Terahertz Phonons in Highly Disordered Crystals
The relaxation of non-equilibrium THz phonons in the mixed-crystal systems Cal−xSrxF2 and Cal−2LaxF2+x has been studied at low temperature. In the Ca1−zSrxF2 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 Cal−xLaxF2+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.
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- U. Happek, R.O. Pohl. Renk in Phonons 89, eds. S. Hunklimger, W. Ludwig, and G. Weis ( World Scientific, Singapore 1990 ), 925Google Scholar