Electron whirlpool seen for the first time

Physicists at MIT and the Weizmann Institute of Science have visualized whirlpools in electron fluid. This is the first time they have observed electrons flowing in eddies, or whirlpools, the hallmark of hydrodynamic flow.

Theorists have long predicted electron vortexes or vortexes but had not seen one until now. Now, physicists have seen it, and it is a clear sign that electrons are in this new regime, where electrons behave as liquids, not as individual particles.

Leonid Levitov, professor of physics at MIT, said, “We know that when an electron enters a liquid state, [energy] dissipation drops, and that’s what’s interesting in designing low-power electronics. This new observation is another step in that direction.”

In 2017, Levitov and colleagues at the University of Manchester detected signs of fluid-like electron behavior in graphene. They carved thin channels on the graphene sheet with multiple pinch points. Sending current through a conduit can also flow through a constriction with less resistance.

This suggests that the current electrons can enter through the pinch points collectively, like a liquid, rather than clogging like individual grains of sand.

This discovery inspired Levitov to discover another electron fluid phenomenon. In this new study, physicists looked to visualize the vortex of electrons.

The author notes, “The most conspicuous and ubiquitous features in ordinary fluid flow, the formation of vortices and turbulence, have not been observed in electron fluids despite many theoretical predictions.”

Description: In most materials such as gold (left), electrons flow with an 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

For the visualization, the team used tungsten ditelluride (WTe2), one of the new quantum materials in which electrons interact strongly and behave as quantum waves rather than particles. They started by synthesizing pure single crystals of tungsten ditelluride and exfoliating thin flakes of the material.

Then, using e-beam lithography and plasma etching techniques, they patterned each flake into a central channel connected to circular spaces on either side. The same pattern was engraved into thin gold flakes.

They then passed a current through the patterned sample at a very low temperature of 4.5 kelvin (about -450 degrees Fahrenheit). Using a nanoscale scanning superconducting quantum interference device (SQUID) on the tip, they were able to measure the current flow at specific points in each sample. SQUID allowed the team to see the flow of electrons through patterned channels.

They found that electrons flowing through patterned channels in gold flakes did so without reversing direction, even as some of the current passed through each side chamber before recombining with the main current. Instead, electrons flowing through the tungsten ditelluride flow through the channels and spin into each side chamber, just as water does when emptying into a bowl. The electrons create tiny whirlpools in each chamber before flowing back into the main channel.

Levitov said, “We observed a change in the direction of flow in space, where the flow direction reversed compared to in the middle lane. That’s a very striking thing, and it’s the same physics as ordinary liquids but happens with electrons at the nanoscale. That’s a clear sign of the electrons being in a liquid-like regime.”

“This finding represents an experimental confirmation of a fundamental property in electron behavior. They can also explain how engineers can design low-power devices that conduct electricity more fluidly, less resistively.”

Klaus Ensslin, professor of physics at ETH Zurich in Switzerland, who was not involved in the study, said, “Signs of viscous electron flow have been reported in several experiments on different materials. The theoretical expectation of eddy-like current flow has now been confirmed experimentally, which adds an important milestone in the investigation of this new transport regime.”

Journal Reference:

1. Aharon-Steinberg, A., Völkl, T., Kaplan, A. et al. Direct observation of vortices in electron fluid. Nature 607, 74–80 (2022). DOI: 10.1038/s41586-022-04794-y

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