Shear viscosity expression for a graphene system in relaxation time approximation

TL;DR

This paper derives a microscopic expression for shear viscosity in graphene within the relaxation time approximation and compares it with other fluid systems to understand the effects of dimensionality and dispersion relations on fluid properties.

Contribution

It provides a detailed shear viscosity expression for 2D graphene and analyzes its differences from other systems, highlighting the impact of dispersion relations and dimensionality.

Findings

Shear viscosity and entropy density decrease from Fermi liquid to Dirac fluid regimes.

The viscosity to entropy density ratio approaches saturation in these regimes.

Similar behavior may occur in high-energy physics systems like quark matter.

Abstract

We have gone through the detailed microscopic calculation of the shear viscosity of a 2-dimensional graphene system in the relaxation time approximation-based kinetic theory framework. After getting its final expressions, we compared it with the shear viscosity expressions of other possible 2-dimensional as well as 3-dimensional nonrelativistic and ultra-relativistic fluid systems. The aim of the comparison is to reveal how their different one-body dispersion relations affect their many-body fluid properties like shear viscosity and the viscosity to entropy density ratio. It is also aimed to reveal the 3-dimension to the 2-dimension transformation of their mathematical structures. We have numerically explored the differences in their order of magnitude and dependence on thermodynamical parameters-temperature and chemical potential. Marking two thermodynamical domains-Dirac fluid and Fermi liquid-for a 2-dimensional graphene system, we have noticed that shear viscosity, entropy density as well as their ratios decrease toward saturated values when one goes from Fermi liquid to Dirac fluid domain. When one shifts from mili-electron volt scales of temperature and chemical potential in condensed matter physics location to their mega-electron volt scales in high energy physics location, then the same results may be expected for hot quark matter case, where the transition from the neutron star to early universe domains may be considered as Fermi liquid to Dirac fluid transition.