Persistence of charge density wave fluctuations in the absence of long-range order in a hole-doped kagome metal

TL;DR

This study reveals that charge density wave (CDW) fluctuations persist in hole-doped kagome metals even when long-range order disappears, indicating their potential role in the material's electronic properties and superconductivity.

Contribution

It demonstrates the persistence of CDW fluctuations beyond the long-range order phase boundary in hole-doped kagome metals using ultrafast spectroscopy.

Findings

CDW fluctuations survive beyond the long-range order phase boundary

Fluctuations are enhanced near a quantum phase transition at x* ≈ 0.15

Fluctuation behavior is intrinsic to hole doping, not disorder

Abstract

The kagome metals $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) exhibit a complex interplay between charge density wave (CDW) order and superconductivity. In this study, we use ultrafast coherent phonon spectroscopy to probe the evolution of CDW order in hole-doped CsV$_3$Sb$_{5-x}$Sn$_x$ across a broad range of compositions ($0 \leq x \leq 0.68$). While thermodynamic and diffraction measurements show long-range CDW order vanishes above $x \approx 0.05$, we observe persistent signatures of CDW fluctuations up to the highest doping levels, with correlation times on the order of several picoseconds. These results indicate the presence of robust fluctuating charge order that survives well beyond the established CDW phase boundary. Furthermore, these fluctuations are enhanced near a doping-tuned quantum phase transition at $x^* \approx 0.15$, which coincides with a local minimum in the superconducting $T_\mathrm{c}$ double-dome. Additional measurements on Ti- and K-substituted samples confirm that this behavior is intrinsic to hole doping and not tied to disorder. Overall, our findings suggest that CDW fluctuations play a central role in the electronic phase diagram of $A$V$_3$Sb$_5$ and may mediate or compete with superconductivity.