Long predicted but never before observed, this fluid-like behavior of electrons could be exploited for efficient, low-power next-generation electronics. Water molecules, although separate particles, flow collectively as fluids, creating currents, waves, eddies, and other classic fluid phenomena. It is not the same as electricity. While an electric current is similarly made of different 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 ordinary metals. However, in certain materials and under certain conditions, such phenomena disappear and electrons can directly affect each other. In these particular cases, the electrons can flow collectively like a fluid. Now, physicists at MIT and the Weizmann Institute of Science have finally observed electrons flowing in eddies, or eddies—a feature of fluid flow that theorists predicted electrons should exhibit, but that has never been seen before. “Electron vortices are theoretically expected, but there is no direct proof, and seeing is believing,” says Leonid Levitov, a professor of physics at MIT. “Now we’ve seen it, and it’s a clear signature of existence in this new regime, where electrons behave as a fluid, not as individual particles.” Reported July 6, 2022 in the journal Nature, the observations could help design more efficient electronics. “We know when electrons go into a liquid state, [energy] The dissipation is reduced, and this is interesting in the effort to design low-power electronics,” says Levitov. “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 at Denver. In most materials such as gold (left), electrons flow with the electric field. But MIT physicists discovered that in the exotic tungsten ditelluride (right), the particles can reverse direction and swirl like a liquid. Credit: Courtesy of the researchers
A collective squeeze
When electricity flows through most common metals and semiconductors, the momentum and trajectories of the electrons in the current are affected by impurities in the material and vibrations between the material’s atoms. These processes dominate the behavior of electrons in ordinary materials. But theorists have predicted that in the absence of such ordinary, classical processes, quantum effects should dominate. That is, electrons should pick up on each other’s subtle quantum behavior and move collectively, as a thick, honey-like electron fluid. This liquid-like behavior should occur in ultrapure materials and near-zero temperatures. In 2017, Levitov and colleagues at the University of Manchester reported signatures of such fluid-like electron behavior in graphene, an atomically thin sheet of carbon on which they etched a thin channel with multiple pinch points. They observed that a current sent through the channel could flow through the constrictions with little resistance. This suggests that the electrons in the current were able to squeeze through the pinch points collectively, like a fluid, rather than being blocked, like individual grains of sand. This first clue prompted Levitov to explore other electron flow phenomena. In the new study, he and his colleagues at the Weizmann Institute of Science tried to visualize the electron vortices. As they write in their paper, “the most striking and ubiquitous feature in the flow of normal fluids, the formation of eddies and turbulence, has not yet been observed in electron fluids despite numerous theoretical predictions.”
Stream channeling
To image the electron vortices, the team looked at tungsten ditelluride (WTe2), an ultrapure metallic compound that has been found to exhibit exotic electronic properties when isolated in a thin, one-atom, two-dimensional form. “Tungsten ditelluride is one of the new quantum materials where electrons interact strongly and behave as quantum waves rather than particles,” says Levitov. “Furthermore, the material is very clean, which makes liquid-like behavior readily accessible.” The researchers synthesized pure tungsten ditelluride single crystals and exfoliated thin flakes of the material. They then used electron beam lithography and plasma etching techniques to design each flake in a central channel connected to a circular chamber on either side. They etched the same pattern on thin flakes of gold — a typical metal with ordinary, classical electronic properties. They then ran a current through each patterned sample at extremely low temperatures of 4.5 Kelvin (about -450 degrees Fahrenheit) and measured the current flow at specific points on each sample using a nanoscale superconducting quantum interference device (SQUID) on one tip . This device was developed in Zeldov’s laboratory 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 flowed through the patterned channels in each material. The researchers observed that electrons flowing through patterned channels in gold flakes did so without reversing direction, even when part of the current passed through each side chamber before joining the main current. Instead, electrons flowing through the tungsten ditelluride flow through the channel and swirl into each side chamber, just as water would when poured into a bowl. The electrons created small vortices in each chamber before flowing back into the main channel. “We observed a change in flow direction in the chambers, where the flow direction reversed direction compared to that in the central lane,” says Levitov. “This is a very impressive thing, and it’s the same physics as in ordinary fluids, but it’s happening with electrons at the nanoscale. This is a clear signature of electrons being in a fluid-like regime.” The team’s observations are the first direct imaging of swirling eddies in an electric current. The findings represent an experimental confirmation of a fundamental property in the behavior of electrons. They may also offer clues to how engineers might design low-power devices that carry electricity in a more fluid and less resistant way. “Signatures of viscous electron flow have been reported in various experiments in different materials,” says Klaus Ensslin, professor of physics at ETH Zurich in Switzerland, who was not involved in the study. “The theoretical expectation of eddy-like current flow has now been confirmed experimentally, which adds an important milestone to the investigation of this new transport regime.” Reference: “Direct observation of vortices in an electron fluid” by A. Aharon-Steinberg, T. Völkl, A. Kaplan, AK Pariari, I. Roy, T. Holder, Y. Wolf, AY Meltzer, Y. Myasoedov, ME Huber, B. Yan, G. Falkovich, LS 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-Israel Foundation for Scientific Research and Development, and the Israel Science Foundation.
