Collective behavior of electrons beyond hydrodynamic transport

The collective behavior of electrons in hydrodynamic transport regime has been a topic of growing interest due to experimental observations in ultraclean materials. The fluid-like properties in this regime, such as formation of current vortices are attributed to dominance of electron-electron (e-e) scattering. The detection of negative nonlocal resistance is often regarded as the hallmark signature of hydrodynamic regime. Through nonlocal resistance measurements and high-resolution kinetic simulations performed in large-scale ultraclean (electron mean-free ~ 65 μm at 4.2 K) GaAs/AlGaAs devices, hosting numerous point contacts, we demonstrate that both current vortices and negative nonlocal resistance are not exclusive to hydrodynamic regime. Despite the absence of dominant e-e scattering, current vortices and negative nonlocal resistance can form even in the ballistic transport regime. These striking similarities in ballistic and hydrodynamic regimes stem from total electron system momentum conservation in the bulk in both regimes. The collective effects observed in ballistic regime, which is often understood from a single-particle picture, shed a new light on the dynamics of electrons in mesoscopic transport.