Frustrated phonon with charge density wave in vanadium Kagome metal.

The formation of a star-of-David charge density wave 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 AVSb (where A represents K, Rb, or Cs). However, understanding of the underlying mechanism behind charge density wave 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 cesium ions' out-of-plane motion, becomes frustrated in the charge density wave phase. Furthermore, we observed the photoinduced emergence of a metastable charge density wave 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 insights into the competition between phonons and periodic lattice distortions, a phenomenon widespread in other correlated quantum materials including layered high-temperature superconductors.