Observation of Γ -point phonon frequency in ultrathin metallic films confirmed by ab initio calculation and lattice dynamics

Vibrations in ultrathin metallic films excited by ultrafast light pulses have been studied based on continuum mechanics. However, this paper shows that they are Γ -point phonon vibrations of plate-phonon modes. Ab initio and lattice dynamics calculations are made to compare Γ -point phonon vibrational frequencies with measurements obtained by the picosecond ultrasound spectroscopy. The standing-wave frequencies of specific Γ -point phonon modes of the slab model show good agreement with measurements without any fitting parameters. This study informs us of a limitation of the continuum-mechanics theory for explaining the mechanics of ultrathin metallic films.