Critical behavior of the shear resistance of solid 4He near a structural phase transition

We describe experiments on plastic flow of solid He-4 near the first order bcc-hcp transition. We find that the resistance to shear of the solid diminishes strongly near the transition. We are able to distinguish between plastic flow due to vacancies and due to dislocations, and show chat the critical behavior of the solid near the transition is controlled by vacancy diffusion. The enhancement of the diffusion is consistent with a coupling of the vacancies to a phonon which softens near the transition. Possible implications of our findings on the understanding of melting are discussed.