Quantum critical diffusion and thermodynamics in Lifshitz holography

Abstract

We present the full charge and energy diffusion constants for the Einstein-Maxwell dilaton (EMD) action for Lifshitz spacetime characterized by a dynamical critical exponent $z$. Therein we compute the fully renormalized static thermodynamic potential explicitly, which confirms the forms of all thermodynamic quantities including the Bekenstein-Hawking entropy and Smarr-like relationship. All thermodynamics are based on a direct computation of the free energy. Our exact computation demonstrates a modification to the Lifshitz-Ward identity for the EMD theory. For transport, we target our analysis at finite chemical potential and include axion fields to generate momentum dissipation. While our exact results corroborate anticipated bounds, we are able to demonstrate that the diffusivities are governed by the engineering dimension of the diffusion coefficient, $[D]=2\ensuremath{-}z$. Consequently, a $\ensuremath{\beta}$ function defined as the derivative of the trace of the diffusion matrix with respect to the effective lattice spacing changes sign precisely at $z=2$. At $z=2$, the diffusion equation exhibits perfect scale invariance and the corresponding diffusion constant is the pure number $1/{d}_{s}$ for both the charge and energy sectors, where ${d}_{s}$ is the number of spatial dimensions. Further, we find that as $z\ensuremath{\rightarrow}\ensuremath{\infty}$, the charge diffusion constant vanishes, indicating charge localization. Deviation from universal decoupled transport obtains when either the chemical potential or momentum dissipation are large relative to temperature, an echo of strong thermoelectric interactions.