Einfluß der Umklapp-Prozesse auf die hydrodynamischen Bewegungsformen der Phononen in Kristallen

The influence of U-processes on second sound and Poiseuille flow of the phonons in dielectric solids is investigated. Longitudinal and transverse phonons are allowed for throughout. In the first two sections the mean free time approximation is used. Various ratios of the mean free times for U- and N-processes are considered. In the third section the linearized Boltzmann-Peierls equation is solved by means of a double perturbation calculation, whereU-frequencies as well as hydrodynamic frequencies are considered as small, and the damping of second sound is calculated.