Coherent phonon and unconventional carriers in the magnetic kagome metal Fe$_3$Sn$_2$

TL;DR

This study investigates the ultrafast carrier and phonon dynamics in the magnetic kagome metal Fe$_3$Sn$_2$, revealing unconventional carriers and a large-amplitude coherent phonon, suggesting a link to charge-density-wave phenomena.

Contribution

It provides the first ultrafast optical analysis of Fe$_3$Sn$_2$, identifying unconventional carriers and a significant coherent phonon, indicating complex electron-phonon interactions and potential CDW-like behavior.

Findings

Short electron relaxation time (~1 ps) due to electron-phonon scattering.

Unconventional (localized) carriers with longer relaxation (~25 ps).

Large-amplitude coherent A$_{1g}$ phonon comparable to CDW systems.

Abstract

Temperature- and fluence-dependent carrier dynamics of the magnetic Kagome metal Fe$_3$Sn$_2$ were studied using the ultrafast optical pump-probe technique. Two carrier relaxation processes ($\tau_1$ and $\tau_2$) and a laser induced coherent optical phonon were observed. By using the two-temperature model for metals, we ascribe the shorter relaxation $\tau_1$ (~1 ps) to hot electrons transferring their energy to the crystal lattice via electron-phonon scattering. $\tau_2$ (~25 ps), on the other hand, cannot be explained as a conventional process and is attributed to the unconventional (localized) carriers in the material. The observed coherent oscillation is assigned to be a totally symmetric A$_{1g}$ optical phonon dominated by Sn displacements out of the Kagome planes, and possesses a prominently large amplitude, on the order of 10$^{-3}$, comparable to the maximum of the reflectivity change ($\Delta$R/R). This amplitude is equivalent to charge-density-wave (CDW) systems, although no signs of such an instability were hitherto reported in Fe$_3$Sn$_2$. Our results set an unexpected connection between Fe$_3$Sn$_2$ and kagome metals with CDW instabilities, and suggest a unique interplay between phonon and electron dynamics in this compound.