Intriguing Low-Temperature Phase in the Antiferromagnetic Kagome Metal FeGe

The kagome lattice is a focal point for exploring diverse quantum phenomena, with recent attention on the antiferromagnetic kagome metal FeGe showing a unique interplay between magnetic and charge ordering phenomena. However, the interpretation of the low-temperature phase of FeGe remains unclear, considering that the putative charge-density-wave (CDW) transition does not lead to any anomaly in the electrical resistivity.

In a comprehensive infrared spectroscopy study, complemented by ab-initio calculations, we address this enigmatic low-temperature phase. Key findings of our research include the discovery of significant changes in the low-energy interband absorption during the structural phase transition previously associated with the formation of a CDW. Aided by density functional theory calculations, we reveal that this effect originates in the reconstruction of the bands due to minuscule Fe displacements in the kagome planes, resulting in parallel bands near the Fermi level. Importantly, our findings demonstrate that, unlike in conventional CDW materials, the spectral weight shifts to low energies, thus ruling out the opening of a CDW gap in FeGe.

Our work calls for a reconsideration of FeGe physics. We show that the low-temperature phase, previously believed to be a CDW, in fact lacks optical signatures of the CDW. We interpret this phase merely as a superstructure that arises from a mostly structural phase transition, where dimerization of the non-kagome Ge atoms plays a crucial role.