Vórtices electrónicos vistos por primera vez: el flujo de fluidos podría impulsar la electrónica de próxima generación

Los vórtices electrónicos similares a líquidos, predichos durante mucho tiempo pero nunca observados, podrían usarse para la próxima generación de productos electrónicos de baja potencia. Crédito: Christine Daniloff, MIT

Este comportamiento de los electrones, similar a un líquido, predicho durante mucho tiempo pero nunca antes observado, podría explotarse para la próxima generación de productos electrónicos de baja eficiencia energética.

Las moléculas de agua, aunque son partículas separadas, fluyen juntas como líquidos, creando corrientes, olas, remolinos y otros fenómenos fluidos clásicos.

No es lo mismo con la electricidad. Aunque la electricidad también se compone de diferentes partículas, en este caso, electrones, las partículas son tan pequeñas que cualquier comportamiento colectivo entre ellas se ve ahogado por efectos más grandes a medida que los electrones viajan a través de los metales ordinarios. Sin embargo, en materiales específicos y bajo condiciones específicas, tales efectos desaparecen y los electrones pueden afectarse directamente entre sí. En estos casos particulares, los electrones pueden fluir juntos como un líquido.

Ahora, los físicos[{» 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 dispersión disminuye, y esto es interesante para los experimentos de diseño de electrónica de baja potencia”, dice Levitov. «Esta nueva observación es otro paso en esa dirección».

Levitov fue coautor del nuevo artículo con Eli Zeldov y otros en el Instituto de Ciencias Weizmann en Israel y la Universidad de Colorado en Denver.

En muchos materiales, como el oro (izquierda), los electrones fluyen a través de un campo eléctrico. Pero los físicos del MIT han descubierto que las partículas en el exótico ditellurida de tungsteno (derecha) pueden cambiar de dirección y girar como un líquido. Crédito: Investigadores

Compresión colectiva

Cuando la electricidad pasa a través de los metales y semiconductores más comunes, el impulso y las trayectorias de los electrones en la corriente se ven afectados por las impurezas del material y las vibraciones de los átomos del material. Estos procesos dominan el comportamiento de los electrones en los materiales ordinarios.

Pero los teóricos han predicho que en ausencia de tales procesos clásicos ordinarios, los efectos cuánticos deben hacerse cargo. Específicamente, los electrones deben percibir el comportamiento cuántico sutil de los demás y moverse juntos como un fluido electrónico viscoso similar a la miel. El comportamiento similar al líquido debería aparecer en materiales ultrapuros y a temperaturas cercanas a cero.

En 2017, Levitov y sus colegas de la Universidad de Manchester informaron la presencia de un electrón similar a un líquido. comportamiento en grafenoél[{» 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.