  Electron transport in clean 2D systems with weak electron-phonon (e-ph)
coupling can transition from an Ohmic to a ballistic or a hydrodynamic regime.
The ballistic regime occurs when electron-electron (e-e) scattering is weak
whereas the hydrodynamic regime arises when this scattering is strong. Despite
this difference, we find that vortices and a negative nonlocal resistance
believed to be quintessentially hydrodynamic are equally characteristic of the
ballistic regime. These non-Ohmic regimes cannot be distinguished in DC
transport without changing experimental conditions. Further, as our kinetic
calculations show, the hydrodynamic regime in DC transport is highly fragile
and is wiped out by even sparse disorder and e-ph scattering. We show that
microwave-frequency AC sources by contrast readily excite hydrodynamic modes
with current vortices that are robust to disorder and e-ph scattering. Indeed,
current reversals in the non-Ohmic regimes occur via repeated vortex generation
and mergers through reconnections, as in classical 2D fluids. Crucially, AC
sources give rise to strong correlations across the entire device that
unambiguously distinguish all regimes. These correlations in the form of
nonlocal current-voltage and voltage-voltage phases directly check for the
presence of a nonlocal current-voltage relation signifying the onset of
non-Ohmic behavior as well as also for the dominance of bulk interactions,
needed to confirm the presence of a hydrodynamic regime. We use these probes to
demarcate all regimes in an experimentally realizable graphene device and find
that the ballistic regime has a much larger extent in parameter space than the
hydrodynamic regime.
