Low frequency acoustic oscillations of twin superstructures in ferroelastics near the phase transition

Low frequency collective oscillations in a superlattice consisting of alternating highly anisotropic layers are considered. This superstructure may be formed in the ferroelastic near the structural phase transition by alternation of twins. For the surface waves propagating along the layers, the conditions and the range of existence of those with the dispersion law ω ∼ k½, characteristic for two-dimensional plasmons, have been analyzed for a solid-state system with consideration for elastic anisotropy and retardation of acoustic waves. It is shown that such oscillations exist in a frequency range which becomes markedly wider near the ferroelastic phase transition and the “optimum” geometry of the phase matching for which there arise collective excitations of the type under consideration is found. For transverse and longitudinal waves propagating across the layers the existence is proved of low frequency acoustic branches separated by a wide gap from the nearest optical branches.