Using a simple one-dimensional model, we show that the existence of the energy gap for excitations in an isotropic BCS superfluid leads to strongly nonlinear mechanical behavior of the liquid in the ballistic quasiparticle limit. The nonlinear damping of a vibrating wire in ${}_{}{}^3{}_{}{}^{}\mathit{B}$ below 200 μK is explained, both in its velocity dependence and magnitude. At modest velocities (υ>kT/${\mathrm{p}}_{\mathrm{F}}$), the damping force on an object moving through the superfluid becomes independent of velocity, an unexpected result with several interesting implications.

• Received 7 August 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.2566