LEARNING ABOUT 3D MELTING WITH SOLID HELIUM

Theories of bulk melting rely on a sudden proliferation of crystalline defects in order to destabilize the lattice. However, these scenarios were never confirmed experimentally, as it is impossible to approach the transition sufficiently to see critical behaviour. In experiments investigating plastic pow of solid He-4, we observed such critical beh aviour involving a dramatic decrease of the resistance to shear near t he first order bcc-hcp transition at 1.772K on the melting curve. By m easuring the plastic flow over a wide range of sheer stress, we were a ble to differentiate between the contribution of vacancies and of disl ocations to this process. We argue that the mechanism by which the cry stal loses its shear resistance involves coupling of vacancy diffusion with a transverse phonon which softens as the transition is approache d. Dislocations seem to play no fundamental role in this process. Thes e results suggest a refinement of the Lindemann picture, as the phonon mode which plays a dominant role in this process is near the edge of the Brillouin zone.