Frustrated phonon with charge density wave in vanadium Kagome metal

The formation of a star of David CDW superstructure, resulting from the coordinated displacements of vanadium ions on a corner sharing triangular lattice, has garnered significant attention to comprehend the influence of electron phonon interaction within geometrically intricate lattice of Kagome metals, specifically AV3Sb5 (where A represents K, Rb, or Cs). However, understanding of the underlying mechanism behind CDW formation, coupled with symmetry protected lattice vibrations, remains elusive. Here, from femtosecond time resolved X ray scattering experiments, we reveal that the phonon mode, associated with Cs ions out-of-plane motion, becomes frustrated in the CDW phase. Furthermore, we observed the photoinduced emergence of a metastable CDW phase, facilitated by alleviating the frustration. By not only elucidating the longstanding puzzle surrounding the intervention of phonons but introducing the phononic frustration, this research offers fresh insights into the competition between phonons and periodic lattice distortions, a phenomenon widespread in other correlated quantum materials including layered high TC superconductors.