Debutto dei vortici elettronici: i flussi di fluido potrebbero consentire la prossima generazione di elettronica

A lungo attesi ma mai osservati, i vortici di elettroni simili a fluidi possono essere sfruttati per la prossima generazione di elettronica a bassa energia. Credito: Kristen Danilov, MIT

Previsto a lungo ma mai osservato prima, il comportamento fluido dell’elettrone può essere sfruttato per un’elettronica efficiente a bassa energia di prossima generazione.

Sebbene le molecole d’acqua siano molecole distinte, scorrono collettivamente come liquidi, creando correnti, onde, vortici e altri fenomeni fluidi classici.

Non è lo stesso con l’elettricità. Sebbene la corrente elettrica sia composta in modo simile da particelle distinte – in questo caso, elettroni – le particelle sono così piccole che qualsiasi comportamento collettivo tra di loro è soffocato da effetti più grandi quando gli elettroni passano attraverso i metalli di base. Tuttavia, in determinati materiali e in condizioni specifiche, questi effetti svaniscono e gli elettroni possono influenzarsi direttamente a vicenda. In questi casi specifici, gli elettroni possono fluire collettivamente come liquidi.

Ora, fisici dentro[{” attribute=””>MIT and the Weizmann Institute of Science have finally observed electrons flowing in vortices, or whirlpools — a hallmark of fluid flow that theorists predicted electrons should exhibit, but that has never been seen before now.

“Electron vortices are expected in theory, but there’s been no direct proof, and seeing is believing,” says Leonid Levitov, professor of physics at MIT. “Now we’ve seen it, and it’s a clear signature of being in this new regime, where electrons behave as a fluid, not as individual particles.”

Reported on July 6, 2022, in the journal Nature, the observations could inform the design of more efficient electronics.

“We know when electrons go in a fluid state, [energy] La dissipazione diminuisce, il che è importante nel tentativo di progettare elettronica a bassa potenza”, afferma Levitov. “Questa nuova osservazione è un altro passo in quella direzione”.

Levitov è coautore del nuovo documento di ricerca, insieme a Eli Zeldov e altri presso il Weizmann Institute of Science in Israele e l’Università del Colorado a Denver.

Nella maggior parte dei materiali come l’oro (a sinistra), gli elettroni scorrono con un campo elettrico. Ma i fisici del MIT hanno scoperto che nell’esotico decloruro di tungsteno (a destra), le particelle possono invertire la direzione e ruotare come un liquido. Credito: per gentile concessione dei ricercatori

pressione collettiva

Quando l’elettricità passa attraverso la maggior parte dei metalli e dei semiconduttori ordinari, la coppia e i percorsi degli elettroni nella corrente sono influenzati dalle impurità nel materiale e dalle vibrazioni tra gli atomi del materiale. Questi processi dominano il comportamento degli elettroni nei materiali ordinari.

Ma i teorici hanno ipotizzato che in assenza di tali processi classici ordinari, gli effetti quantistici dovrebbero prendere il sopravvento. Vale a dire, gli elettroni devono captare il preciso comportamento quantistico l’uno dell’altro e muoversi collettivamente, come un liquido di elettroni viscoso, simile al miele. Questo comportamento simile a un liquido dovrebbe apparire nei materiali ultrapuri ea temperature vicine allo zero.

Nel 2017, Levitov e colleghi dell’Università di Manchester hanno riportato le firme di un elettrone simile a un fluido comportamento in grafeneE il[{” attribute=””>atom-thin sheet of carbon onto which they etched a thin channel with several pinch points. They observed that a current sent through the channel could flow through the constrictions with little resistance. This suggested that the electrons in the current were able to squeeze through the pinch points collectively, much like a fluid, rather than clogging, like individual grains of sand.

This first indication prompted Levitov to explore other electron fluid phenomena. In the new study, he and colleagues at the Weizmann Institute for Science looked 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.”

Channeling flow

To visualize electron vortices, the team looked to tungsten ditelluride (WTe2), an ultraclean metallic compound that has been found to exhibit exotic electronic properties when isolated in single-atom-thin, two-dimensional form.

“Tungsten ditelluride is one of the new quantum materials where electrons are strongly interacting and behave as quantum waves rather than particles,” Levitov says. “In addition, the material is very clean, which makes the fluid-like behavior directly accessible.”

The researchers synthesized pure single crystals of tungsten ditelluride, and exfoliated thin flakes of the material. They then used e-beam lithography and plasma etching techniques to pattern each flake into a center channel connected to a circular chamber on either side. They etched the same pattern into thin flakes of gold — a standard metal with ordinary, classical electronic properties.

They then ran a current through each patterned sample at ultralow temperatures of 4.5 kelvins (about -450 degrees Fahrenheit) and measured the current flow at specific points throughout each sample, using a nanoscale scanning superconducting quantum interference device (SQUID) on a tip. This device was developed in Zeldov’s lab 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 some of the current passed through each side chamber before joining back up with the main current. In contrast, electrons flowing through tungsten ditelluride flowed through the channel and swirled into each side chamber, much as water would do when emptying into a bowl. The electrons created small whirlpools in each chamber before flowing back out into the main channel.

“We observed a change in the flow direction in the chambers, where the flow direction reversed the direction as compared to that in the central strip,” Levitov says. “That is a very striking thing, and it is the same physics as that in ordinary fluids, but happening with electrons on the nanoscale. That’s a clear signature of electrons being in a fluid-like regime.”

The group’s observations are the first direct visualization of swirling vortices in an electric current. The findings represent an experimental confirmation of a fundamental property in electron behavior. They may also offer clues to how engineers might design low-power devices that conduct electricity in a more fluid, less resistive manner.

“Signatures of viscous electron flow have been reported in a number of experiments on different materials,” says Klaus Ensslin, professor of physics at ETH Zurich in 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 novel transport regime.”

Reference: “Direct observation of vortices in an electron fluid” by A. Aharon-Steinberg, T. Völkl, A. Kaplan, A. K. Pariari, I. Roy, T. Holder, Y. Wolf, A. Y. Meltzer, Y. Myasoedov, M. E. Huber, B. Yan, G. Falkovich, L. S. 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-Israeli Foundation for Scientific Research and Development, and by the Israel Science Foundation.