Violation of the viscosity/entropy bound in translationally invariant non-Fermi liquids

TL;DR

This paper investigates shear viscosity in a solvable strongly interacting model of coupled SYK islands, revealing violations of the viscosity/entropy bound in certain regimes, indicating nearly perfect fluidity and challenging existing bounds.

Contribution

It demonstrates the violation of the KSS bound in a translationally invariant model with known gauge-gravity duality, highlighting new fluid behavior in non-Fermi liquids.

Findings

Violates KSS bound in incoherent metal regime

Satisfies KSS bound in marginal Fermi liquid regime

Provides the first translationally invariant example violating the KSS bound

Abstract

The shear viscosity is an important characterization of how a many-body system behaves like a fluid. We study the shear viscosity in a strongly interacting solvable model, consisting of coupled Sachdev-Ye-Kitaev (SYK) islands. As temperature is lowered, the model exhibits a crossover from an incoherent metal with local criticality to a marginal fermi liquid. We find that while the ratio of shear viscosity to entropy density in the marginal Fermi liquid regime satisfies a Kovtun-Son-Starinets (KSS) like bound, it can strongly violate the KSS bound in a robust temperature range of the incoherent metal regime, implying a nearly perfect fluidity of the coupled local critical SYK model. Furthermore, this model also provides the first translationally invariant example violating the KSS bound with known gauge-gravity correspondence.