‘Electron whirlpools’ observed for the first time

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The phenomenon was predicted by theory for a very long time. For the first time, researchers from the Weizmann Institute of Science and MIT are providing experimental evidence for its existence: they have witnessed the formation of electron vortices, a characteristic behavior of liquids. This discovery could lead to the development of next-generation low-power electronics.

Water molecules can flow collectively and form currents, waves and vortices or any other fluid behavior. Like water, electricity consists of separate particles (electrons); we could therefore expect it to behave in the same way, that is, as a liquid. However, electrons are much smaller than water molecules and are therefore affected more by their environment (the metal they pass through) than by their counterparts – so they cannot behave collectively.

The theory predicts, however, that under certain conditions – at extremely low temperatures, close to absolute zero, and in pure, flawless materials – the movement of electrons is controlled by quantum effects. From then on, they could flow like a liquid without exercising any resistance. By exploiting this phenomenon, it would therefore be possible to design more efficient electronic devices. But until now, there had never been any direct evidence of this particular behavior. gold,” to see is to believe “reminiscent of Leonid Levitov, professor of physics at MIT.

Extremely conductive electrons

In a new study, a team of researchers from the Weizmann Institute of Science and MIT reports that they have for the first time observed electrons circulating in the form of vortices in crystals of tungsten-ditelluride. These structures are common in liquids, but electrons generally cannot produce them.

As electrons pass through most ordinary metals and semiconductors, their moments and paths are affected by impurities in the material and the vibrations between the atoms that make it up. These processes control their behavior. But in their absence, quantum effects theoretically prevail: the electrons then no longer behave like individual particles, but manage to “capture” the quantum behavior of their congeners in order to move together. They thus form a viscous electronic liquid.

A few years ago, Levitov and his colleagues at the University of Manchester reported the first evidence of liquid-like behavior of electrons in graphene – an atom-thick sheet of carbon. In this experiment, they etched a thin channel with several bottlenecks through which they passed a stream. They then measured the electrical potential drop at each choker to measure the flow: they found that the conductivity of the electrons exceeded the maximum possible conductivity of free electrons, known as the Landauer ballistic limit.

In other words, they had proven that the electrons were able to float collectively, like a liquid, instead of clumping around the constriction, like individual grains of sand. Because of this first result, the researchers wanted to go further and try to observe electronic vortices – which they consider to be the most striking and ubiquitous property in the flow of regular fluids.

A new hydrodynamic flow mechanism

To do this, they used tungsten ditelluride (WTe2), an ultra-pure metallic compound that exhibits special properties in its two-dimensional shape (one layer one atom thick). ” Tungsten ditelluride is one of the new quantum materials where electrons interact strongly and behave like quantum waves rather than particles said Levitov.

In most materials like gold (left), electrons always flow in the same direction (that of the electric field). In two-dimensional tungsten ditelluride (right), particles can reverse direction and swirl like a liquid. © A. Aharon-Steinberg et al.

They produced fine flakes of this material, and then etched a narrow channel into it, connected by circular chambers on each side. By comparison, they similarly etched fine flakes of gold, a metal with common electronic properties. They then conducted a current through the channel at ultra-low temperatures (-268.6 ° C), and then measured the current of electrons at specific points.

The team found that in gold, electrons always flow in the same direction, even though they scatter out through the chambers before returning to the central channel. In contrast, the electrons in tungsten ditelluride created small vortices in the circular chambers, reversing their direction before returning to the main channel. ” The direction of flow reversed relative to the central band. […] It is the same physics as ordinary liquids, but it happens with nanoscale electrons. “, Explains the physicist.

The researchers therefore got a clear signature that the electrons had assumed a liquid-like behavior. These results suggest a new hydrodynamic flow mechanism in thin pure crystals and open up new possibilities for exploring and using electronic fluid in electronic systems with high mobility, the team writes in Nature. In fact, in the liquid state, energy dissipation decreases, which is particularly interesting for the development of low-power electronic devices. ” This new observation is another step in that direction. Levitov concludes.

Source: A. Aharon-Steinberg et al., Nature

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