The evolution of a pulse of noninteracting quasiparticles, caused by their different velocities and angular distribution of momenta, is studied theoretically. Equations are found that describe the shape of the pulse surface at any time. The times of the beginning and end and the duration of the quasiparticle energy flux density is determined at a general spatial point. The quasiparticle energy density is considered at all times and positions, and it is shown that in the region of high energy density, in the middle of the pulse, it is equal to the initial energy density under certain conditions. These theoretical results are discussed in relation to experimental data on the evolution of a pulse of noninteracting phonons in superfluid helium.

Topics
Phonons, Superfluids, Quasiparticle, Energy flux density
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