Order-disorder Peierls instability in the kagome metal (Cs,Rb)V$_3$Sb$_5$

TL;DR

This study reveals that the charge density wave in kagome metal AV$_3$Sb$_5$ arises from an order-disorder transition involving quasi-static domains, challenging classical Peierls transition models and highlighting strong electron-phonon interactions.

Contribution

It provides experimental evidence of an order-disorder type Fermi surface instability in kagome metals, combining diffuse and inelastic scattering to elucidate the transition mechanism.

Findings

Charge order condenses via a first order phase transition.

Diffuse scattering indicates a quasi-elastic central peak without phonon softening.

The transition involves strong electron-phonon coupling and non-adiabatic effects.

Abstract

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.