Charge Dynamics of an Unconventional Three-Dimensional Charge Density Wave in Kagome FeGe

TL;DR

This study investigates the charge dynamics of kagome FeGe, revealing a complex three-dimensional charge density wave driven by structural transition, magnetic effects, and charge order interplay, distinct from other kagome metals.

Contribution

It provides the first detailed analysis of the unconventional 3D CDW in FeGe, highlighting the role of structural transition and magnetic splitting in charge dynamics.

Findings

Pronounced optical anisotropy along a- and c-axes.

Spectral weight transfer associated with CDW transition.

Emergence of new excitations around 1200 cm$^{-1}$.

Abstract

We report on the charge dynamics of kagome FeGe, an antiferromagnet with a charge density wave (CDW) transition at $T_{\mathrm{CDW}} \simeq 105$ K, using polarized infrared spectroscopy and band structure calculations. We reveal pronounced optical anisotropy along the $a$- and $c$-axis, as well as an unusual response associated with three-dimensional CDW order. Above $T_{\mathrm{CDW}}$, there is a notable transfer of spectral weight (SW) from high to low energies, promoted by the magnetic splitting-induced shift in bands. Across the CDW transition, we observe a sudden SW transfer from low to high energies over a broad range, along with the emergence of new excitations around 1200 cm$^{-1}$. These results contrast with observations from other kagome metals like CsV$_3$Sb$_5$, where the nesting of VHSs leads to a clear CDW gap feature. Instead, our findings can be accounted for by a $2\times2\times2$ CDW ground state driven by a first-order structural transition involving large partial Ge1-dimerization. Our study thus unveils a complex interplay among structure, magnetism, and charge order, offering valuable insights for a comprehensive understanding of CDW order in FeGe.