Hydrodynamic transport and violation of the viscosity-to-entropy ratio bound in nodal-line semimetals

TL;DR

This paper demonstrates that in nodal-line semimetals, the ratio of shear viscosity to entropy density violates the universal bound at low temperatures, indicating extremely strong correlations.

Contribution

The study provides the first quantum kinetic theory analysis showing $ ext{η/s}$ violation in nodal-line semimetals, challenging the universality of the viscosity bound.

Findings

$ ext{η/s}$ scales linearly with temperature and violates the bound at low T.

Hydrodynamic scattering time remains nearly temperature independent, with logarithmic corrections.

Nodal-line semimetals can exhibit extremely short scattering times, indicating strong correlations.

Abstract

The ratio between the shear viscosity and the entropy $\eta/s$ is considered a universal measure of the strength of interactions in quantum systems. This quantity was conjectured to have a universal lower bound $(1/4\pi)\hbar/k_{B}$, which indicates a very strongly correlated quantum fluid. By solving the quantum kinetic theory for a nodal-line semimetal in the hydrodynamic regime, we show that $\eta/s\propto T$ violates the universal lower bound, scaling towards zero with decreasing temperature $T$ in the perturbative limit. We find that the hydrodynamic scattering time between collisions is nearly temperature independent, up to logarithmic scaling corrections, and can be extremely short for large nodal lines, near the Mott-Ragel-Ioffe limit. Our finding suggests that nodal-line semimetals can be very strongly correlated quantum systems.