Evrim Yazgin
Cosmos science journalist
Ever watched the vortex that forms when you drain a bath or sink and thought, “I wonder if electrons can flow like this”? No? Well, you’re probably not alone, but physicists have been thinking about it for decades. Now, for the first time, scientists have observed electrons in a whirlpool – and it’s created a whirlwind of excitement.
Water molecules influence each other to produce the collective behaviour of classical fluid mechanics.
Because electrons are so small, any collective behaviour is usually drowned out when they conduct through metals. But, under specific conditions and in particular materials, electrons can be shown to behave like other fluids.
In fact, theorists have predicted for a while now that electrons should exhibit tornado-like flow. Now, a team of physicists from the Massachusetts Institute of Technology (MIT) in the US and the Weizmann Institute of Science in Israel are the first to actually observe electrons flowing in vortices.
The team’s findings, published in Nature, are believed to be able to help in the production of next-generation, more efficient electronics.
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“Electron vortices are expected in theory, but there’s been no direct proof, and seeing is believing,” says co-author Leonid Levitov, a professor of physics at MIT. “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.”
Normally, when electrons flow in a metal or semiconductor, their path is determined above all by impurities and vibrations in the material.
But when these classical considerations are removed, quantum effects take over and the electrons begin influencing each other’s quantum behaviour. The electrons then begin to move collectively as a viscous, honey-like electron fluid.
Such behaviour should be apparent in metals with virtually no impurities, known as “ultraclean” metals, and at temperatures approaching absolute zero.
It’s not the first time that liquid-like behaviour has been seen in electrons. Levitov and his colleagues at the University of Manchester reported signatures of fluidity in electron flow in graphene back in 2017.
This prompted Levitov to explore other fluid phenomena in electrons. And vortices are the big one. As the authors write in the Nature paper: “The most striking and ubiquitous feature in the flow of regular fluids, the formation of vortices and turbulence, has not yet been observed in electron fluids despite numerous theoretical predictions.”
So, the team turned to a single-atom layer of tungsten ditelluride (WTe2), an ultraclean metallic compound where fun electron effects have been seen.
“Tungsten ditelluride is one of the new quantum materials where electrons are strongly interacting and behave as quantum waves rather than particles,” Levitov says. “In addition, the material is very clean, which makes the fluid-like behaviour directly accessible.”
Etching a path with side chambers for the electrons in the tungsten ditelluride, the team did the same on gold flakes to compare the flow in a standard metal with ordinary electron behaviour. Cooling both samples to 4.5°C above absolute zero, the researchers passed a current through them and measured the flow at specific points.
As expected, the electrons in the gold flakes didn’t reverse direction, even when confronted with the side chambers. The electrons in the tungsten ditelluride, however, flowed through the main channel and swirled into the side chamber creating small whirlpools, as you would see with water, before re-joining the central path.
“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 says. “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.”
Apart from being the first direct observation of electron whirlpools, the findings present opportunities for engineering low-power devices with less resistance in current flow.
“We know when electrons go in a fluid state, [energy] dissipation drops, and that’s of interest in trying to design low-power electronics,” Levitov continues. “This new observation is another step in that direction.”