Raman scattering study of charge density wave order in the magnetic kagome metal FeGe

The kagome lattice with corner shared triangle network is a fertile platform to study the exotic orders at the interplay between band topology and strong correlations. Recently, V-based kagome metal systems AV₃Sb₅ have arisen great interest for the observation of a giant anomalous Hall effect and chiral charge density order (CDW) as well as signatures of time-reversal symmetry-breaking in the absence of magnetic ordering. These systems are usually nonmagnetic and show weak electronic correlations. A strongly correlated version of AV₃Sb₅ is discovered in the hexagonal kagome metal FeGe which exhibits a CDW coexisting with a collinear A-type antiferromagnetic order. Here, we use polarization-resolved Raman spectroscopy to study the nature of the CDW order in FeGe. We detect a weak symmetry-forbidden fully-symmetric phonon mode in the collinear A-type antiferromagnetic phase. The existence of this mode indicates a doubling of the unit cell along the c-axis possibly due to strong spin-lattice coupling, forming dimerizations with short correlation lengths in the anti-ferromagnetic phase. In the subsequent CDW phase, we find that this mode intensity is significantly enhanced and that several additional phonon modes emerge below the CDW transition, indicating that FeGe establishes a 2 × 2 × 2 CDW order via a 2 × 2 × 1 transition and that the short-ranged dimerizations become long-ranged. Our results provide a solid basis for future studies of the nontrivial band topology and exotic orders in kagome metal systems.