Universal flow of charge and heat near quantum criticality in ultraclean graphene

Over the past decade, electron fluids in graphene have garnered significant attention because of their unique electrical and magnetic properties. Near the charge neutrality point, graphene is expected to behave like a “Dirac fluid” - a fluid of massless Dirac fermions described by a universal electrical conductivity σ_Q. Despite several experimental reports on the fluid-like electronic transport in graphene, there has been no direct experimental evidence of σ_Q. In this work, we have fabricated hBN-encapsulated ultraclean graphene devices with high electron mobilities (~100 m² V^-1 s^-1) and performed electrical and thermal transport from room temperature down to 20 K. We observe that the quantum critical conductivity of our devices approaches the universal bound of (4±1) e²/h near room temperature. Additionally, we report a giant violation of the Wiedemann-Franz Law near the charge neutrality point across a range of temperatures and find the η/s ratio for our devices, from independent measurements of shear viscosity and entropy density, to be within a factor of four of the holographic bound. Our experimental observations are also quantitatively consistent with that of a non-Galilean-invariant Dirac fluid.