The kagome material AV$_3$Sb$_5$ exhibits multiple exotic orders, including
an unconventional charge density wave (CDW). Elucidating the underlying
mechanism behind the CDW transition is crucial for unraveling the complex
interactions among these phases. Cependant, the driving force of the CDW remains
a topic of debate due to the intertwined interactions among the system’s
various excitations. Here we investigated the CDW transition in KV$_3$Sb$_5$ by
isolating the ultrafast electronic phase transition using time- et
angleresolved photoemission spectroscopy. An ultrafast electronic phase
transition was observed at a critical photoexcitation fluence, Fc, without
reduction in CDW lattice-distortion-induced band folding. This folded band
persisted up to 150 K under equilibrium heating, well above the CDW
condensation temperature of Tc = 78 K. Notably, the pump-induced band shifts at
Fc were comparable to those caused by thermal effects at Tc. These findings
suggest that in KV$_3$Sb$_5$, a fluctuating lattice-driven in-plane CDW emerges
above 150 K, with out-of-plane electronic correlations leading to the $2\times2
\times 2$ CDW near Tc, offering key insights into the interplay between the
electronic and structural dynamics in AV$_3$Sb$_5$.

Cet article explore les excursions dans le temps et leurs implications.

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