Hydrodynamic and Ballistic AC transport in two-dimensional Fermi Liquids

Electron transport in clean two-dimensional 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. Current reversals in the non-Ohmic regimes occur via repeated vortex generation and mergers through reconnections (movie : vimeo.com/261891439). Crucially, AC sources give rise to strong spatiotemporal correlations across the entire device that unambiguously distinguish all regimes.