Observation of Electronic Viscous Dissipation in Graphene Magneto-thermal Transport

Hydrodynamics describes the collective transport of strongly-interacting particles. Due to enhanced electron-electron interactions at elevated temperatures, the behavior of electrons in clean graphene can be depicted as a hydrodynamic flow of charge. In this new regime, the well-known rules of Ohmic transport no longer apply, necessitating the consideration of collective electron dynamics. In particular, the hydrodynamic analogues of Joule heating and thermal transport require consideration of the electronic viscosity and associated energy dissipation, but remain unexplored. In this work, we probe graphene via thermal transport measurement in small magnetic fields and find an unexpected enhancement of cooling in Corbino geometries. We construct a theory that identifies the origin of this effect in viscous dissipation of the electron fluid, enabling a new measurement of the electronic viscosity and underlying microscopic thermal and electrical conductivities. This analysis reveals the Lorenz ratio of the graphene electronic fluid, which is shown to be strongly suppressed away from charge neutrality compared to the Wiedemann-Franz value, in agreement with longstanding expectations for the hydrodynamic regime. Our results demonstrate viscous electronic heating in an electron fluid, offering a new, transport-based methodology for identifying hydrodynamic states in other material systems, and providing insight for thermal management in electronic hydrodynamic devices.