Hydrodynamic halos in graphene

In graphene, several experimental observations at low temperatures - in which electrons and holes are in near-equilibrium with the crystal lattice - have attributed unusual device response to the apparent Dirac hydrodynamic plasma. While it is expected that energy may be transported with remarkable efficiency in the hydrodynamic regime, how this plasma phase contributes to hot carrier cooling remains unknown. Here, we report extremely efficient, anomalous quenching of the photoexcited state in neutral graphene. Quenching is found to be enhanced at an intermediate sample temperature, despite the orders-of-magnitude disparity between the initial hot electronic temperature Te ~ 1000 K and the sample temperature T = 50 K. Importantly, this enhanced quenching process - which is highly sensitive to sample temperature - is unexpected based on known energy relaxation pathways. Based on comprehensive temperature dependent photoconductance measurements, we attribute this remarkable quenching to the formation of hydrodynamic halos surrounding the high temperature hot carrier distribution. A new model of hot carrier cooling that includes this hydrodynamic halo contribution can successfully explain this sample temperature dependence along with other experimental details. Rapid quenching in high-mobility graphene heterostructures indicates an emergent cooling regime within the Dirac excited state and raises important questions about hydrodynamic energy transport in graphene.