Temperature Pulses in Dilute LiquidHe3-He4Mixtures near 0°K

The speed of second sound in dilute liquid ${\mathrm{He}}^3$-${\mathrm{He}}^4$ mixtures in concentrations up to 0.35% ${\mathrm{He}}^3$ has been measured from 0.2 to 1°K at different pressures up to 20 atm. A pulsed time-of-flight technique was used. The second-sound values have been analyzed on Pomeranchuk's model, and the effective mass of the ${\mathrm{He}}^3$ excitations has been obtained at different pressures, as well as different values of the roton parameters $\Delta$ and $\mu$. The ratios of the effective mass of ${\mathrm{He}}^3$ to the mass of a ${\mathrm{He}}^3$ atom were found to be 2.55 at 46 cm Hg, 2.67 at 5.4 atm, 2.84 at 11.0 atm, and 3.03 at 19.8 atm. At the lowest temperature and especially at high pressures, the observations were complicated by poor coupling between phonons and ${\mathrm{He}}^3$ excitations in the liquid, giving rise to two received pulses. From these measurements an estimate was made of a quantity ${\lambda }^{*}$ that is thought to be closely related to the mean free path of phonons scattered by ${\mathrm{He}}^3$ atoms. It was found that ${\lambda }^{*}\propto T^{-4\mathrm{}}$.