After Years of Searching, Physicists Observe Electrons Flow Into Whirlpools Like Liquids

For the first time, physicists have witnessed something very interesting: electrons form eddies like liquids.

This behavior is one that scientists have long predicted, but never observed before. And that could be the key to developing next-generation electronics that are more efficient and faster.

“Electron vortex is expected in theory, but there is no direct evidence yet, and seeing is believing,” said one of the researchers behind the new study, physicist Leonid Levitov of MIT.

“Now we’ve seen it, and it’s a clear sign of being in this new regime, where electrons behave as liquids, not as individual particles.”

While electrons flowing in a vortex might not sound like a breakthrough, it’s a big deal because flowing like a liquid results in more energy being sent to the end point, instead of being lost on the way while the electrons are pushed around by things like impurities in matter or vibrations in atoms.

“We know that when an electron enters a liquid state, [energy] dissipation drops, and it’s interesting to try to design low-power electronics,” Levitov said. “These new observations are another step in that direction.”

The work is a joint experiment between MIT, the Weizmann Institute of Science in Israel, and the University of Colorado in Denver.

Of course, we already know that electrons can bounce off each other and flow unimpeded in superconductors, but this is the result of the formation of something known as a ‘Cooper pair’, and not an actual example of electrons flowing collectively like liquids.

Take water, for example. Water molecules are individual particles, but they move as one according to the principles of fluid dynamics, carrying each other across surfaces, creating streams and eddies as they go.

An electric current should be able to do basically the same thing, but any collective behavior of electrons is usually overridden by impurities and vibrations in normal metals and even semiconductors. This ‘disturbance’ knocks electrons out as they move and stops them from exhibiting fluid-like behavior.

It has long been predicted that in special materials at near-zero temperatures, this interference will dissipate so electrons can move like liquids… but the problem is that no one has really been able to prove this, until now.

There are two fundamental features of fluids: linear flow, in which separate particles all flow in parallel as one; and the formation of eddies and eddies.

The first was observed by Levitov and his colleagues at the University of Manchester in 2017 using graphene. In an atomic-thin sheet of carbon, Levitov and his team showed that an electric current can flow through a pinch point like a liquid, not like a grain of sand.

But no one saw the second feature. “The most conspicuous and ubiquitous features in ordinary fluid flow, the formation of vortices and turbulence, have not been observed in electron fluids despite many theoretical predictions,” the researchers wrote.

To find this out, the team took pure single crystals of an ultra-clean material known as tungsten ditelluride (WTe₂) and slices into atomic-thin flakes.

They then carved a pattern into a central channel with circular spaces on either side, creating a ‘labyrinth’ for electric current to flow. They etched the same pattern on gold flakes, which do not have the same ultra-clean properties as tungsten ditelluride and therefore act as a control.

(Aharon-Steinberg et al., Natural2022)

Above: The diagram on the left shows how electrons flow in an experiment in gold (Au) flakes. The image on the right shows a simulation of how they would expect a liquid-like electron to behave.

After cooling the material to about -269 degrees Celsius (4.5 Kelvin or -451.57 Fahrenheit) they ran an electric current through it and measured the flow at specific points throughout the material, to map how electrons flow.

In gold flakes, electrons flow through the maze without changing direction, even when the current has passed through each side chamber before returning to the main stream.

In contrast, in tungsten ditelluride, electrons flow through channels and then spin into each side chamber creating a whirlpool, before flowing back into the main channel – as you would expect from a liquid.

“We observed a change in the direction of flow in space, where the flow direction reversed compared to in the middle lane,” Levitov said.

“That’s a very striking thing, and it’s the same physics as ordinary liquids, but it happens with electrons at the nanoscale. It’s a clear sign of electrons being in a liquid-like regime.”

(Aharon-Steinberg et al., Natural2022)

Above: The column on the left shows how electrons flow through tungsten ditelluride (WTe₂) compared to the hydrodynamic simulation on the left column.

Of course, these experiments were carried out at very cold temperatures with special materials – this is not something that will be happening in your home gadget anytime soon. There are also size constraints in the space and center channel.

But this is “the first direct visualization of a swirling eddy in an electric current” as the press release describes it. Not only this confirmation that electrons can behave as a fluid, advancements can also help engineers to better understand how to harness this potential in their devices.

This research has been published in Natural.

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