In their Letter, Haziot et al. report a novel phenomenon of giant plasticity for hcp 4He quantum crystals. They assert that 4He exhibits mechanical properties not found in classical plasticity theory. Specifically, they examine high-quality crystals as a function of temperature T and applied strain ϵapp, where the shear modulus µ =Tapp/ϵapp reaches a plateau and dissipation 1=Q becomes close to zero; both quantities are reported to be independent of applied stress Tapp and strain ϵapp, implying a reversible dissipation process and suggesting dislocation motion by quantum tunneling. At lower T, an increase in µ and 1/Q is found, which is argued to be caused by 3He atoms binding to dislocations, thus pinning them and stiffening the solid.

In this Comment, we show that these signatures can be explained with a classical model of thermally activated dislocation glide without the need to invoke quantum tunneling or dissipationless motion. Recently, we proposed a dislocation glide model in solid 4He containing the dissipation contribution in the presence of other dislocations with qualitatively similar behavior.