Picosecond ultrasonic study of localized phonon surface modes in Al/Ag superlattices

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Abstract
We present the results of picosecond ultrasonic studies of localized acoustic-phonon surface modes in Al/Ag superlattices. By making measurements on a series of superlattices with different periods, we have been able to study surface modes with frequencies in the range from 110 to 670 GHz. The experiment reveals interesting variations in the surface-mode structure as the geometry of the cap layers is varied. We compare these results with the predictions of a transfer-matrix theory of the localized modes. The measured damping rate of the modes varies linearly with frequency and is nearly independent of temperature in the temperature range from 70 to 300 K. This variation of the attenuation with frequency and the order of magnitude of the attenuation are consistent with the assumption that the attenuation rises from an interaction between the phonon mode and the electrons, but the damping rate is about a factor of 5 larger than expected based upon Pippard’s theory for bulk materials. It is likely that the enhancement in the damping rate is primarily due to losses occurring at the free surface of the structure.