Finite amplitude wave propagation in solid and liquid 4He

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 4He as well as normal fluid and superfluid 4He. 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 β of these phases was evaluated. It is found that the magnitude β in solid 4He 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 4He should be attributed to the unusually large compressibility, i.e., small elastic constants (second order), which, in turn, induces in solid 4He much larger strain amplitudes for a given applied stress than in ordinary solids