Long-predicted, but never observed, liquid-like swirls of electrons could be harnessed for next-generation low-power electronics. Photo credit: Christine Daniloff, MIT
This liquid-like electron behavior, long predicted but never observed before, could be exploited for efficient, low-power next-generation electronics.
Although water molecules are distinct particles, they flow collectively as liquids, creating streams, waves, eddies, and other classic fluid phenomena.
This is not the case with electricity. While an electric current is also made up of different particles – in this case electrons – the particles are so small that any collective behavior between them is drowned out by larger influences when electrons pass through ordinary metals. With certain materials and under certain conditions, however, such effects subside and electrons can directly influence each other. In these special cases, electrons can flow collectively like a liquid.
Well, physicists at[{” attribute=””>MIT and the Weizmann Institute of Science have finally observed electrons flowing in vortices, or whirlpools — a hallmark of fluid flow that theorists predicted electrons should exhibit, but that has never been seen before now.
“Electron vortices are expected in theory, but there’s been no direct proof, and seeing is believing,” says Leonid Levitov, 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.”
Reported on July 6, 2022, in the journal Nature, the observations could inform the design of more efficient electronics.
“We know when electrons go in a fluid state, [energy] power dissipation decreases, and this is of interest when trying to design low-power electronics,” says Levitov. “This new observation is another step in that direction.”
Levitov is co-author of the new paper, along with Eli Zeldov and others at the Weizmann Institute for Science in Israel and the University of Colorado in Denver.
In most materials, such as gold (left), electrons flow with the electric field. But MIT physicists have found that the particles in exotic tungsten ditelluride (right) can reverse direction and swirl like a liquid. Credit: Courtesy of the researchers
A collective crush
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 between the atoms of the material. These processes dominate electronic behavior in ordinary materials.
But theorists have predicted that in the absence of such ordinary, classical processes, quantum effects should take over. Namely, electrons should pick up each other’s sensitive quantum behavior and move together as a viscous, honey-like electronic fluid. This liquid-like behavior should occur in ultra-pure materials and at near-zero temperatures.
In 2017, Levitov and colleagues from the University of Manchester reported signatures of such liquid-like electron behavior in graphene[{” attribute=””>atom-thin sheet of carbon onto 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 suggested that the electrons in the current were able to squeeze through the pinch points collectively, much like a fluid, rather than clogging, like individual grains of sand.
This first indication prompted Levitov to explore other electron fluid phenomena. In the new study, he and colleagues at the Weizmann Institute for Science looked to visualize electron vortices. As they write in their 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.”
Channeling flow
To visualize electron vortices, the team looked to tungsten ditelluride (WTe2), an ultraclean metallic compound that has been found to exhibit exotic electronic properties when isolated in single-atom-thin, two-dimensional form.
“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 behavior directly accessible.”
The researchers synthesized pure single crystals of tungsten ditelluride, and exfoliated thin flakes of the material. They then used e-beam lithography and
The researchers observed that electrons flowing through patterned channels in gold flakes did so without reversing direction, even when some of the current passed through each side chamber before joining back up with the main current. In contrast, electrons flowing through tungsten ditelluride flowed through the channel and swirled into each side chamber, much as water would do when emptying into a bowl. The electrons created small whirlpools in each chamber before flowing back out into the main channel.
“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.”
The group’s observations are the first direct visualization of swirling vortices in an electric current. The findings represent an experimental confirmation of a fundamental property in electron behavior. They may also offer clues to how engineers might design low-power devices that conduct electricity in a more fluid, less resistive manner.
“Signatures of viscous electron flow have been reported in a number of experiments on different materials,” says Klaus Ensslin, professor of physics at ETH Zurich in Switzerland, who was not involved in the study. “The theoretical expectation of vortex-like current flow has now been confirmed experimentally, which adds an important milestone in the investigation of this novel transport regime.”
Reference: “Direct observation of vortices in an electron fluid” by A. Aharon-Steinberg, T. Völkl, A. Kaplan, A. K. Pariari, I. Roy, T. Holder, Y. Wolf, A. Y. Meltzer, Y. Myasoedov, M. E. 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-Israeli Foundation for Scientific Research and Development, and by the Israel Science Foundation.