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

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.

Comments: 4 pages; minor modifications in the text, new references added