  Electron-electron interactions in high-mobility conductors can give rise to
transport signatures resembling those described by classical hydrodynamics.
Using a nanoscale scanning magnetometer, we imaged a distinctive hydrodynamic
transport pattern - stationary current vortices - in a monolayer graphene
device at room temperature. By measuring devices with increasing characteristic
size, we observed the disappearance of the current vortex and thus verify a
prediction of the hydrodynamic model. We further observed that vortex flow is
present for both hole- and electron-dominated transport regimes, while
disappearing in the ambipolar regime. We attribute this effect to a reduction
of the vorticity diffusion length near charge neutrality. Our work showcases
the power of local imaging techniques for unveiling exotic mesoscopic transport
phenomena.
