EQuAL Seminar: Canxun Zhang Imaging Electron Hydrodynamics in a Graphene Flat Band System

Hydrodynamic electron transport arises when carrier kinetics is dominated by inter-electron collisions rather than scattering of electrons by impurities or phonons. Signatures of electron hydrodynamics have been reported in ultraclean conductors such as semiconductor quantum wells, graphene, WTe , and PdCoO . In all of these experiments, however, hydrodynamic transport emerges as a high temperature correction to the kinetics of light carriers in a weakly interacting ground state. This sets a practical lower bound on the electron–electron scattering length (l ) above which the hydrodynamic description applies. The advent of dual-gated rhombohedral graphene multilayers introduces a new route toward enhanced hydrodynamic behavior owing to their large and tunable effective mass. Here, we employ a scanning superconducting magnetic sensor to image local current flow in dual-gated bilayer graphene. Exploiting a sample geometry sensitive to both laminar and vortical flow, we identify three distinct transport regimes — ballistic, hydrodynamic, and ohmic — across the full phase space spanned by carrier density and displacement field. The hydrodynamic regime coincides with previously reported regions of flat band magnetism, consistent with enhanced scattering mediated by magnetic fluctuations above the Curie temperature. Quantitatively fitting our results to a unified Boltzmann transport model reveals the electron–electron scattering length to be no greater than 50 nm in a wide parameter range, while high-current measurements reveal striking nonlinearities in the flow pattern. Our results highlight the potential of ultraclean tunable flat band systems for miniaturized electronic devices based on mechanisms unique to hydrodynamic transport.