The origin of strings and rings in the atomic dynamics of disordered systems

TL;DR

This paper challenges the traditional view of atomic strings in disordered systems, proposing that they are propagating density perturbations called densitons, which migrate via local jumps or small rearrangements, unifying disordered and crystalline behaviors.

Contribution

It introduces the densiton model, reinterpreting atomic strings as propagating density perturbations rather than collective rearrangements, based on molecular dynamics simulations.

Findings

Strings are propagating local density perturbations, not collective events.

Densitons migrate via single-atom jumps or small rearrangements.

Similar string-like features are found in both disordered and crystalline structures.

Abstract

It has long been believed that the atomic dynamics in disordered structures, such as undercooled liquids and pre-melted interfaces, are characterized by collective atomic rearrangements in the form of quasi-one-dimensional chains of atomic displacements (strings) and their closed forms (rings). Here, we show by molecular dynamics (MD) simulations that strings involving more than a few atoms do not form by a single collective event. Instead, they represent trajectories of propagating local density perturbations, which we call densitons. The atoms on this trajectory are almost indistinguishable from their environments except for the moving head of the string (densiton). A densiton migrates by either single-atom jumps or a concerted rearrangement of 2-3 atoms. The simulations reveal a remarkable similarity between the strings in disordered and crystalline structures, in which the densitons localize into point defects. This work calls for a significant reinterpretation of the string concept and instead proposes a densiton model of the atomic dynamics.