Despite of the strong Coulomb interaction, electrons in simple metal behave as a non-interacting Fermi gas with long-lived quasi-particle excitation according to Landau Fermi liquid theory. However, weak screening near the charge neutrality point of the massless Dirac fermions in graphene can lead to a new collective behavior described by hydrodynamics. By listening to the Johnson noise of the electrons, we are able to probe simultaneously the thermal and electrical transport of the Dirac fluid and observe how it departs from Fermi liquid physics. At high temperature near the neutrality point, we find a strong enhancement of the thermal conductivity and breakdown of Wiedemann-Franz law in graphene. This is attributed to the non-degenerate electrons and holes forming a strongly coupled Dirac fluid.
Ref: Science 351, 1058 (2016)