Electron hydrodynamics at the boundaries

When electron-electron interactions dominate in mesoscopic solid-state devices, current flow can remarkably resemble that of hydrodynamic flow in a continuum fluid. Exciting glimpses of this behavior have emerged in recent experiments; in graphene, for example, negative local resistances arise due to viscous sheer forces (Geim, et. al. 2015). While a modified Hall-bar geometry—as was used in those experiments—can give us some important insights, we need new device architectures and measurement schema that take control of the boundaries, which become critically important in the hydrodynamic regime.

By controlling electron densities and fluxes at device boundaries, we probe the transition from ballistic to hydrodynamic flow in novel device geometries, measuring (1) the broadening of narrowly focused ballistic jets with temperature, and (2) the changing of the angular flow-field of constrictions with the microscopic reflectivity of edges. In both cases we strategically drain current at ohmic contacts to allow flow to mimic that of a larger bulk. Using a homebuilt lock-in method, we control potentials and measure currents through all ohmic drains in parallel. This crucially eliminates artefacts that arise from unwanted backflows from high-impedance contacts.