We have performed picosecond ultrasonic studies on the surface and bulk acoustic phonons in amorphous $\mathrm{M}\mathrm{o}/\mathrm{S}\mathrm{i}$ superlattices. Localized surface modes within the first, second, and sixth frequency gaps of the zone-folded phonons are observed. A selection rule derived from symmetry considerations provides new understanding of why certain modes are seen and not the others. The excitation strengths and detailed spectral features of these lines are studied, and the results are well explained by an elastic-continuum theory. It is found that the line shapes are significantly modified by the presence of bulk modes near the zone center.

• Received 9 May 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.076101