Critical dislocation speed in helium-4 crystals

Our experiments show that in He-4 crystals, the binding of He-3 impurities to dislocations does not necessarily imply their pinning. Indeed, in these crystals, there are two different regimes of the motion of dislocations when impurities bind to them. At low driving strain epsilon and frequency omega, where the dislocation speed is less than a critical value (45 mu m/s), dislocations and impurities apparently move together. Impurities really pin the dislocations only at higher values of epsilon omega. The critical speed separating the two regimes is two orders of magnitude smaller than the average speed of free He-3 impurities in the bulk crystal lattice. We obtained this result by studying the dissipation of dislocation motion as a function of the frequency and amplitude of a driving strain applied to a crystal at low temperature. Our results solve an apparent contradiction between some experiments, which showed a frequency-dependent transition temperature from a soft to a stiff state, and other experiments or models where this temperature was assumed to be independent of frequency. The impurity pinning mechanism for dislocations appears to be more complicated than previously assumed.