Instability Modes Prior to Melting

An unexpected rise in the constant-volume heat capacity of bcc ${\mathrm{He}}^4$ is observed in the 0.02°K interval below the discontinuity at melting, giving thermodynamic evidence for the appearance of new modes of motion associated with the break up of the crystal. The appearance of slipping motion in molecular-dynamics computer experiments with hard-particle systems allows the nature of these instabilities to be examined.