Ultrasound propagation in polycrystalline solid $^4$He

We are carrying out measurements of 10 MHz longitudinal ultrasound propagation in polycrystalline solid $^4$He samples grown by the blocked capillary method. Temperature dependence of the velocity and attenuation of ultrasound are measured. The observed temperature dependence during cooling runs at $T >$ 200 mK can be described qualitatively in terms of the effects of the motion of dislocation lines present in the samples. At $T <$ 100 mK significant deviations from the higher temperature behavior are observed. Sharp anomalous changes in the velocity and attenuation appear near 70 mK. At the end of a cooling run at 20 mK, if the ultrasound excitation pulse amplitude is decreased below a threshold level, the attenuation decreases to a minimum and it remains constant at the minimum as the pulse amplitude is increased back up. The anomalous temperature dependence and the hysteretic behavior are discussed as possible consequences of $^3$He impurity atoms being ``condensed" onto dislocation lines.