Flipover process

Apart from phonon-phonon scattering, which is allowed by the anharmonicity of the interatomic potentials, the umklapp or flipover process is another effect that decreases the mean free path of the phonons and the conductivity two short-wavelength waves may interfere to form one long-wavelength wave going back (Figure 4.38). The lattice periodicity contributes to the umklapp process, a reflection of drifting phonons. 

Figure 4.38. Schematic of the flipover process of phonon scattering at high temperatures. Two phonons (wave packets) interfere and form a phonon with a shorter wavelength. In a periodic lattice this is the same as a long-wavelength phonon traveling in the opposite direction. The number of short-wavelength phonons in the lattice increases with temperature.

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