Localised acoustic waves associated with a planar defect in a superlattice

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Abstract
The authors present a theoretical study of the existence of shear horizontal vibrations localised at a planar defect in a superlattice. Such a defect, which breaks the translational symmetry along the axis of the superlattice, is formed by a layer whose elastic parameters and thicknesses are different from those of the normal layers; it may result from inhomogeneities in the properties of the perfect superlattice. They show that localised shear horizontal modes associated with the planar defect may exist in the gaps of the superlattice or even below the bottom of the bulk bands and may in this case extend down to k/sub ///=0 (k/sub /// is the wavevector parallel to the layers). The dispersion relations of these waves are discussed as functions of the relative parameters of the superlattice and the planar defect. These results are qualitatively compared with those obtained in other surface and interface problems in which the existence of shear horizontal excitations have been investigated.