Phonon spectra and local vibrations in layered crystals

We have analyzed the spectral density and conditions for appearance of local vibrational frequencies in layered crystals from a microscopic viewpoint, using the method of Peresada. In particular, we demonstrate that the threshold for the appearance of local vibrations of isotopic impurities is considerably lowered, with respect to that of usual non-layered crystals, for vibrations in the basal plane, while it is considerably increased for impurity vibrations perpendicular to the plane.

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