Anharmonic scattering processes and relaxation of quasi-transverse modes in cubic crystals with positive or negative anisotropy of second-order elastic moduli

Abstract

Relaxation of long-wavelength quasi-transverse modes due to anharmonic scattering processes is considered. An anisotropic continuum model is used to analyze the dependence of the relaxation in cubic crystals on the direction of the wave vector of quasi-transverse phonons for the Landau-Rumar mechanism both in the quasi-isotropic approximation and in the case where the cubic anisotropy is rigorously taken into account. The quasi-isotropic approximation is shown to be inadequate for quantitative description of the anisotropy in the relaxation rates of quasi-transverse phonons. It is found that not only the spectra and polarization vectors of phonons but also the angular dependences of the relaxation rates of quasi-transverse vibrational modes are different in cubic crystals with positive and negative anisotropy in the second-order elastic moduli.