Viscosity to entropy density ratio: violation of the lower bound at small temperatures

TL;DR

This paper demonstrates that in theories with continuum excitations rather than quasiparticles, the shear viscosity to entropy density ratio can approach zero at low temperatures, violating previously proposed lower bounds.

Contribution

It reveals that the shear viscosity to entropy density ratio can vanish at low temperatures in certain theories, challenging the AdS/CFT-based lower bound.

Findings

Viscosity to entropy ratio approaches zero at low temperatures.

Continuum excitations lead to no lower bound for viscosity ratio.

Contrasts with AdS/CFT predictions.

Abstract

We show that in theories where the lowest energy excitations are not quasiparticles but they form a continuum, the shear viscosity to entropy density ratio goes to zero as the temperature goes to zero. In these theories therefore there is no lower bound for the shear viscosity to entropy density ratio, in contrast to the predictions coming from the AdS/CFT correspondence.