Ultrafast coherent phonon dynamics in the kagome magnet Gd V6 S n6

The kagome intermetallic compound RV6Sn6 (R=rare earth) not only supports unique lattice geometries and nontrivial topological band structures but also exhibits long-range magnetic order at low temperatures, providing an ideal platform for studying interactions among electrons, phonons, and magnetism. Phonons play a crucial role in the coupling between magnetic order and topological fermions, whose dynamics are essential for understanding the complex multiple degrees of freedom interaction in RV6Sn6. In this work we systematically investigate the temperature-dependent coherent phonon dynamics of GdV6Sn6, and two characteristic temperature points have been identified (i.e., Te at ∼100K and Tm at ∼5K). Specifically, we have observed the anomalous softening and suppressed decaying of the Ag4 phonon mode when approaching Te, which may result from the change of density of states near the Fermi surface, while around Tm, the hardening and accelerated decaying of the Ag4 phonon mode emerges since the buildup of magnetic order alters the phonon energy renormalization and opens an additional scattering channel.