Critical dislocation speed in helium-4 crystals

TL;DR

This study reveals that in helium-4 crystals, dislocation-impurity interactions depend on a critical speed, challenging previous assumptions about impurity pinning and explaining frequency-dependent mechanical behavior.

Contribution

It demonstrates the existence of a critical dislocation speed in helium-4 crystals, clarifying the conditions under which impurities pin dislocations versus when they move freely.

Findings

Dislocation speed below 45 μm/s results in impurities moving with dislocations.

Impurities pin dislocations only at higher dislocation speeds and frequencies.

The critical speed is much lower than the impurity's free movement speed in the lattice.

Abstract

Our experiments show that in $^4$He crystals, the binding of $^3$He 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 lowdriving 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 $\omega$. The critical speed separating the two regimes is two orders of magnitude smaller than the average speed of free $^3$He 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.