Quasiequilibrium distribution function of anisotropic phonon systems and the interaction of pulses of low-energy phonons in superfluid helium

Abstract

The phenomenon of a hot line forming in liquid helium was observed in experiments carried out in the University of Exeter (UK). It arises when two phonon pulses interact and this is theoretically investigated in this paper. To develop the theory we start from the exact quasiequilibrium distribution function that describes anisotropic phonon systems such as a phonon pulse in superfluid helium. This is related to the approximate distribution function, which is more physically intuitive and was used earlier. The local equilibrium distribution function for phonons in the region of a hot line is obtained from the distribution functions for the phonons in the two interacting pulses. In order to explain the results of experiments, we analyze the effect of different pressures when the angle between the two moving pulses in superfluid helium is constant and also the effect of different angles at the saturated vapor pressure. The conditions suitable for the creation of a hot line are found. The results of the calculations are compared with the experimental data.