The giant plasticity in $^4$He crystals

We have applied very small shear stresses (down to 1 nanobar) to oriented single $^4$He crystals, and directly measured their response as a function of temperature (from 15 mK to 1 K), orientation, crystal quality, $^3$He concentration, frequency and shear stress magnitude. For particular orientations, we have found a giant plasticity that is reversible, associated with the elastic coefficient c$_{44}$ which nearly vanishes around 200 mK. Other elastic coefficients show no measurable anomaly. The strong reduction of c$_{44}$ (80\% in high quality crystals with no impurities) shows that dislocations glide in the basal plane of the hexagonal structure with no dissipation. This plasticity disappears as soon as traces of $^3$He impurities bind to the dislocations (at low T) or if their motion is damped by collisions with thermal phonons (at higher T). It has no equivalent in classical crystals.