Thermal fluctuation driven structural relaxation in undeformed glasses: Unraveling the evolution of mechanical stability.

Abstract

Glasses are mechanically rigid, but still undergo structural relaxation that changes their properties and affects potential technological applications. Understanding the underlying physical processes is a problem of broad theoretical and practical interest. We investigate intermittent structural relaxation events or "avalanches" occurring inside a glassy regime. Contrary to the more well-known avalanches due to shear, here they are induced by thermal fluctuations in undeformed glass. By analyzing changes in the structural, mechanical, dynamical, topological, and vibrational properties of the system, we provide a multifaceted characterization of avalanches. Overall we find that the system softens due to avalanches. Further, we extend a formalism to extract local measures of nonaffine displacement and tensorial strain for thermal amorphous solids in the absence of any external deformation. Our analysis highlights a key difference between two types of driving: while the shear deformation response is dominated by volume preserving deviatoric strain, changes in local density must be considered to model the response of undeformed glass under thermal noise. The observations suggest the idea of Generalized Strain Transformation Zones, where coupled shear and volume-changing deformations govern thermally mediated plasticity. Our work paves the way for a unified description of the elastoplastic response of (athermal) mechanically deformed and thermally driven undeformed glasses.