Physics of glassy and granular materials

Title

Exploring the Potential Energy Landscape of glass-forming systems, from microscopic mechanisms to dynamic properties

Speaker

Venessa Kay de Souza (University of Granada)

Abstract

The Potential Energy Landscape (PEL) provides a description of the complicated many-particle effects in disordered systems, with information about individual particle configurations and the relationships between different configurations. The properties of a glassy system at low temperature are dominated by long residences near local minima (inherent structures) but at higher temperatures, many dynamic properties are still related to properties of the inherent structures.

In glassy systems, the landscape contains many disordered minima with similar energies and has a hierarchical, frustrated appearance with relatively high barriers separating a large number of potential energy funnels. When following an individual trajectory, revisits to individual minima are common. For this reason it is useful to be able to coarse-grain the landscape, for example by defining metabasins. Metabasins are groups of minima within which forward-backward correlations occur and can be identified via the total potential energy of the system (Doliwa and Heuer, Phys. Rev. E 67, 030501 (2003)). Alternatively, on the microscopic level, important transitions, such as cage-breaks, where an atom leaves its cage of nearest-neighbours can be identified (de Souza and Wales, J. Chem. Phys. 129, 164507 (2008)).

In this work, we reconcile the two approaches, providing a microscopic description for metabasins within the PEL in the form of productive cage-breaks. Productive (or successful) cage-breaks are cage-breaks that are not later reversed in the course of the trajectory and can be identified for each individual atom. The link between metabasins and cage-breaks allows us to extend the metabasin description to larger systems and to identify lengthscales of cooperative motion. We can also examine in detail the effects of system size on the dynamics.

In a coarse-grained, metabasin description of the PEL, correlation is removed, and the dynamics can be described by a random walk. A temperature-independent elementary lengthscale is uncovered, the hopping distance between metabasins, which corresponds to the distance moved by a cage-breaking atom and its neighbours. The temperature-dependent quantity is the waiting time within each metabasin, the time during which atoms are not able to successfully escape their nearest-neighbour cages. The waiting time determines both the diffusion constant and relaxation timescales.In this way, we have a complete description of the overall dynamics, starting from the atomic level.