  Interactions between particles in quantum many-body systems can lead to
collective behavior described by hydrodynamics. One such system is the
electron-hole plasma in graphene near the charge neutrality point which can
form a strongly coupled Dirac fluid. This charge neutral plasma of
quasi-relativistic fermions is expected to exhibit a substantial enhancement of
the thermal conductivity, due to decoupling of charge and heat currents within
hydrodynamics. Employing high sensitivity Johnson noise thermometry, we report
the breakdown of the Wiedemann-Franz law in graphene, with a thermal
conductivity an order of magnitude larger than the value predicted by Fermi
liquid theory. This result is a signature of the Dirac fluid, and constitutes
direct evidence of collective motion in a quantum electronic fluid.
