Sound conversion in impure superfluids - art. no. 140507

We consider the conversion of first sound into second sound and the reverse in different impure superfluid systems. We calculate the coupling between temperature (entropy) oscillations and pressure (density) oscillations for impure superfluids (including He-3-He-4 mixtures) and for superfluid He (bo th He-3 and He-4) in aerogel and show that sound conversion in these system s is governed by c(partial derivative rho/partial derivativec) or by sigma rho (a)rho (s) (instead of partial derivative rho/partial derivativeT, whic h is enormously small in pure He). This replacement plays a fundamental rol e in the sound conversion phenomenon in impure superfluids and superfluids in aerogel. It enhances the coupling strengths of the two sounds and decrea ses the threshold values for nonlinear processes in homogeneous superfluids . Some phenomena such as the slow mode of superfluid He-3 and He-4 in aerog el, and heat pulse propagation with the velocity of first sound in HeII in aerogel can be understood as a double sound conversion phenomena.