Vortices and para-hydrodynamics in ultraclean WTe2 flakes
Electron hydrodynamics, which is the viscous flow of current in mesoscopic high-mobility devices, has become a valuable tool in characterizing quantum materials, and can be used to (re)create a number of hydrodynamic phenomena, both old and new. However, the formation of a whirlpool - arguably the most striking hydrodynamic phenomenon - has so far been elusive.I discuss recent results on ultrapure, thin WTe2 flakes in which it is for the first time possible to directly observe hydrodynamic vortices using spatially resolved current-imaging techniques. From a theory perspective, this observation is rather challenging: Both from the measured bulk mean free path and from ab-initio calculations, one would instead expect ballistic transport, and not hydrodynamic flow. Here, I show how this discrepancy can be resolved in a kinetic theory that incorporates weak surface scattering from the top and bottom surfaces, thereby leading to a novel para-hydrodynamic regime where ballistic and hydrodynamic mechanisms coexist. Surprisingly, our findings imply that para-hydrodynamic transport appears generically in ultraclean devices, meaning it is probably observable in a range of other currently studied materials.