Instability of the charge density wave in the kagome magnet FeGe

Kagome metals show rich competing quantum phases due to geometry frustration, flat bands, many-body effects, and nontrivial topology. Recently, a novel charge density wave (CDW) was discovered deep inside the antiferromagnetic phase of FeGe, attracting intense attention due to the close relation with magnetism. Here, via a scanning tunneling microscope (STM), we find the 2 x 2 CDW in FeGe is very fragile and can be readily disrupted into the initial 1 x 1 phase; small /3 x /3 CDW puddles are found to coexist with the 2 x 2 CDW in as-grown samples, and can also be induced in the intermediate process of CDW disruption, which will eventually transform into the initial 1 x 1 phase. Moreover, an exotic intermediate CDW state and standalone CDW nuclei appear unexpectedly during the disruption process. Our first-principles calculations find equal softening of a flat optical phonon mode in a large momentum region around the CDW wave vector, corresponding to numerous competing CDWs with close energies. This might lead to strong instability of the CDW ground state, responsible for STM observations. Our findings provide more interesting experimental aspects to understand the CDW in FeGe and suggest that FeGe-like kagome metals are ideal platforms for studying the physics of competing CDW instabilities.