When teaching electricity, a popular but much-criticized analogy is of water flowing through pipes, with voltage being the counterpart of a change in height and current the amount of flow in a circuit. Some students find the analogy helpful, but many physicists consider it misleading owing to differences in the way electrons and water molecules behave.
In certain materials, however, the analogy comes true; electrons influence each other in ways that more closely resemble the interactions between water molecules, leading to fluid-like behavior. One form of such behavior is the creation of whirlpools, which have now been described in Nature for the first time, having previously proven elusive.
“Electron vortices are expected in theory, but there’s been no direct proof, and seeing is believing,” said MIT Professor Leonid Levitov in a statement. “Now we’ve seen it, and it’s a clear signature of being in this new regime, where electrons behave as a fluid, not as individual particles.”
Among the strange phenomena witnessed under such conditions are negative resistance and “superballistic electron flow” where electrons appear to cooperate to pass through narrow gaps.
An individual electron moving as part of an electric current is subject to a wide array of forces. These include the movement of the atoms in the conducting material and impurities that might affect its flow, as well as the voltage causing it to move in the first place. Other electrons that are also part of the stream have an influence too, but in most materials, this is minor compared to everything else. Superconducting materials, where electron pairs move more smoothly than would be possible for a single electron, represent a partial exception.
However, if you can damp everything else down, quantum interactions between electrons become dominant. The electrons move as a viscous fluid. To achieve this state the materials in which they travel need to be cleaned of impurities and cooled to near absolute zero, so the atoms' movements almost disappear.
Using these properties Levitov and colleagues achieved almost resistance-less flow of electrons through graphene in 2017. However, water does not always flow smoothly. Instead, it can become turbulent, and even create vortices. The authors have not observed comparable behavior in graphene, so they turned to single-atom-thick sheets of tungsten ditelluride (WTe2) instead.
Not only does WTe2 bring out electrons' wave-like properties, Levitov noted: “The material is very clean, which makes the fluid-like behavior directly accessible.” The authors etched a channel running between two circular chambers onto sheets of WTe2 and gold for comparison. When currents were run through the patterning at 4.5°K (-451°F) magnetic fields revealed the electrons' behavior.
The authors were able to witness electrons swirling in and out of the side chambers making tiny whirlpools on their way, but only in the tungsten ditelluride, not the gold.
“We observed a change in the flow direction in the chambers, where the flow direction reversed the direction as compared to that in the central strip,” Levitov said.“That is a very striking thing, and it is the same physics as that in ordinary fluids, but happening with electrons on the nanoscale. That’s a clear signature of electrons being in a fluid-like regime.”
The conditions needed to be carefully controlled to produce these whirlpools – if the gaps through which the electrons flowed were widened the votrices, and indeed any turbulence – disappeared. Close to the transition point where smooth flow replaces turblence a vortex was seen to split into two, one of the predicted behaviors Levitov and team hoped to witness.
It is hoped the observations will have real-world applications like leading to ways to power low-energy electronics more efficiently.
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