Migration of 3He impurities along dislocation lines in 4He single crystals

We have studied acoustic resonances in He-4 single crystals in the temperature region 20 < T < 300 mK where a large softening occurs due to the unbinding of dislocations from He-3 impurities. Here we present an analysis of the dependence of resonance frequencies on the sound amplitude, that is on the acoustic stress in the crystal. Above a threshold of order 10(-6) bar, this stress is able to unbind dislocations from impurities. Our results indicate that He-3 impurities diffuse along dislocations at 60 mK but not at 23 mK.