Longitudinal Acoustic Impedance and the Squashing Mode in SuperfluidHe3-B

Abstract

We have observed an unexpectedly strong effect of the squashing mode in $^{3}\mathrm{He}$-$B$ on the longitudinal acoustic impedance $Z$. Most peculiar is the behavior of the reactive part ${Z}^{\ensuremath{'}\ensuremath{'}}$ which displays both large positive and negative excursions. Our analysis illustrates the role of the boundary conditions on the superfluid order parameter and of the crossover with temperature of the squashing mode from a propagating wave with velocity ${c}_{\mathrm{sq}}$ (which is measured) to a nonpropagating surface wave (whose imaginary velocity accounts for the anomalies of ${Z}^{\ensuremath{'}\ensuremath{'}}$).