Hydrodynamic flow of topological electrons

The electron transport in metals and semiconductors is conventionally determined by the momentum-relaxing scattering, e.g., electron-phonon and electron-impurity scattering. However, an exotic hydrodynamic flow of topological electrons is experimentally observed in three-dimensional topological insulator HgTe, where the transport seems to be dominated by the momentum conserved electron-electron scattering. In these experiments, HgTe narrow channels with different length-to-width ratios (1:3, 1:5, and 1:10) were fabricated via a refined wet etching process, which defines the physical boundary and maintains high electron mobility. In particular, to modulate the strength of the electron-electron scattering, a DC bias current was applied to heat up the channel by Joule heating. As the DC heating current increases, the differential resistances of the HgTe narrow channels initially increases and then decreases, which is reminiscent of the Gurzhi effect reported by M.J.M. de Jong and L.W. Molenkamp.