Terahertz Phonons in Highly Disordered Crystals

Abstract

The relaxation of non-equilibrium THz phonons in the mixed-crystal systems Ca_l−xSr_xF₂ and Ca_l−2La_xF_2+x has been studied at low temperature. In the Ca_1−zSr_xF₂ system we find only a moderate (up to a factor of two) increase of the relaxation rate, even at high Sr concentrations, and a v⁵ frequency dependence of the relaxation rate, as is characteristic for spontaneous phonon decay due to lattice anharmonicity, similar to that found in pure CaF₂ and SrF₂. In Ca_l−xLa_xF_2+x however, La concentrations in the order of even 1% reduce the lifetime by up to two orders of magnitude. Furthermore, the relaxation rate increases with the cube of the frequency, indicating the existence of a new mechanism dominating the relaxation of the phonons.