Two-Fluid Theory of Phonon Scattering in Dielectric Crystals
In order to describe the transport characteristics of thermal phonons, we propose to divide them into two categories. The first consists of relatively nondispersive phonons with interactions dominated by anharmonic normal (N) processes. This category defines a propagation group that transports energy. The second category is dominated by resistive processes and constitutes a group that extracts momentum and energy from the propagation group, and passes it on to an external heat bath. Thus, this second group acts as a thermal reservoir, thermal resistance appearing through the scattering of propagating phonons into these reservoir modes. Towards high temperature (T), the resistive processes that determine the reservoir are principally umklapp (U) processes.
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