טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentDanzig Almog
SubjectThe Effective Surface Shear Modulus of Solid Gallium near
Surface Melting
DepartmentDepartment of Physics
Supervisor Professor Emeritus Emil Polturak


Abstract

Historically, melting is one of the most extensively studied physical phenomena. The attributes of melting include the loss of the crystallographic order, a latent heat, vanishing of the shear modulus and a discontinuous change of the density. Potential mechanisms of melting include the enhancement of atomic vibrations and surface melting. Intuitively, melting should begin in a weak region of the solid, such as the surface or a grain boundary, from where the liquid phase will propagate into the bulk. For that reason, surface melting is the most likely precursor of melting. It is often called premelting, as it takes place at a lower temperature than melting of the bulk. Until now, surface melting was investigated exclusively by diffraction or other non-contact techniques, revealing how the surface becomes progressively disordered with temperature. Those previous measurements show that unlike bulk melting, surface melting is a gradual transition. We designed a method to measure an effective shear modulus of the surface, another property which should change at melting. This property was not measured before. We applied this method to surface melting of Gallium, and found that this effective surface shear modulus vanishes abruptly near the onset of premelting, about 9K below the bulk melting temperature. Vanishing of the shear modulus is the only mechanical attribute of solid-liquid transition in bulk melting.  In that, we are the first to show the appearance of fluidity during surface melting.