We have measured the elasticity of high quality ultrapure ${}^4\mathrm{He}$ single crystals in the low temperature region where supersolidity is supposed to occur. At 20 mK, our results are consistent with elastic coefficients previously measured at 1.2 K. As the temperature increases from 20 to 100 mK, a large softening occurs because dislocations unpin from ${}^3\mathrm{He}$ impurities. In the absence of ${}^3\mathrm{He}$ impurities, dislocations are free to move down to 20 mK; the crystals are soft. The large magnitude of this anomalous softening shows that dislocations form a mobile mosaic structure. It illustrates the remarkable quantum plasticity of ${}^4\mathrm{He}$ crystals.

• Received 24 June 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.145302