Physicists see electron whirlpool for the first time

Even though they are separate particles, water molecules flow collectively as a liquid, producing streams, waves, whirlpools, and other classic fluid phenomena.

Not so with electricity. While electric current is also a different construction of particles – in this case, electrons – the particles are so small that any collective behavior between them is drowned out by a greater influence when electrons pass through ordinary metals. However, in certain materials and under certain conditions, these effects fade, and electrons can directly affect each other. In this case, the electrons can flow collectively like a liquid.

Now, physicists at MIT and the Weizmann Institute of Science have observed electrons flowing in eddies, or whirlpools — a fluid flow feature that theorists predicted electrons would exhibit, but that had not been seen until now.

“Electronic vortexes are expected in theory, but there is no direct evidence yet, and seeing is believing,” said Leonid Levitov, professor of physics at MIT. “Now we’ve seen it, and it’s a clear sign of being in this new regime, where electrons behave as liquids, not as individual particles.”

The observations, reported in the journal Nature, could inform the design of more efficient electronics.

“We know that when an electron is in a liquid state, [energy] dissipation goes down, and it’s interesting to try to design low-power electronics,” Levitov said. “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 of Science in Israel and the University of Colorado in Denver.

A collective blackmail

When electricity flows through most metals and ordinary semiconductors, the momentum and trajectory of electrons in the current are affected by impurities in the material and vibrations between the atoms of the material. This process dominates the behavior of electrons in ordinary materials.

But theorists had predicted that in the absence of such a usual classical process, quantum effects would take over. That is, the electrons must capture each other’s subtle quantum behavior and move collectively, as a liquid electron thick like honey. This fluid-like behavior should occur in ultraclean materials and at near-zero temperatures.

In 2017, Levitov and colleagues at the University of Manchester reported signs of fluid-like electron behavior in graphene, an atomic-thin sheet of carbon in which they carve thin channels with multiple pinch points. They observed that the current sent through the conduit could flow through the constriction with little resistance. This suggests that electrons in the current are able to penetrate the pinch point collectively, like a liquid, 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 his colleagues at the Weizmann Institute for Science looked to visualize electron vortices. As they write in their paper, “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.”

Dispensing flow

To visualize electron vortices, the team looked to tungsten ditelluride (WTe2), an ultraclean metal 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 in which electrons strongly interact and behave as quantum waves rather than particles,” Levitov said. “Additionally, the material is very clean, which makes its 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 plasma etching techniques to pattern each splinter into a central channel that connects to circular spaces on either side. They carved the same pattern into thin gold flakes — a standard metal with the usual classic electronic properties.

They then passed a current through each patterned sample at a very low temperature of 4.5 kelvin (about -450 degrees Fahrenheit) and measured the current flow at specific points throughout the sample, using a nanoscale scanning superconducting quantum interference device (SQUID) on the tip. The device was developed in Zeldov’s lab and measures magnetic fields with extremely high precision. Using the device to scan each sample, the team was able to observe in detail how electrons flow through the patterned channels in each material.

The researchers observed that electrons flowing through the patterned channels in the 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.

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

The group observation is the first direct visualization of swirling eddies in an electric current. This finding is an experimental confirmation of a fundamental property in the behavior of electrons. They might also offer pointers on how engineers can design low-power devices that conduct electricity in a more fluid and less resistive way.

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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