Localization and delocationzation of surface disordering in surface mediated melting

Melting of solids usually starts with a thin liquid layer forming on the surface at temperatures below their bulk melting point. While the liquid phase formation in the so-called premelting is understood to be governed by the interface energies among the coexisting solid, liquid, and vapor phases, the influence of the underlying crystal structures on its kinetics, and atomistic mechanisms are unknown, and consequently, ignored in the theory of melting. Here we report the first observation of a strong influence of the underlying crystal structure on melting kinetics with the resulting localization and delocalization behavior of the surface disordering and the liquid layers. With increasing temperature, the surface disordering remains in the localized state with a constant thickness in fcc crystals, whereas it becomes delocalized by growing steadily into the bulk in bcc crystals. In both cases, the surface melting is found to occur with highly correlated atomic motion in the form of extended atomic chains and loops that emit from the disordered surface and transmit to the bulk. This newly discovered mechanism in surface melting is behind the surface melting kinetics: The close packed fcc crystal can effectively impede the correlated atomic motion and limit the surface disordering to only the localized region on surface, while the more openly packed bcc crystal allows for its proliferation by tunneling from the surface into the bulk of the crystals. These findings provide valuable insights for future development of new theories of crystal melting.