STRESS DRIVEN INSTABILITY AT THE CRYSTAL-MELT INTERFACE OF HE-4

We have studied the effect of nonhydrostatic stresses on the equilibri um shape of pure helium-4 crystals[1]. First, by applying small stress es, a macroscopic melting of the crystal is produced which is quadrati c in the strain and can be quantitatively understood using simple ther modynamics and elasticity theory[2]. Secondly above a certain threshol d in strain, easily visible grooves appear on the crystal surface[1][3 ]. This is the first quantitative evidence for the general instability which was first predicted by Grinfeld[4].