Velocity and Absorption Coefficient of Sound Waves in Classical Gases

The velocity and absorption coefficient of plane sound waves in classical gases are obtained by solving the Boltzmann kinetic equation. This is done within the linear response theory as a reaction of the single-particle distribution function to a periodic external field. The nonperturbative dispersion equation is derived in the relaxation time approximation and solved numerically. The obtained theoretical results demonstrate an universal dependence of the sound velocity and scaled absorption coefficient on the variable wт , where w is the sound frequency, and т−1 is the particle collision frequency. In the region of wт ∼ 1, a transition from the frequent- to rare-collision regime takes place. The sound velocity increases sharply, and the scaled absorption coefficient has a maximum – both theoretical findings are in agreement with the data.

Autoresonant Excitation of Spin Wave "Dark" Solitons

In this paper we propose a method to generate pure dark solitons by a periodic external field with a slowly varying frequency, which allows to control amplitude of the excited solitons and period of soliton trains. Our approach to generate dark solitons is based on the effect of autoresonance when the excited wave is phase-locked by the drive after the crossing the resonance. The resonant frequency and the threshold condition on the amplitude of the driving were found.