Out-of-bound hydrodynamics in holographic anisotropic Dirac semimetals

Abstract

We present a version of a strongly correlated 2+1-dimensional condensed matter system that features a thermal phase transition between a semimetal and an insulator through a semi-Dirac quantum critical region using AdS/CFT holography. We introduce backreaction into the bulk equations of motion to measure transport coefficients in the boundary; specifically the shear viscosity η. By explicitly breaking rotational symmetry we find a new instance of violation of the KSS-bound for the η/s ratio in the quantum critical region, as well as a monotone dependence on temperature in the T → 0 regime fixed by a Lifshitz dynamical critical exponent. We find that the Lifshitz critical exponent in the anisotropic direction is approximately equal to 2 for our choice of backreaction parameters. We find explicit T = 0 solutions separated by a quantum critical point in parameter space, showing that the thermal critical phase found in previous work comes from a quantum phase transition at zero temperature.