Transport without quasiparticles in graphene and Weyl semi-metals*

Graphene and Weyl semi-metals have electronic excitations with a massless Dirac spectrum. At charge neutrality, the long-range Coulomb interaction remains unscreened, and electronic quasiparticles are not well defined. In sufficiently clean samples, thermal and electrical transport enters a hydrodynamic regime: insights on the nature of transport in this regime have emerged from holographic connections to the dynamics of black holes. A large enhancement of the Wiedemann-Franz ratio of the thermal and electrical conductivities was predicted, and is in good agreement with observations on graphene. In Weyl semi-metals, an axial-gravitational anomaly was employed to predict magnetotransport properties, and is consistent with a positive longitudinal magnetothermoelectric conductance observed in NbP.