For the first time, physicists have witnessed something incredibly exciting: electrons that form vortices just like a liquid.
This behavior is one that scientists have long predicted, but never observed before. And that could be the key to developing more efficient and faster next-generation electronics.
“Electron vortices are expected in theory, but there has been no direct evidence, and to see is to believe,” said one of the researchers behind the new study, physicist Leonid Levitov from MIT.
“Now we’ve seen it, and it’s a clear signature of being in this new regime, where electrons behave like a liquid, not like individual particles.”
Although electrons flowing in a vortex may not sound as groundbreaking, this is a big deal because flowing as a liquid results in more energy being delivered to the endpoint, rather than being lost along the way while electrons are being pushed around by things like impurities in material or vibrations in atoms.
“We know when electrons are in a liquid state, [energy] “Dissipation is falling, and it’s of interest to try 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 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 without resistance in superconductors, but this is the result of the formation of something known as the ‘Cooper pair’, and is not a true example of electrons floating collectively as a liquid.
Take water, for example. Water molecules are individual particles, but they move as one according to the principles of fluid dynamics, carrying each other over a surface, creating streams and whirlpools as they go.
An electric current should be able to do essentially the same thing, but any collective behavior of electrons is usually overridden by impurities and vibrations in ordinary metals and even semiconductors. These “distractions” turn electrons around as they travel and stop them from exhibiting fluid-like behavior.
It has long been predicted that in special materials at near zero temperatures, these interferences would disappear so that the electrons could move like a liquid … but the problem was that no one had actually been able to prove that this was the case until now.
There are two basic features of a liquid: linear flow, where separate particles all 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 back in 2017 using the graph. In carbon-thin sheets of carbon, Levitov and his team showed that an electric current could flow through a terminal point as a liquid, rather than as a grain of sand.
But no one had seen the second feature. “The most striking and ubiquitous feature of the flow of ordinary fluids, the formation of vortices and turbulence, has not yet been observed in electron fluids despite a number of theoretical predictions,” the researchers write.
To find out, the team took pure, single crystals of an ultra-pure material known as tungsten ditelluride (WTe)2) and cut single-atom flakes.
They then etched a pattern into a central channel with a circular chamber on each side, creating a “maze” for an electric current to flow through. They etched the same pattern on gold flakes, which do not have the same ultra-pure properties as tungsten ditelluride and therefore acted as a control.
Above: The diagram on the left shows how electrons flowed in the experiment in gold (Au) flakes. The image to the right shows a simulation of how they expect liquid-like electrons to behave.
After cooling the material to around -269 degrees Celsius (4.5 Kelvin or -451.57 Fahrenheit), they ran an electric current through it and measured the current at specific points throughout the material, to map how the electrons flowed.
In the gold flakes, the electrons flowed through the maze without changing direction, even when the current had passed through each side chamber before returning to the main current.
In contrast, in tungsten ditelluride, the electrons flowed through the channel and then swirled into each side chamber, creating vortices, before flowing back to the main channel – as you would expect a liquid to do.
“We observed a change in the flow direction in the chambers, where the flow direction reversed the direction compared to that in the central strip,” says Levitov.
“It’s a very striking thing, and it’s the same physics as in ordinary liquids, but that happens with nanoscale electrons. It’s a clear signature that electrons are in a liquid-like regime.”
Above: The column on the left shows how the electrons flowed through tungsten ditelluride (WTe)2) compared to the hydrodynamic simulations on the left column.
Of course, this experiment was done at extremely cold temperatures with a specialized material – it is not something that will happen in your home gadgets right away. There were also size restrictions on the chambers and the central canal.
But this is the “first direct visualization of swirling vortices in an electric current” as the press release explains. Not only is this confirmation that electrons can behaving like a liquid, advances can also help engineers better understand how to exploit this potential in their devices.
The research is published in Nature.