AC signatures of electron hydrodynamics published in Phys. Rev. B
The flow of electrons in most materials is governed by Ohm’s law, which is very different from the flow of liquids for one simple reason: liquids molecules only strike other liquid molecules which are all in motion together, while electrons strike atoms in the material fixed in space. This allows liquids to ‘churn’ and form vortices, while electrons typically do not. Scientists recently discovered that electrons in graphene can be made to interact so weakly with the atoms that they now mostly interact with other electrons. Once this happens, electrons begin to flow like a liquid, with striking consequences including vortices!
This ‘hydrodynamic’ regime of electron flow in graphene has been challenging to access experimentally because it is very sensitive to defects in the material. Previously, experimental tests relied on applying voltage and looking for a negative resistance. We show that oscillating voltages much more readily bring out the hydrodynamic flow in electrons. We can understand this with a very simple analogy. Consider a very thin layer of liquid at the bottom of a jar: it drags along the surface and will move similar to Ohmic electron flow. Once the liquid is thick enough, most of the liquid interacts only with the liquid itself and it exhibits the typical liquid flow that we are intuitively familiar with. How would you distinguish these regimes? Tilting the jar slowly would not produce noticeably different flow patterns. In contrast, shaking the jar with thick enough liquid will easily introduce churning and vorticity, compared to a smoother draggy flow for the thin liquid layer.
The churning induced by an AC source gives rise to an intricate dance of current vortices that are synchronized with the source (see video above). These signatures will facilitate easy experimental identification of the hydrodynamic regime of electron flow, including in candidate materials other than graphene.