Experimental evidence for coupled-mode phonon gaps in superlattice structures

Frequency gaps for acoustic phonons with a propagation direction oblique to the superlattice layers are investigated using phonon transmission spectroscopy in amorphous superlattices. In addition to the gaps at the center and at the boundaries of the superlattice mini-Brillouin-zone, the phonon transmission spectra show gaps associated with stop bands for coupled phonon modes with mixed longitudinal and transverse character. The position of the gaps is well explained by a description of the phonons in the elastic continuum limit.