Routes towards viscous electron flow in mesoscopic devices

Viscous electron flow in graphene and other two dimensional materials results in several exotic effects. In the so-called hydrodynamic regime, which occurs when electron-electron collisions are frequent enough, these phenomena can be described with a model that resembles the Navier-Stokes equation for classical fluids. We thoroughly studied the hydrodynamic requirements and found three meta-hydrodynamic routes to achieve viscous electron flow: favoring inelastic collisions, a magnetic field, or a high-frequency electric field. Increasing the reflectivity of the edges of the material further spans the range of validity of the above conditions. We show that the conventional requirement of frequent electron-electron collisions is too restrictive and, therefore, materials and phenomena to be described using hydrodynamics are widened. We discuss well-known experiments regarding Poiseuille-like flows, superballistic conduction and negative resistances as signatures for viscous flow onset and find that these usual signatures of viscous electron flow are achieved by following alternative meta-hydrodynamic routes. We also carried out experiments in graphene nanostructures where hydrodynamic effects are observed in geometrically engineered samples.