The nature of the charge density wave phases in the kagome metal compound AV$_3$Sb$_5$ has raised many questions and their origin is still under debate. Here, we combine diffuse scattering (DS) and inelastic x-ray scattering (IXS) to identify a 3-dimensional (3D) precursor of the charge order with propagation vector (0.5 0 0.5), which condenses into a 3D-CDW through a first order phase transition. The quasi-elastic critical scattering indicates that the dominant contribution to the diffuse precursor is the elastic central peak without phonon softening. However, the inelastic spectra show a small broadening of the Einstein-type phonon mode on approaching the phase transition. Our results point to the situation where the Fermi surface instability at the \textit{L} point is of order-disorder type with a critical growth of quasi-static domains. The results go beyond the classical weak-coupling Peierls transition dynamics and are discussed within models including strong-electron phonon coupling and non-adiabaticity.

中文翻译：

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Kagome 金属 (Cs,Rb)V$_3$Sb$_5$ 中的有序无序 Peierls 不稳定性

Kagome 金属化合物 AV$_3$Sb$_5$ 中电荷密度波相的性质引发了许多问题，其起源仍在争论中。在这里，我们结合漫散射 (DS) 和非弹性 X 射线散射 (IXS) 来识别具有传播矢量 (0.5 0 0.5) 的电荷级的 3 维 (3D) 前驱体，该前驱体通过 a 凝聚成 3D-CDW一阶相变。准弹性临界散射表明，对漫射前体的主要贡献是没有声子软化的弹性中心峰。然而，非弹性光谱在接近相变时显示出爱因斯坦型声子模式的小幅展宽。我们的结果指出了在 \textit{L} 点处的费米表面不稳定性是有序无序类型的情况，准静态域的临界增长。