Charge Density Wave Coexisting with Amplified Nematicity in the Correlated Kagome Metal CsCr₃Sb₅

Correlated phenomena of flat bands have been extensively studied in kagome lattice materials and twisted bilayer materials. However, the emergent ordered states arising from interactions in intrinsic multi-orbital flat bands remain underexplored. The newly discovered kagome lattice metal CsCr₃Sb₅, with its magnetism, strong correlation, and multiple flat bands around the Fermi surface, provides a fertile platform for exploring multi-orbital physics in density waves and superconductivity under pressure, in contrast to vanadium-based AV₃Sb₅ (A = K, Rb, Cs). Here, using ultrafast optical techniques, we reveal the coexistence of charge density wave (CDW) and electronic nematic orders in CsCr₃Sb₅. Amplitude modes emerge below T^*, whose frequencies agree with a 1×4 CDW texture. Time-resolved birefringence measurements show that the CDW ordering reduces rotational symmetry from C₆ to C₂, creating three inequivalent nematic domains. Anisotropy time-resolved reflectivity measurements uncover an exotic nematicity, distinct from that observed in AV₃Sb₅, but it is amplified by the degeneracy lifting of the multi-orbital flat bands, akin to observations in iron-based superconductors. Our study thus pioneers the investigation of ultrafast dynamics in flat band systems at the Fermi surface, offering new insights into the interactions among charge, orbital and lattice elementary excitations in strongly correlated systems.