# Atomic-scale origin of dynamic viscoelastic response and creep in disordered solids

## TL;DR

This study uncovers the atomic-level origins of viscoelasticity and creep in disordered solids, revealing how local structural disorder and inversion-symmetry breaking influence their mechanical response.

## Contribution

The paper introduces a theoretical framework linking atomic non-affine motions and inversion-symmetry breaking to viscoelastic behavior in disordered solids, supported by numerical validation.

## Key findings

- Viscoelastic responses collapse onto a master curve when normalized by inversion-symmetry breaking.
- Near the isostatic point, power-law creep $G(t) \\sim t^{-1/2}$ is observed.
- Analytical scalings predict the interplay between soft vibrational modes and non-affine dynamics.

## Abstract

Viscoelasticity has been described since the time of Maxwell as an interpolation of purely viscous and purely elastic response, but its microscopic atomic-level mechanism in solids has remained elusive. We studied three model disordered solids: a random lattice, the bond-depleted fcc lattice, and the fcc lattice with vacancies. Within the harmonic approximation for central-force lattices, we applied sum-rules for viscoelastic response derived on the basis of non-affine atomic motions. The latter motions are a direct result of local structural disorder, and in particular, of the lack of inversion-symmetry in disordered lattices. By defining a suitable quantitative and general atomic-level measure of nonaffinity and inversion-symmetry, we show that the viscoelastic responses of all three systems collapse onto a master curve upon normalizing by the overall strength of inversion-symmetry breaking in each system. Close to the isostatic point for central-force lattices, power-law creep $G(t)\sim t^{-1/2}$ emerges as a consequence of the interplay between soft vibrational modes and non-affine dynamics, and various analytical scalings, supported by numerical calculations, are predicted by the theory.