Vortex dynamics and nonlocal phases in hydrodynamic and ballistic AC transport

Transport in conductors is typically dominated by momentum-relaxing (MR) collisions of electrons with the lattice or defects. However, clean systems hosting two-dimensional electron systems allow for the suppression of MR scattering so that the dominant effect is now the collision of electrons either with other electrons (hydrodynamic regime) or with device boundaries (ballistic regime). Unlike MR scattering, (normal) electron-electron and electron-boundary collisions conserve the total momentum of the electrons in the bulk leading to the observation of fluid-like phenomena such as current vortices and negative nonlocal resistance under a DC bias. We investigate AC transport in these regimes and showcase the striking vortex dynamics that occur in hydrodynamic and ballistic transport. The nonlocal current-voltage and voltage-voltage phases allow for an unambiguous discrimination between diffusive, hydrodynamic and ballistic transport.