Rethinking Lindemann criterion: A molecular dynamics simulation of surface mediated melting

Lindemann criterion has been the orthodox view for melting, and its interpretation of both the disordering process of the crystalline solids and formation of the liquid phases involved in melting extends to many other branches of science. However, an increasing number of works rebuke the universal criterion conceived by Lindemann in perfect bulk crystals. Here we examine the criterion in a more realistic setting of surface mediated melting from atomistic modeling. As a defect, surface helps mediating melting both on the surface and in the bulk. While this general relation is known, detailed mechanisms and atomic structural changes remain elusive, especially in relation to the Lindemann criterion. Our extensive molecular dynamics simulations show that in bcc Ta, the surface disorders first but still remains in crystalline state; and the bulk melts when the surface liquid layer forms and subsequently grows inside the bulk. The Lindemann parameter during the entire melting process remains a variable rather than a (universal) constant that supposedly predicts the onset of surface as well as the bulk melting. In addition, we observe surface diffusion and formation of correlated atomic chains and loops that are originated from the surface and permeate into the bulk crystals, causing the bulk to melt.