Localized mixed acoustic modes in non-cubic-axial superlattice with a cap layer

We study theoretically the properties of the localized mixed acoustic phonon modes lying in the interior minigap in a superlattice (SL) grown away from cubic axial of bulk material with a cap layer by using a transfer-matrix method. We find the appearance and the characteristics of the mixed localized modes in the interior minigap strongly depend on the growth direction and the structure parameters of the superlattice. Compared with the case in infinite superlattice with a structural defect layer, the localized modes exhibit some novel features. The dependence of the interior minigap width on the growth direction of the superlattice is also demonstrated. This provides a theoretical basis for experimental observation of the mixed localized acoustic modes and the interior minigap in SLs.