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.