Kitsenko Y. Theory of relaxation processes in isotropic and anisotropic phonon systems of superfluid helium

The thesis is devoted to investigation of relaxation processes in anisotropic and isotropic phonon systems of superfluid helium. The rates of three-phonon processes in the anisotropic and isotropic systems of superfluid helium are calculated. Process of a creation of high-energy phonons by a pulse of low-energy phonons in superfluid helium is considered. The evolution of a short l-phonon pulse which is caused by a creation of high-energy phonons is considered. Processes of one phonon decay into three are investigated and it is shown that due to these processes equilibrium is quickly attained in the momentum region where processes of one phonon decay into two are forbidden. The problem of interaction of phonon pulses in superfluid helium is considered. The damping factor in each of cases is found. The obtained results allow to explain existing experimental data and to offer new experiments. Keywords: superfluid helium, phonon, relaxation, anisotropic phonon systems, three-phonon processes, four-phonon processes, interaction of phonon pulses.