String-like cooperative motion in homogeneous melting

Abstract

Despite the fundamental nature and practical importance of melting, there is still no generally accepted theory of this ubiquitous phenomenon. Even the earliest simulations of melting of hard discs by Alder and Wainwright indicated the active role of collective atomic motion in melting and here we utilize molecular dynamics simulation to determine whether these correlated motions are similar to those found in recent studies of glass-forming (GF) liquids and other condensed, strongly interacting, particle systems. We indeed find string-like collective atomic motion in our simulations of "superheated" Ni crystals, but other observations indicate significant differences from GF liquids. For example, we observe neither stretched exponential structural relaxation, nor any decoupling phenomenon, while we do find a boson peak, findings that have strong implications for understanding the physical origin of these universal properties of GF liquids. Our simulations also provide a novel view of "homogeneous" melting in which a small concentration of interstitial defects exerts a powerful effect on the crystal stability through their initiation and propagation of collective atomic motion. These relatively rare point defects are found to propagate down the strings like solitons, driving the collective motion. Crystal integrity remains preserved when the permutational atomic motions take the form of ring-like atomic exchanges, but a topological transition occurs at higher temperatures where the rings open to form linear chains similar in geometrical form and length distribution to the strings of GF liquids. The local symmetry breaking effect of the open strings apparently destabilizes the local lattice structure and precipitates crystal melting. The crystal defects are thus not static entities under dynamic conditions, such as elevated temperatures or material loading, but rather are active agents exhibiting a rich nonlinear dynamics that is not addressed in conventional "static" defect melting models.

Understanding of the Structural Relaxation of Metallic Glasses within the Framework of the Interstitialcy Theory

“…The coexisting solid and liquid are close to defect free and almost maximally defective states. Several other molecular dynamic experiments noted an important role of interstitialcies in melting [38,[41][42][43] and even in the crystallization of simple metals [44]. Recent molecular dynamics work by Betancourt et al [45] makes sense of the interstitialcy in liquids, and the results seem to accord remarkably with defect concentration estimates given by Granato [18,19].…”

Influence of string-like cooperative atomic motion on surface diffusion in the (110) interfacial region of crystalline Ni

“…42 This scaling relation has also been observed in the homogeneous melting of crystalline Ni, yet another material exhibiting dynamic heterogeneity. 44 We next focus on the particular form of collective motion occurring in the interfacial dynamics of crystalline Ni using computational tools drawn from the field of glass-forming liquids. The van Hove correlation function G s (r,t) describes the distribution function for particle displacements from an arbitrary origin at t = 0 to a distance r(t) at time t and our illustration of this quantity in Fig.…”

String model for the dynamics of glass-forming liquids

“…Dynamical heterogeneity in the form of strings seems to be ubiquitous in strongly-interacting, condensed-phase systems, ranging from simple GF fluids, to the interfacial dynamics of nanoparticles, lipid membranes, crystal melting, and grain boundaries, 24,29,30,[86][87][88] and may be relevant to describe physical aging in glass-forming liquids. 89 Strings have been observed directly experimentally in the amorphous interfacial region of crystals and in particle tracking measurements of granular and colloidal particles.…”

“…Interestingly, an exponent value of θ = 2.5 is consistent with ideal ring equilibrium polymers. Consequently, the increase of θ may reflect a progressive topological transition between open strings at high T, to predominantly ring-like structures at low T, as seen in crystal melting 30 and the polymerization of dipolar fluids. 31 Regardless of whether such a scenario occurs here, it is apparent that the exponential tail is the predominate feature of our data for P(L), a characteristic of equilibrium polymers.…”

“…Moreover, Ref. 30 found that the mobile particles associated with the "activated" species have a concentration that is comparable to the coordination number of the mobile particles. This rough argument makes the activated particles on the order of 50%, as we find here.…”