For the first time, physicists have witnessed something incredibly exciting: electrons forming vortices like a liquid.
Scientists have long predicted this behavior but have never observed it before. And it could be the key to developing more efficient and faster next-generation electronics.
“Electron vortices are theoretically expected, but there was no direct evidence, and seeing is believing,” says one of the researchers behind the new study, MIT physicist Leonid Levitov.
“Now we’ve seen it, and it’s a clear signature that we’re in this new regime where electrons behave like a liquid, not individual particles.”
While electrons flowing in a vortex might not sound all that groundbreaking, it’s a big deal, as flowing like a fluid results in more energy being delivered to the end point, rather than being lost along the way as electrons flow through things like Impurities are pushed around in the vortex matter or vibrations in atoms.
“We know that when electrons go into a liquid state, [energy] power dissipation decreases, and this is of interest when trying to design electronics with low power consumption,” 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 in Denver.
Of course, we already know that electrons in superconductors can bounce off each other and flow without resistance, but this is the result of the formation of what are 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 move as a unit according to the principles of fluid dynamics, carrying one another across a surface, creating streams and eddies as they do so.
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 normal metals and even semiconductors. These “distractions” knock electrons around on their journey and prevent them from exhibiting liquid-like behavior.
It’s long been predicted that these interferences should disappear in special materials at near-freezing temperatures, allowing the electrons to move like a liquid…but the problem was that nobody has actually been able to prove this until now.
There are two basic characteristics of a liquid: linear flow, where separate particles flow all in parallel as one; and the formation of whorls and whorls.
The first was observed using graphs in 2017 by Levitov and colleagues at the University of Manchester. In atomic-thin layers of carbon, Levitov and his team showed that an electric current can flow through a pinch point like a liquid and not like grains of sand.
But nobody had seen the second feature. “The most striking and ubiquitous feature in the flow of normal liquids, the formation of vortices and turbulence, has not yet been observed in electron liquids, despite numerous theoretical predictions,” the researchers write.
To find out, the team took pure single crystals of an ultra-pure material known as tungsten ditelluride (WTe2) and cut into single-atom-thin flakes.
They then etched a pattern into a central channel with a circular chamber on either side, creating a “maze” for an electric current to flow through. They etched the same pattern onto gold flake, which does not have the same ultrapure properties as the tungsten ditelluride, so served as a control.
Above: The diagram on the left shows how electrons flowed in gold (Au) flakes in the experiment. The image at right shows a simulation of how liquid-like electrons would behave.
After cooling the material to about -269 degrees Celsius (4.5 Kelvin or -451.57 Fahrenheit), they passed an electric current through it and measured the flux 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 after the stream had passed through each side chamber before returning to the main stream.
In contrast, the electrons within the tungsten ditelluride flowed through the channel and then swirled into each side chamber, creating eddies before flowing back into the main channel – as one would expect from a liquid.
“We observed a change in flow direction in the chambers, where the flow direction reversed compared to that in the median,” says Levitov.
“This is a very striking thing, and it’s the same physics as in ordinary liquids, but it’s happening with nanoscale electrons. This is a clear signature that electrons are in a liquid-like regime.”
Top: The column on the left shows the electrons passing through tungsten ditelluride (WTe2) compared to the hydrodynamic simulations on the left Pillar.
Of course, this experiment was carried out at ultra-cold temperatures using a special material – something like this won’t be in your home appliances any time soon. There were also size restrictions on the chambers and center canal.
But this is the “first direct visualization of swirling vortices in an electrical flow,” according to the press release. This confirmation is not just that electrons can Behaving like a liquid, the advance could also help engineers better understand how to harness this potential in their devices.
The research was published in Nature.