Anomalous Phonon-Vibration Frequencies in Metallic Ultrathin Films Studied by Resonant Acoustic-Phonon Spectroscopy and Ab-initio Calculations

The phonon frequencies of metallic ultrathin films were studied by the resonant acoustic-phonon spectroscopy using femtosecond pump-probe method. We excite and detect the acoustic-phonon resonances in the thin films thinner than 50 nm using the femtosecond light pulses. The pump light causes the phonon vibrations through the thermal expansion at the film surface. For the ultrathin films, the standing wave occurs, yielding the non-propagating phonon frequencies. The results show that when the film thicknesses are less than 10 nm, the measured phonon frequencies are significantly larger than predictions from the continuum mechanics of elasticity. Our results of ab-initio calculations showed qualitative agreement with observed frequencies.