Theory of microwave absorption in wide-band-gap insulators: The role of thermal phonon lifetimes

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Abstract
Earlier it has been argued that, in insulating materials, a proper calculation of the contributions to the far-infrared and microwave absorption rate from two-phonon difference processes must take explicit account of the influence of the finite lifetime of the thermal phonons which participate. We explore this issue further here. After initial remarks on the means of incorporating these effects into the computation of the absorption rate, we present numerical studies of the frequency and temperature variations of the low-frequency absorption constant for NaCl and KI, through use of a model which accurately reproduces the phonon spectrum of these materials. Our results provide a reasonable account of the existing data on these materials, and contain predictions that can be tested by further experimental work.