Localized folded acoustic phonon modes in coupled superlattices with structural defects

Using a transfer-matrix method, we investigate the property of the localized folded acoustic phonon modes in superlattices (SL's) with structural defect layers in the elastic approximation. We find that there exist localized folded acoustic phonon modes inside the minigaps of the coupled SL's grown along a cubic axis of bulk materials when introducing structural defect layers, and the frequency and the localization degree of these localized modes strongly depend on the widths of the structural cell and the defect layers. Moreover, we also notice an interesting change of the parity of the localized modes in the same minigap for different widths of defect layers.