Disorder-enabled hydrodynamics of charge and heat transport in monolayer graphene*

Hydrodynamic behavior in electronic systems is commonly associated with extremely clean samples, such that momentum-conserving collisions between electrons dominate over momentum-non-conserving electron-impurity and electron-phonon collisions. In this talk I show that semimetals like graphene support two distinct hydrodynamics regimes: an ambipolar hydrodynamic regime near the charge neutrality point (i.e., with nearly equal densities of electrons and holes), and a unipolar hydrodynamic regime when a single species of carriers dominates. The Lorenz ratio between thermal and electric conductivity is enhanced in the ambipolar regime (i.e., it is larger than the universal value of the Wiedemann-Franz law), and is suppressed in the unipolar regime. The crossover between the two regimes, as well as the magnitude of the violation of the Wiedemann-Franz law, is sensitive to the amount of disorder. In particular, the ambipolar regime, characterized by vanishing thermal resistivity and finite electric resistivity is observable only for carrier densities lower than a crossover value n_c, which tends to zero in the limit of zero disorder, i.e., when momentum-non-conserving collisions are absent. In this sense the ambipolar hydrodynamics can be viewed as a disorder-enabled hydrodynamics.