# Theory of Shear Modulus in Glasses

## TL;DR

This paper develops a linear response theory for shear deformations in glasses, linking shear modulus to stress and particle correlations, and confirms predictions through numerical simulations, revealing long-range correlations and sound amplification effects.

## Contribution

It introduces a general linear response framework for glasses under shear, incorporating boundary effects and inhomogeneous responses, with numerical validation and application to sound propagation.

## Key findings

- Shear modulus expressed via stress-force correlations.
- Presence of long-range, long-lived force-displacement correlations.
- Resonant amplification of transverse sound near mode frequency.

## Abstract

We construct a linear response theory of applying shear deformations from boundary walls in the film geometry in Kubo's theoretical scheme. Our method is applicable to any solids and fluids. For glasses, we assume quasi-equilibrium around a fixed inherent state. Then, we obtain linear-response expressions for any variables including the stress and the particle displacements, even though the glass interior is elastically inhomogeneous. In particular, the shear modulus can be expressed in terms of the correlations between the interior stress and the forces from the walls. It can also be expressed in terms of the inter-particle correlations, as has been shown in the previous literature. Our stress relaxation function includes the effect of the boundary walls and can be used for inhomogeneous flow response. We show the presence of long-ranged, long-lived correlations among the fluctuations of the forces from the walls and the displacements of all the particles in the cell. We confirm these theoretical results numerically in a two-dimensional model glass. As an application, we describe propagation of transverse sounds after boundary wall motions using these time-correlation functions We also find resonant sound amplification when the frequency of an oscillatory shear approaches that of the first transverse sound mode.