Abstract
The validity of a four-parameter semi-quantum (FPSQ) model to fit infrared reflectivity spectra of crystals having a large number of phonon branches is discussed in view of the results of anharmonic quantum theories. The form of the quantum damping function may indeed be simplified when the two-phonon density of states resembles a broad continuum in the vicinity of the resonance frequency. Infrared reflectivity data of corundum obtained at temperatures ranging from 77 to 1775K are fitted with the aid of the FPSQ model. The temperature dependence of damping of TO and LO modes is analysed and it is found that phonon lifetimes are essentially limited by anharmonic three-phonon coupling described by the cubic hamiltonian consistent with two qualitative arguments.
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