Universality in quantum critical flow of charge and heat in ultra-clean graphene

Close to the Dirac point, graphene is expected to exist in quantum critical Dirac fluid state, where the flow of both charge and heat can be described with a dc electrical conductivity $σ_\mathrm{Q}$, and thermodynamic variables such as the entropy and enthalpy densities. Although the fluid-like viscous flow of charge is frequently reported in state-of-the-art graphene devices, the value of $σ_\mathrm{Q}$, predicted to be quantized and determined only by the universality class of the critical point, has not been established experimentally so far. Here we have discerned the quantum critical universality in graphene transport by combining the electrical ($σ$) and thermal ($κ_\mathrm{e}$) conductivities in very high-quality devices close to the Dirac point. We find that $σ$ and $κ_\mathrm{e}$ are inversely related, as expected from relativistic hydrodynamics, and $σ_\mathrm{Q}$ converges to $\approx (4\pm 1)\times e^2/h$ for multiple devices, where $e$ and $h$ are the electronic charge and the Planck's constant, respectively. We also observe, (1) a giant violation of the Wiedemann-Franz law where the effective Lorentz number exceeds the semiclassical value by more than 200 times close to the Dirac point at low temperatures, and (2) the effective dynamic viscosity ($η_\mathrm{th}$) in the thermal regime approaches the holographic limit $η_\mathrm{th}/s_\mathrm{th} \rightarrow \hbar/4πk_\mathrm{B}$ within a factor of four in the cleanest devices close to the room temperature, where $s_\mathrm{th}$ and $k_\mathrm{B}$ are the thermal entropy density and the Boltzmann constant, respectively. Our experiment addresses the missing piece in the potential of high-quality graphene as a testing bed for some of the unifying concepts in physics.