of the charge order and directly observe its amplitude mode, imposing a fundamental limit for the fastest possible lattice rearrangement time. These observations together with ab-initio calculations provide clear evidence for a structural rather than electronic mechanism of the charge density wave, providing a path towards better understanding of the unconventional phases hosted on the kagome lattice.