Viscosity bound violation in holographic solids and the viscoelastic response

TL;DR

This paper demonstrates that the Kovtun--Son--Starinets viscosity bound is violated in solid-like holographic materials with elastic properties, challenging its universality and suggesting a relation to viscoelastic response.

Contribution

It provides explicit holographic examples of bound violation in solid-type theories and links the violation to viscoelastic properties, proposing a generalized bound involving elastic modulus.

Findings

KSS bound holds for fluids but not solids with elastic modulus

Explicit holographic models show violation of the bound in solid phases

Proposes a generalized bound involving shear elastic modulus and pressure

Abstract

We argue that the Kovtun--Son--Starinets (KSS) lower bound on the viscosity to entropy density ratio holds in fluid systems but is violated in solid materials with a non-zero shear elastic modulus. We construct explicit examples of this by applying the standard gauge/gravity duality methods to massive gravity and show that the KSS bound is clearly violated in black brane solutions whenever the massive gravity theories are of solid type. We argue that the physical reason for the bound violation relies on the viscoelastic nature of the mechanical response in these materials. We speculate on whether any real-world materials can violate the bound and discuss a possible generalization of the bound that involves the ratio of the shear elastic modulus to the pressure.