Although they are small particles, water molecules can flow together as liquids. This creates streams, waves and other classic fluid phenomena.
But electricity is different. An electric current can also be a construction of distinct particles, in this case electrons. However, the particles are so small that their collective behavior is drowned by larger influences when electrons pass through metals. However, these effects disappear in some materials and under certain conditions. Electrons can influence one another. These electrons can flow together like fluid.
Now, physicists from MIT and the Weizmann Institute of Science observed electrons moving in vortices or whirlpools. This is a hallmark of fluid flows that theorists predicted electrons would exhibit but has not been seen until now.
Professor of Physics at MIT Leonid Levitov says that although electron vortices seem to be expected in theory, there has been no direct evidence. "Seeing is believing," he said. "Now, we have seen it. It's a clear sign of being in the new regime, where electrons behave like fluids, not individual particles.
These observations were published in Nature and could be used to design more efficient electronics.
Levitov states that when electrons are in fluid state, energy dissipation falls. This is something Levitov believes is important in designing low-power electronics. This new observation is another step forward in that direction."
Levitov, Eli Zeldov, and other scientists at the Weizmann Institute for Science in Israel are co-authors of the new paper.
A collective squeeze
The momenta and trajectories electrons take when electricity flows through ordinary metals and semiconductors are affected by impurities and vibrations between the material's elements. These factors dominate electron behavior in ordinary materials.
Theorists predict that quantum effects will take over in the absence conventional processes. In other words, electrons should be able to pick up each other's delicate quantum behavior, and then move together as a viscous, honeylike electron fluid. This liquid-like behavior should be visible in ultraclean materials at near-zero temperatures.
Levitov and his colleagues from the University of Manchester discovered fluid-like electron behavior in graphene. This thin sheet of carbon is atom-thin. They etched a channel with many pinch points onto it. The constrictions could be passed through by a current that was sent through the channel. This result was not surprising. This suggests that electrons in the current could squeeze through the pinch points together, much like fluids, and not like individual grains of sand.
Levitov was inspired by this first sign to investigate other electron fluid phenomena. He and his colleagues from the Weizmann Institute for Science attempted to visualize electron vortices in their new study. "The most striking and universal feature in the flow regular fluids is the formation of vortices or turbulence," they wrote in their paper. This is despite many theoretical predictions.
The team used tungsten ditelluride, (WTe2) to visualize electron vortices. This ultraclean metallic compound has been shown to have exotic electronic properties when isolated in a single-atom-thin two-dimensional form.
Levitov states that tungssten ditelluride, one of the latest quantum materials, is where electrons interact strongly and behave like quantum waves instead of particles. The material's fluid-like behavior is also easily accessible due to its purity.
Researchers synthesized single crystals pure of tungsten ditelluride and exfoliated thin flakes. The researchers then created each flake using plasma etching and e-beam technology. Each flake was connected to a central channel that is connected to a circular chamber on the opposite side. The same pattern was then etched into thin gold flakes, a standard metal with classical electronic properties.
The current was then run through the patterned samples at ultralow temperatures of about 4.5 kelvins (or -450 Fahrenheit). They measured the current flow at certain points in each sample using a tip with a nanoscale scanning supraconducting quantum interference (SQUID). The device, which measures magnetic fields with high precision, was created in Zeldov’s lab. The team used the scanner to scan each sample and was able to see in detail how electrons flowed through each material's channels.
Researchers observed that electrons flowed through gold flakes patterned channels without changing direction. Even though some of the current passed through each chamber, it joined up with the main current. Instead, electrons flowing through tungsten dialuride flowed through each channel and swirled into the side chambers, just like water when it emptys into a bowl. Each chamber was filled with electrons, which created small whirlpools before returning to the main channel.
Levitov states that there was a shift in flow direction between chambers and central strips. It is striking and the same physics that occurs in fluids of ordinary size, but it is happening with electrons at the nanoscale. This is a clear sign that electrons are in fluid-like conditions.
These observations represent the first direct visualization of swirling vortices within an electric current. These findings are an experimental confirmation of a fundamental property of electron behavior. These findings may provide clues as to how engineers might design low power devices that conduct electricity more fluidly and less resistively.