Elucidating the structure of the charge density wave in the topological kagome metal CsV₃Sb₅

The recent discovery of the AV₃Sb₅ (A = K, Rb, Cs) material family offers an exciting opportunity to investigate the interplay of correlations, topology, and superconductivity in kagome metals. Emerging from a topologically nontrivial band structure, an unusual charge density wave phase dominates the low energy physics of these materials. The observation of a giant anomalous Hall effect and chiral charge order suggest that this charge density wave may spontaneously break time reversal symmetry, even while there is no evidence of local moment magnetism. A combination of ultrafast coherent phonon spectroscopy and first-principles density functional theory calculations is used to examine the structure of the charge density wave order in CsV₃Sb₅. It is found that the charge density wave results from a simultaneous condensation of three optical phonon modes and can be described as tri-hexagonal ordering with an interlayer transverse shift. This distortion breaks C₆ rotational symmetry of the crystal and may offer a natural explanation for reports of uniaxial order in this material family. These results highlight the important role of characterization and modeling in deciphering the exotic properties of topological kagome metals.