Transport coefficients in non-quasiparticle systems

TL;DR

This paper investigates transport coefficients in non-quasiparticle systems, revealing that the shear viscosity to entropy density ratio lacks a universal lower bound and can violate the conjectured $1/4 extpi$ limit.

Contribution

It demonstrates that in systems without quasiparticles, the shear viscosity to entropy density ratio varies with system and temperature, challenging previous bounds.

Findings

$ ext{eta}/s$ has no universal lower bound

The minimal $ ext{eta}/s$ depends on system and temperature

Models constructed violate the $1/4 extpi$ bound

Abstract

Transport coefficeints, in particular the shear viscosity to entropy density ratio is studied in systems where the small-width quasiparticle assumption is not valid. It is found that $\eta/s$ has no unversal lower bound, the minimal value depends on the system and the temperature, and can be even zero. We construct models where the $1/4\pi$ conjectured bound is violated.