Physicists see electron vortices for the first time

Although they are separate particles, water molecules flow together like liquids, producing streams, waves, eddies, and other classical fluid phenomena.

Not so with electricity. Although an electric current is also a construction of different particles – in this case electrons—the particles are so small that each collective behavior among them is drowned out by greater influences as electrons pass through common metals. But in certain materials and under specific conditions, such effects fade and electrons can directly influence each other. In these cases, electrons can collectively flow like a liquid.

Now, physicists at MIT and the Weizmann Institute of Science have observed electrons flowing in eddies or eddies — a feature of fluid flow that theorists predicted electrons should exhibit, but has never been observed until now.

“Electron vortices are theoretically expected, but there is no direct evidence, and seeing is believing,” said Leonid Levitov, a physics professor at MIT. “Now we’ve seen it, and it’s a clear signature of being in this new regime, where electrons behave like a liquid, not individual particles.”

The sightings, reported in the journal Naturecould inform the design of more efficient electronics.

“We know when electrons go into a liquid state, [energy] The dissipation is decreasing, which is important when designing low-power electronics,” Levitov says. “This new observation is another step in that direction.”

Levitov is co-author of the new paper, along with Eli Zeldov and others from the Weizmann Institute for Science in Israel and the University of Colorado in Denver.

A collective pressure

When electricity flows through most common metals and semiconductors, the moments and trajectories of electrons in the current are affected by impurities in the material and vibrations between the atoms of the material. These processes dominate the electron behavior in ordinary materials.

But theorists have predicted that in the absence of such ordinary, classical processes, quantum effects should take over. Electrons should pick up on each other’s delicate quantum behavior and move collectively, like a viscous, honey-like electron liquid. This liquid-like behavior should manifest itself in ultra-clean materials and at temperatures close to zero.

In 2017, Levitov and colleagues at the University of Manchester reported signatures of such a liquid-like electron behavior in graphene, an atom-thin layer of carbon on which they’ve etched a thin channel with several pinch points. They noted that a current sent through the canal could flow through the strictures with little resistance. This suggested that the electrons in the flow could collectively squeeze through the pinch points, much like a liquid, rather than clog, like individual grains of sand.

This first indication prompted Levitov to investigate other electron liquid phenomenon. In the new study, he and colleagues at the Weizmann Institute for Science attempted to visualize electron vortices. As they write in their paper, “the most striking and ubiquitous feature in the flow of regular liquids, the formation of vortices and turbulence, has not yet been observed in electron liquids despite numerous theoretical predictions.”

Channeling flow

To visualize electron vortices, the team looked to tungsten ditelluride (WTe2), an ultra-clean 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 where electrons interact strongly and behave like quantum waves instead of particles,” Levitov says. “In addition, the material is very clean, making the liquid-like behavior immediately 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 form each flake in a center channel connected to a circular chamber on either side. They etched the same pattern in thin gold flakes – a standard metal with ordinary, classic electronic properties.

They then ran a current through each patterned sample at ultra-low temperatures of 4.5 Kelvin (about -450 degrees Fahrenheit) and measured the current at specific points in each sample, using a nanoscale scan. superconducting quantum interference device (squid) on a tip. Developed in Zeldov’s laboratory, this device measures magnetic fields with extremely high precision. By using the device to scan each sample, the team was able to observe in detail how electrons flowed through the patterned channels in each material.

The researchers noted that electrons flowing through patterned channels in gold flakes did so without changing direction, even when some of the current passed through each side chamber before rejoining the main stream. In contrast, electrons flowing through tungsten ditelluride flowed through the channel and swirled in each side chamber, much like water would when emptying into a bowl. The electrons created small vortices in each chamber before flowing back into the main channel.

“We saw a change in the flow direction: in the chambers, where the flow direction reversed compared to that in the central strip,” Levitov says. “That’s very striking, and it’s the same physics as that in regular liquids, but happens with electrons at the nanoscale. That’s a clear signature of electrons that are in a liquid-like regime.”

The group’s observations are the first direct visualization of swirling vortices in a electric current† The findings represent an experimental confirmation of a fundamental property in electron behavior. They may also provide clues as to how engineers can design low-power devices that conduct electricity in a lake liquidless resistant way.