Novel parity violating transport coefficients in 2 + 1 dimensions from holography

We construct a 3+1 dimensional holographic model dual to a parity violating hydrodynamic system in 2+1 dimensions. Our model contains gravitational and electrodynamic Chern-Simons terms coupled to a neutral pseudo scalar θ, and a potential composed of quadratic and quartic terms in θ. The background is a charged black brane. We study the hydrodynamics to first order in spacetime derivatives near the probe limit of the pseudo scalar, by extracting the transport coefficients from the scalar, vector, and tensor modes of bulk perturbations. We study two mechanisms for breaking the parity of the boundary fluid: the parity is either spontaneously broken by the nonzero vev of the dual pseudo scalar operator, or by the pseudo scalar source on the boundary. We discover some novel temperature-dependent behaviors of the transport coefficients. It would be interesting to find these behaviors being realized in the real world materials.