Finite amplitude wave propagation in solid and liquidHe4

Abstract

The amplitude response (i.e., relation between input and output amplitudes of a wave transmitted through a medium) of 10-MHz longitudinal wave is investigated in hcp and bcc solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ as well as normal fluid and superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. In all four phases the acoustic saturation in the amplitude response curve was observed. The results are well described by an equation for amplitude loss based on second-harmonic generation. The nonlinear parameter $\beta$ of these phases was evaluated. It is found that the magnitude $\beta$ in solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is comparable to that of classical solids. Dislocation contributions to the amplitude response were also investigated and were ruled out as a significant source of the observed effects. It is concluded, therefore, that the large nonlinear amplitude response, which leads to saturation observed in solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ should be attributed to the unusually large compressibility, i.e., small elastic constants (second order), which, in turn, induces in solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ much larger strain amplitudes for a given applied stress than in ordinary solids.