The role of point defects in melting of solid He

We have recently observed that the resistance to shear of solid He-4 decrea ses dramatically near the first-order BCC-HCP transition. In our view, the solid shears via a diffusive counter-flow of atoms and point defects. The m echanism of self diffusion couples point defects with one specific phonon w hich softens as the transition is approached. Since such a scenario can pos sibly lead to melting, it is important to understand (a) which type of poin t defect is associated with the reduction of sheer resistance, and tb can t he presence of point defects lead to the softening of phonons. We report he re the results of numerical simulations and analytic modeling. Our results indicate that split interstitials are much more effective than vacancies in lowering the resistance to shear. We suggest that these excitations can he generated as a result of a "local mode" excited in the crystal. (C) 2000 E lsevier Science B.V. All rights reserved.