“Seeing is believing”, Leonid Levitov, professor of Physics at the Massachusetts Institute of Technology (MIT), expresses this old saying. Physicists have finally seen what they suspected: that electrons form eddies. A theoretical assumption that, for the first time, is confirmed by experience. These observations, which had been pre-published in February, definitely see the light this wednesday in the magazine Nature after being reviewed by independent experts.
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Separate (discrete, in physics parlance) particles are known to behave collectively. This happens, for example, with water molecules, which flow in liquid form forming currents, waves, eddies... The theory foresaw that electrons – negatively charged subatomic particles – also behave like this when they are free. The problem is that electrons are rarely free from external influences: they form the electric current and are so small that, when passing through metals –as when they run through a copper wire– they impose a certain behavior on them.
However, in certain materials and under specific conditions, these effects disappear and the electrons can directly influence each other. In these cases they can flow collectively, like a liquid. A first confirmation of this statement came in 2017, when it was possible to observe electrons flowing like a fluid in contact with a graphene plate.
Now physicists at MIT and the Weizmann Institute of Science have gone a step further and observed that electrons can not only flow in different directions, but also flow in vortices or eddies, a feature of fluid flow that Theorists predicted that these particles should present, but that it had never been observed. “Electron vortices were predicted in theory, but there has been no direct proof, and seeing is believing. Now we have seen it, and it is a clear example of this new regime in which electrons behave as a fluid, not as individual particles”, says Leonid Levitov, professor of physics at MIT, in the institution's press release.
Electron vortices were predicted in theory, but there has been no direct proof, and seeing is believing. Now we have seen
Leonid Levitov — MIT Physics Professor
"We know that when electrons behave like a fluid, the dissipation [de energía] decreases, and that is interesting to try to design low consumption electronics. This new observation is a further step in that direction,” he adds. Levitov is a co-author of the new work, along with Eli Zeldov and others from the Israel Weizmann Institute of Science and the University of Colorado Denver.
A honey-like fluid
When electricity passes through most ordinary metals and semiconductors, the moments—the rate of rotation of the particles about themselves—and the paths of the electrons in the current are influenced by impurities in the material and vibrations between the atoms in the material. same. These processes dominate the behavior of electrons in ordinary materials.
But theorists have predicted that, in the absence of these ordinary, classical processes, quantum effects should take over. That is, the electrons should pick up each other's delicate quantum behavior and move collectively, like a viscous, honey-like fluid of electrons. This liquid-like behavior should arise in ultralight materials and at near-zero temperatures.
In 2017, Levitov and colleagues at the University of Manchester reported the existence of this fluid behavior of electrons in graphene, an atomic sheet of carbon in which they etched a fine channel. They observed that a current sent through the channel could flow. This suggested that the electrons in the current were able to pass through certain points collectively, like a fluid, instead of getting stuck, like individual sand grains.
This first hint led Levitov to explore other electron fluid phenomena. In the new study, he and his colleagues at the Weizmann Institute for Science tried to visualize electron vortices. As they wrote then in their article, "the most striking and pervasive feature in regular fluid flow, the formation of vortices and turbulence, has yet to be observed in electron fluids despite numerous theoretical predictions," notes the MIT in its press release.
channel the flow of electrons
To visualize electron vortices, the team looked at tungsten diteluride (WTe2), an ultralight semimetallic compound that, when isolated in the two-dimensional form of a single atom, exhibits exotic electronic properties (i.e., different from those predicted). the laws of physics).
"Tungsten diteluride is one of the new quantum materials in which electrons interact strongly and behave like quantum waves instead of particles," says Levitov. "In addition, the material is very clean, which makes fluid-like behavior directly accessible."
The researchers synthesized pure tungsten diteluride crystals and exfoliated fine flakes of the material. They then used electron beam lithography and plasma etching techniques to create a pattern of each flake in a central channel connected to a small circular chamber-shaped space on each side. They etched the same pattern into fine flakes of gold, a standard metal with classic electronic properties.
They then passed a current through each patterned sample at ultra-low temperatures of 4.5 kelvins (about -269 degrees Celsius) and measured current flow at specific points on each sample using a superconducting quantum interference device (SQUID). -on-tip or SOT, in its English acronym). 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 the electrons flowed through the channels patterned in each material.
The researchers found that electrons flowing through the channels stamped into the gold flakes did so without reversing direction, even as part of the current passed through each side chamber before rejoining the main current.
Instead, the electrons flowing through the tungsten diteluride flowed down the channel and swirled into each side chamber, much as water would flow into a bowl. The electrons created small eddies 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 of the central strip," says Levitov. “It is something very striking, and it is the same physics as in ordinary fluids, but what happens with electrons at the nanoscale. It is a clear sign that the electrons are in a regime similar to that of fluids”.
The group's observations are the first direct visualization of eddies in an electrical current. The findings represent experimental confirmation of a fundamental property in the behavior of electrons. They may also offer clues as to how engineers could design low-power devices that conduct electricity more smoothly and with less resistance.