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Sunday, December 04, 2022

After years of research, physicists observe the flow of electrons in fluid-like eddies

For the first time, physicists have witnessed something incredibly exciting: electrons swirling like a fluid.

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

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

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

While electrons flowing in a vortex might not seem all that innovative, this is important because flowing like a fluid results in more energy being delivered to the endpoint, rather than being lost along the way as electrons are pushed around by things like impurities in the material. or vibrations in atoms.

“We know when electrons are in a fluid state, [energy] dissipation drops, and that’s interesting for trying to design low-power electronics,” says Levitov. “This new observation is another step in that direction.”

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

Of course, we already know that electrons can bounce off each other and flow without resistance in superconductors, but this is the result of the formation of something known as ‘Cooper Pairs’, and is not a true example of electrons flowing collectively like a fluid.

Take water, for example. Water molecules are individual particles, but they travel as one according to the principles of fluid dynamics, transporting each other across a surface, forming streams and eddies as they go.

An electric current should essentially be able to do the same, but any collective behavior of electrons is often replaced by impurities and vibrations in normal metals and even semiconductors. These ‘distractions’ push electrons around as they travel and prevent them from exhibiting fluid-like behavior.

It has long been predicted that in special materials at temperatures close to zero, these interferences should disappear, allowing electrons to move like a fluid…

There are two fundamental characteristics of a fluid: linear flow, where all separate particles flow in parallel as one; and the formation of vortices and vortices.

The first was observed by Levitov and colleagues at the University of Manchester in 2017 using graphene. In thin sheets of carbon, Levitov and his team showed that an electric current can flow through a pinch point like fluid, rather than grains of sand.

But no one had seen the second feature. “The most striking and ubiquitous feature in regular fluid flow, the formation of vortices and turbulence, has not yet been observed in electron fluids, despite numerous theoretical predictions,” the researchers write.

To find out, the team took pure, simple crystals of an ultra-clean material known as tungsten ditelluride (WTetwo) and cut thin flakes from a single atom.

They then etched a pattern into a central channel with a circular chamber on either side, creating a “labyrinth” for the electrical current to pass through. They etched the same pattern onto gold flakes, which did not have the same ultra-clean properties as tungsten ditelluride and therefore acted as a control.

Above: The diagram on the left shows how electrons flowed in the gold (Au) flake experiment. The image on the right shows a simulation of how they expect fluid-like electrons to behave.

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

In the gold flakes, electrons flowed through the labyrinth without changing direction, even as the current passed through each side chamber before returning to the main current.

In contrast, within the tungsten ditelluride, electrons flowed through the channel and then rotated in each side chamber creating eddies, before flowing back into the main channel – as you would expect a fluid to do.

“We observed a change in flow direction in the chambers, where the flow direction reversed direction with respect to the central strip,” says Levitov.

“This is a very impressive thing, and it’s the same physics as in ordinary fluids, but happening with nanoscale electrons. That’s a clear signature of electrons in a fluid-like regime.”

Above: The left column shows how electrons flowed through tungsten ditelluride (WTetwo) compared to the hydrodynamic simulations on the left column.

Of course, this experiment was done in ultra-cold temperatures with a specialized material – it’s not something that’s going to happen in your home appliances anytime soon. There were also size restrictions on the chambers and the middle channel.

But this is the “first direct view of whirling vortices in an electrical current,” as the press release explains. It is not just this confirmation that electrons I can behave like a fluid, the advancement could also help engineers better understand how to harness this potential in their devices.

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