CRITICAL-BEHAVIOR OF THE SHEAR RESISTANCE OF SOLID HE-4 NEAR A STRUCTURAL PHASE-TRANSITION

We describe experiments on plastic flow of solid He-4 near the first o rder bcc-hcp transition. We find that the resistance to shear of the s olid diminishes strongly near the transition. We are able to distingui sh between plastic flow due to vacancies and due to dislocations, and show chat the critical behavior of the solid near the transition is co ntrolled by vacancy diffusion. The enhancement of the diffusion is con sistent with a coupling of the vacancies to a phonon which softens nea r the transition. Possible implications of our findings on the underst anding of melting are discussed.