Mechanistic view of the relaxation dynamics of a simple glass-former. A bridge between the topographic and the dynamic approaches

We provide a link between the two main approaches to the relaxation dynamics of glassy systems: The 'real dynamics' scheme and the inherent dynamics or topographic formalism. The first approach is based on molecular dynamics (MD) simulations, whilst the second one reflects the underlying influence of the energy 'landscape' (within a timescale separation and activated dynamics scenario) and constitutes a widespread picture within the realm of complex systems ranging from glasses to biopolymers. For a model glass-former (a binary Lennard-Jones system), MD studies which characterized in detail the movements of the different particles led to the discovery of dynamical heterogeneities. On the other hand, the topographic approach identified activated events on the potential energy surface of this system corresponding to transitions between different basins of attraction or inherent structures. In this work we demonstrate that at low temperature the relevant events identified by both methods conform to a basic mechanistic phenomenology with elementary steps involving ballistic string-like particle movements. We also show that as temperature increases and the timescales characterizing events of different range become comparable, these elemental steps loose their nature of rare activated events. Concurrently, the system looses diversity and complexity, signatures of glassy behavior. This fact enables us to furnish for the first time the microscopic structural and dynamical basis and conditions for the prevalence of the 'landscape paradigm' for this class of systems. (C) 2004 Elsevier B.V. All rights reserved.