Nonlinear wave propagation in Fermi liquids with resonant excitations across an energy gap: Application to superfluid 3He

The propagation of waves in a superfluid Fermi liquid is discussed in the quasiclassical regime on the basis of a nonlinear kinetic equation. We show that the dominant nonlinearity appears in the distribution function (rather than the mean fields) due to resonant excitations across the gap in a small regime of momentum space. The dynamics of these resonantly excited quasiparticles is governed by a Bloch-type equation for a pseudospin vector in particle-hole space. We consider saturation effects and soliton formation in the sound propagation in 3He-A in some detail and comment briefly on the B phase