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

• Published in: Physical review research Vol. 3; no. 3; p. 033003
• Main Authors: Kim, Sang Wook, Jose, Geo, Uchoa, Bruno
• Format: Journal Article
• Language: English
• Published: American Physical Society 01.07.2021
• ISSN: 2643-1564; 2643-1564
• DOI: 10.1103/PhysRevResearch.3.033003

The ratio between the shear viscosity and the entropy η/s is considered a universal measure of the strength of interactions in quantum systems. This quantity was conjectured to have a universal lower bound (1/4π)ℏ/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 η/s∝T violates the universal lower bound, scaling toward 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.