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

We report on the charge dynamics of kagome FeGe, an antiferromagnet with a charge density wave (CDW) transition at T_{CDW}≃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_{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 2×2×2 CDW ground state driven by a first-order structural transition involving large partial Ge1 dimerization. Our Letter thus unveils a complex interplay among structure, magnetism, and charge order, offering valuable insights for a comprehensive understanding of CDW order in FeGe.