Electron Whirlpools Seen for the First Time – Fluid Flow Could Enable Next-Generation Electronics

Long predicted but never observed, fluid-like electron whirlpools could be leveraged for next-gen low-power electronics. Credit: Christine Daniloff, MIT

Long predicted but never observed before, this fluid-like electron behavior could be leveraged for efficient low-power next-generation electronics.

Water molecules, although being distinct particles, flow collectively as liquids, creating streams, waves, whirlpools, and other classic fluid phenomena.

It isn’t the same with electricity. While an electric current is likewise constructed of distinct particles — in this case, electrons — the particles are so small that any collective behavior among them is drowned out by larger influences as electrons pass through ordinary metals. However, in particular materials and under specific conditions, such effects fade away, and electrons can directly influence each other. In these specific instances, electrons can flow collectively like a fluid.

Now, physicists at

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] dissipation drops, and that’s of interest in trying to design low-power electronics,” Levitov says. “This new observation is another step in that direction.”

Levitov is a 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 at Denver.

In most materials like gold (left), electrons flow with the electric field. But MIT physicists have found that in exotic tungsten ditelluride (right), the particles can reverse direction and swirl like a liquid. Credit: Courtesy of the researchers

A collective squeeze

When electricity runs through most ordinary metals and semiconductors, the momenta and trajectories of electrons in the current are influenced by impurities in the material and vibrations among the material’s atoms. These processes dominate electron 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 on each other’s delicate quantum behavior and move collectively, as a viscous, honey-like electron fluid. This liquid-like behavior should emerge in ultraclean materials and at near-zero temperatures.

In 2017, Levitov and colleagues at the University of Manchester reported signatures of such fluid-like electron behavior in graphene, an

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.

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