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

Area of Science:

• Condensed Matter Physics
• Materials Science
• Statistical Mechanics

Background:

• Viscoelasticity bridges purely viscous and elastic responses in materials.
• The microscopic, atomic-level mechanisms governing viscoelasticity in solids remain poorly understood.
• Disordered solids present unique challenges due to local structural irregularities.

Purpose of the Study:

• To elucidate the atomic-level origins of viscoelasticity in model disordered solids.
• To establish a quantitative link between nonaffine atomic motions, structural disorder, and viscoelastic response.
• To predict and explain emergent phenomena like power-law creep.

Main Methods:

• Studied three model disordered solids: random, bond-depleted fcc, and fcc lattices with vacancies.
• Applied sum rules for viscoelastic response within the harmonic approximation for central-force lattices.
• Utilized a quantitative measure of nonaffinity and inversion symmetry to analyze atomic motions.

Main Results:

• Viscoelastic responses of diverse disordered solids collapse onto a universal master curve when normalized by inversion symmetry breaking.
• Nonaffine atomic motions, directly linked to local disorder and lack of inversion symmetry, are identified as the key mechanism.
• Power-law creep (G(t)∼t^{-1/2}) emerges near the isostatic point due to the interplay of soft vibrational modes and nonaffine dynamics.

Conclusions:

• A unified atomic-level understanding of viscoelasticity in disordered solids is achieved.
• The strength of inversion symmetry breaking quantitatively predicts viscoelastic behavior.
• The theory provides analytical scalings and predictions for power-law creep, supported by numerical calculations.