HELIUM CRYSTALS UNDER STRESS - THE GRINFELD INSTABILITY

We present the first quantitative experiment on the equilibrium shape of crystals under nonhydrostatic stress. By studying very pure helium-4 crystals, evidence was obtained that small nonhydrostatic stresses produce a macroscopic melting, as predicted by calculations based on elastic theory [1]. Furthermore, above a certain threshold in strain, large grooves appear on the crystal surface, in agreement with the instability first predicted by Grinfeld [2]. Our results also agree with a more qualitative study done by M. Thiel et al. [3]. We finally discuss the relaxation of stresses as a possible consequence of the motion of crystal defects.