Observation of the Mechanical Response of the Condensate in He-Bin the Regime of Negligible Normal Fluid Density

Abstract

Experiments are described in which superfluid $^{3}\mathrm{He}$-$B$ is cooled to sufficiently low temperatures that the normal-fluid fraction becomes so small ($\frac{{\ensuremath{\rho}}_{n}}{\ensuremath{\rho}}<{10}^{\ensuremath{-}6}$) that the quasiparticle properties become undetectable. The liquid $^{3}\mathrm{He}$ is then effectively a thermal vacuum. In this regime nonlinear effects are observed in the behavior of a vibrating wire, effects which are not observed in an actual vacuum and which are evidently associated with the mechanical response of the $B$-phase superfluid texture.