Condensed matter physicists often speak of the "electronic fluid" in conductors, and we use a lot of vocabulary that makes analogies between the motion of liquids and the flow of charge (current, vortices, Fermi liquids, and in the UK vacuum triodes were called "valves"). In recent years, there has been a lot of talk about electron flow in the hydrodynamic regime (e.g.
this highly cited paper and
this recent review and
this preprint). In this regime, electron-electron scattering is strong, and it makes sense to think less about a single-electron picture and more like the electrons acting as a viscous fluid, which can have
vortices and a
velocity profile transverse to a channel. (The
boundary conditions at the channel walls are a subtle question for sure.)
This preprint from last week is a great example of pushing this similarity to new limits. Through a combination of electronic transport measurements and scanning Kelvin probe microscopy, the authors observe what really looks like a
hydraulic jump in the flow of electrons through a constriction. I've written about hydraulic jumps before,
here and
here. They're the incompressible flow analog of a standing shockwave, when local fluid velocity goes from supersonic (relative to some sound-like excitation) to subsonic. Very cute.
(I hope to write more soon, about the role of fluctuations in condensed matter and nanoscale physics.)
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