J. Phys. Colloques
Volume 42, Numéro C6, Décembre 1981International Conference on Phonon Physics
|Page(s)||C6-122 - C6-124|
J. Phys. Colloques 42 (1981) C6-122-C6-124
ANHARMONICITY EFFECTS IN CaF2 AT LOW TEMPERATUREP.F. Tua et G.D. Mahan
Physics Department, Indiana University, Bloomington, IN 47405, U.S.A.
Using the framework of the rigid ion model, we have calculated the lifetime of longitudinal acoustic phonons in CaF2 at low temperature. The decay mode 1→1 + t is strongly anisotropic and of one order of magnitude smaller than the decay mode 1→t + t. For each single decay mode, the long-wavelength approximation which yields the ν5 dependence of 1/τ, is valid for ν ≤ 2.5 THz. At higher frequencies, the competing decay modes show different deviances which, curiously, cancel each other so that the total life-time can be well approximated by the long-wavelength limit up to 5.5 THz. We have also calculated the temperature dependence of the second and third order elastic constants and a good agreement is found with the available experimental data.