Heterogeneous Dynamics, Marginal Stability and Soft Modes in Hard Sphere Glasses

TL;DR

This paper analyzes the dynamics of hard sphere glasses by studying their free energy landscape and normal modes, revealing that structural relaxation occurs mainly along a few nearly-unstable modes whose number decreases with density and approaches the jamming transition.

Contribution

It introduces a novel approach linking free energy landscapes to normal modes in hard sphere glasses, elucidating the role of marginal stability and soft modes in structural relaxation.

Findings

Structural relaxation occurs along a small number of nearly-unstable modes.

The number of these modes decreases with increasing density.

Mode extension length scale diverges near the jamming transition as $(\,\phi_c - \phi\,)^{-1/2}$.

Abstract

In a recent publication we established an analogy between the free energy of a hard sphere system and the energy of an elastic network [1]. This result enables one to study the free energy landscape of hard spheres, in particular to define normal modes. In this Letter we use these tools to analyze the activated transitions between meta-bassins, both in the aging regime deep in the glass phase and near the glass transition. We observe numerically that structural relaxation occurs mostly along a very small number of nearly-unstable extended modes. This number decays for denser packing and is significantly lowered as the system undergoes the glass transition. This observation supports that structural relaxation and marginal modes share common properties. In particular theoretical results [2, 3] show that these modes extend at least on some length scale $l^*\sim (\phi_c-\phi)^{-1/2}$ where $\phi_c$ corresponds to the maximum packing fraction, i.e. the jamming transition. This prediction is consistent with very recent numerical observations of sheared systems near the jamming threshold [4], where a similar exponent is found, and with the commonly observed growth of the rearranging regions with compression near the glass transition.