Long predicted but never observed, liquid-like electronic vortices could be used for next-generation low-power electronics. Credit: Christine Daniloff, MIT
Long predicted but never before observed, this liquid-like behavior of electrons could be exploited for efficient next-generation low-power electronics.
Water molecules, although they are individual particles, flow collectively as fluids, creating currents, waves, eddies, and other classical fluid phenomena.
It’s not the same with electricity. While electric current is also made up of individual particles—in this case, electrons—the particles are so small that any collective behavior among them is drowned out by larger influences as electrons flow through ordinary metals. However, in specific materials and under specific conditions, such effects fade and electrons can directly influence each other. In these specific cases, the electrons can flow collectively as a liquid.
Now physicists at MIT and the Weizmann Institute of Science have finally observed electrons flowing in vortices, or eddies—a hallmark of fluid flow that theorists predicted electrons should exhibit but that had never been seen before.
“Electron vortices are expected in theory, but there is no direct proof, and seeing is believing,” says Leonid Levitov, a physics professor at MIT. “Now we’ve seen it, and it’s a clear sign that we’re in this new regime where electrons behave like a liquid rather than individual particles.”
Reported July 6, 2022 in the journal Nature, the observations could help design more efficient electronics.
“We know when electrons go into the liquid state, [energy] the dissipation drops, and this is of interest when trying to design low-power electronics,” says Levitov. “This new observation is another step in that direction.”
Levitov co-authored the new paper with Eli Zeldov and others at the Weizmann Institute of Science in Israel and the University of Colorado at Denver.
In most materials such as gold (left), electrons flow along with the electric field. But MIT physicists have discovered that in the exotic tungsten ditelluride (right), the particles can reverse direction and spin like a liquid. Credit: Courtesy of the researchers
Collective squeeze
When electricity flows through most common metals and semiconductors, the momenta and trajectories of the electrons in the current are affected by impurities in the material and vibrations among the material’s atoms. These processes dominate the behavior of electrons in ordinary materials.
But theorists have predicted that in the absence of such ordinary, classical processes, quantum effects should take over. Namely, the electrons must adopt their delicate quantum behavior and move collectively, like a viscous, copper-like electronic fluid. This liquid-like behavior should appear in ultrapure materials and at near-zero temperatures.
In 2017, Levitov and colleagues at the University of Manchester reported signs of such liquid-like electron behavior in graphene, an atom-thin sheet of carbon, into which they etched a thin channel with several pinch points. They observed that a current sent through the channel could flow through the constrictions with little resistance. This suggests that the electrons in the current were able to slip through the pinch points collectively, much like a liquid, rather than being clogged up, like individual grains of sand.
This first indication prompted Levitov to investigate other electronic liquid phenomena. In the new study, he and his colleagues at the Weizmann Institute of Science tried to visualize electron vortices. As they write in their paper, “the most surprising and ubiquitous feature in the flow of ordinary liquids, the formation of eddies and turbulence, has not yet been observed in e-liquids despite numerous theoretical predictions.”
Channeling stream
To visualize the electron vortices, the team looked to tungsten ditelluride (WTe2), an ultrapure metallic compound that has been found to exhibit exotic electronic properties when isolated in a single-atom-thin, two-dimensional form.
“Tungsten ditelluride is one of the new quantum materials where electrons interact strongly and behave as quantum waves rather than particles,” says Levitov. “In addition, the material is very pure, making liquid-like behavior directly accessible.”
The researchers synthesized pure single crystals of tungsten ditelluride and exfoliated thin flakes of the material. They then used electron beam lithography and plasma etching techniques to pattern each flake in a central channel connected to a circular chamber on either side. They etched the same pattern into thin flakes of gold, a standard metal with ordinary, classical electronic properties.
They then passed a current through each patterned sample at ultra-low temperatures of 4.5 Kelvin (about -450 degrees Fahrenheit) and measured the current flow at specific points in each sample using a nanoscale scanning superconducting quantum interference device (SQUID) on the tip. This device was developed in Zeldov’s laboratory and measures magnetic fields with extremely high accuracy. Using the device to scan each sample, the team was able to observe in detail how electrons flow through the structured channels in each material.
The researchers note that electrons flowing through structured channels in gold flakes do so without reversing direction, even as part of the current passes through each side chamber before rejoining the main current. Conversely, electrons flowing through the tungsten ditelluride pass through the channel and spin into each side chamber, much like water when emptied into a bowl. The electrons created small eddies in each chamber before flowing back into the main channel.
“We observed a change in flow direction in the chambers, where the flow direction reversed direction compared to that in the central strip,” says Levitov. “It’s a very amazing thing, and it’s the same physics as in ordinary liquids, but it’s happening with electrons at the nanoscale. This is a clear sign that the electrons are in a liquid-like regime.
The group’s observations are the first direct visualization of swirling vortices in an electric current. The findings represent experimental confirmation of a fundamental property in the behavior of electrons. They may also offer pointers on how engineers can design low-power devices that conduct electricity in a smoother, less resistive manner.
“Signs of viscous electron flow have been reported in a number of experiments on different materials,” says Klaus Enslin, professor of physics at ETH Zurich in Switzerland, who was not involved in the research. “The theoretical expectation of eddy current flow has now been confirmed experimentally, adding an important milestone in the study of this new transport mode.”
Reference: “Direct observation of vortices in an electron fluid” by A. Aharon-Steinberg, T. Völkl, A. Kaplan, AK Pariari, I. Roy, T. Holder, Y. Wolf, AY Meltzer, Y. Myasoedov, ME Huber , B. Yan, G. Falkovich, L. S. Levitov, M. Hücker, and E. Zeldov, 6 July 2022, Nature.DOI: 10.1038/s41586-022-04794-y
This research was supported in part by the European Research Council, the German-Israel Foundation for Scientific Research and Development, and the Israel Science Foundation.
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