Although they are discrete particles, water molecules collectively flow like liquids, producing currents, waves, eddies and other classical fluid phenomena.
Not so with electricity. Although an electric current is also a construction of discrete particles – in this case, electrons – the particles are so small that any collective behavior between them is drowned out by larger influences as electrons pass through base metals. But in certain materials and under specific conditions, these effects disappear and electrons can directly influence each other. In such cases, electrons can collectively flow like a fluid.
Now, physicists at MIT and the Weizmann Institute of Science have observed electrons flowing in vortices, or whirlpools — a hallmark of fluid flow that theorists predicted electrons should exhibit, but this has never been seen until now.
“Electron vortices are expected in theory, but there is no direct evidence, and seeing is believing,” says Leonid Levitov, a professor of physics at MIT. “Now we’ve seen that, and it’s a clear signature of being in this new regime, where electrons behave like a fluid, not like individual particles.”
The observations, reported today in the journal Nature, could inform the design of more efficient electronics.
“We know when electrons enter a fluid state, [energy] dissipation drops, and that’s interesting 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 of Science in Israel and the University of Colorado at Denver.
a collective grip
When electricity passes through most common metals and semiconductors, the moments and trajectories of electrons in the current are influenced by impurities in the material and vibrations between atoms in the material. These processes dominate the behavior of electrons in common materials.
But theorists predicted that in the absence of these common classical processes, quantum effects should prevail. That is, the electrons must pick up on each other’s delicate quantum behavior and move collectively, like a honey-like, viscous electron fluid. This liquid-like behavior should appear in ultra-clean materials and at near-zero temperatures.
In 2017, Levitov and colleagues at the University of Manchester reported signatures of this fluid-like electron behavior in graphene, a thin sheet of carbon from an atom on which they etched a thin channel with multiple points of compression. They observed that a current sent through the channel could flow through the constrictions with little resistance. This suggested that the electrons in the stream were able to squeeze through the pinch points collectively, like a fluid, rather than clogging up, like individual grains of sand.
This first indication led Levitov to explore other electron fluid phenomena. In the new study, he and colleagues at the Weizmann Institute for Science sought 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.”
To visualize electron vortices, the team looked for tungsten ditelluride (WTe2), an ultra-clean metallic compound that exhibits exotic electronic properties when isolated in the two-dimensional form of a thin atom.
“Tungsten ditelluride is one of the new quantum materials where electrons interact strongly and behave like quantum waves rather than particles,” says Levitov. “Also, the material is very clean, which makes fluid behavior directly accessible.”
The researchers synthesized pure tungsten ditelluride single crystals and exfoliated fine flakes of the material. They then used electron beam lithography and plasma etching techniques to pattern each flake into a central channel connected to a circular chamber on either side. They etched the same pattern into thin flakes of gold – a standard metal with both common and classical electronic properties.
They then ran a current through each standardized sample at ultra-low temperatures of 4.5 kelvins (about -450 degrees Fahrenheit) and measured current flow at specific points on each sample using a nanoscale scanning superconducting quantum interference device. (SQUID) at one end. This device was developed in Zeldov’s laboratory and measures magnetic fields with very high precision. 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 observed that electrons flowing through channels patterned in gold flakes did so without reversing direction, even as some of the current passed through each side chamber before joining the main current. In contrast, the electrons flowing through the tungsten ditelluride flowed through the channel and rotated in each side chamber, just as water would when emptying into a bowl. The electrons created little eddies in each chamber before flowing back into the main channel.
“We observed a change in the flow direction in the chambers, where the flow direction reversed the direction with respect to the central strip,” says Levitov. “This is a very impressive thing, and it’s the same physics as ordinary fluids, but happening with electrons at the nanoscale. This is a clear signature of electrons in a fluid regime.”
The group’s observations are the first direct visualization of swirling vortices in an electrical current. The findings represent experimental confirmation of a fundamental property in electron behavior. They can also offer clues to how engineers can design low-power devices that conduct electricity more fluidly and less resistively.
“Signatures of viscous electron flow have been reported in several experiments on different materials,” says Klaus Ensslin, a physics professor at ETH Zurich, 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 new transport regime.”
This research was supported, in part, by the European Research Council, the German-Israeli Foundation for Scientific Research and Development and the Israel Science Foundation.