Observation of Electronic Viscous Dissipation in Graphene Magneto-thermal Transport

The recent discovery of interaction-driven viscous electronic hydrodynamics in graphene has inspired new devices and insights about other materials. In this new regime, the well-known rules of Ohmic transport no longer apply, and a number of effects have been identified in electronic transport. Despite these advancements, the hydrodynamic analogue of Joule heating remains unexplored, and the thermal properties of hydrodynamic electronic devices are unknown. In this work, we probe graphene hydrodynamics with thermal transport and find two distinct, qualitative signatures: thermal conductivity suppression below the Wiedemann-Franz value and negative thermal magnetoresistance. These signatures arise from two distinct aspects of this new regime: microscopic momentum conservation due to electron-electron scattering, and geometry-dependent viscous dissipation. We find they are coincident in temperature and density, providing new and robust qualitative signatures of hydrodynamics in a simple, two-terminal global transport setup. Our results mark the first observation of viscous electronic heating in an electron fluid, which may influence the design of hydrodynamic devices and offers a new methodology to identify hydrodynamic states in other systems.