Free-flowing electrons lead to cooler electronics

Can a fluid capable of flowing through materials without any electrical resistance lead to improved electronics that do not heat up as much as current electronics do? Prof. Shahal Ilani of the Weizmann Institute’s Department of Condensed Matter Physics has demonstrated just that, together with colleagues from Manchester University and University of California, Irvine.

For any given conductive material, the number of quantum channels is finite and is determined by its physical width. Thus, even a “perfect” electronic device, devoid of any imperfections, always has a type of resistance known as Landauer-Sharvin resistance. In the absence of interactions between electrons, Landauer-Sharvin resistance is unavoidable, putting a fundamental lower bound on the heating of computer chips.

When the current flow involves frequent collisions between electrons, the flow changes its nature from that of a diffusive motion of gas molecules to that of a collective motion of a liquid, a phenomenon known as “electron hydrodynamics.”

When electrons are forced to flow through a constriction between two confining walls, they can conduct somewhat better than the Landauer-Sharvin limit. This was theoretically explained to result from the lubrication of these walls by the electronic fluid, naturally raising a fundamental question – how would hydrodynamic electrons behave when there are no walls at all?

In an article published in Nature in September 2022, Prof. Ilani and his colleagues probed the behavior of hydrodynamic electrons in conditions where there are no confining walls.

The researchers imaged electronic flows in devices made of graphene encapsulated in hexagonal boron nitride (hBN) and patterned into the geometry of a DVD-like “Corbino disk.” However, because there are no contacts in the bulk of the Corbino disk, transport experiments can only measure the device’s total resistance and not how it is distributed in space.

To overcome this problem, the researchers used a unique nanotube-based scanning single-electron transistor (SET) technique that allowed them to directly image the spatial distribution of the device’s resistance throughout the device.

The group first imaged the resistance distribution at liquid helium temperature (4.2 degrees Kelvin), and discovered that there is significant resistance spread across the entire bulk of the disk, increasing as the current gets closer to the center of the disk.

However, an even more striking phenomenon occurred when these devices were warmed up to temperatures above 100 degrees Kelvin.

In a complementary theory paper, the researchers explained that the answer lies in the “Landauer highway”: electrons starting at the outer perimeter of the disk have many quantum channels available, but as they propagate toward the center of this disk, these channels get gradually blocked, forcing the flowing electrons back.

Although there is no physical barrier that scatters the electrons, these electrons still experience fundamental Landauer-Sharvin resistance coming from the termination of conduction channels.

In contrast to conventional rectangular devices, in which the number of quantum channels changes abruptly at the contacts, in a Corbino disk change in the number of channels is gradual, spreading resistance over the entire bulk of the disk.

While ballistic electrons cannot avoid Landauer-Sharvin resistance, the theory suggests that the collisions between electrons can help them completely evade it. The key observation is that such collisions allow electrons to transfer from channels that are about to be blocked to propagating channels, rather than being scattered back.

According to the Weizmann-Manchester-Irvine team, these findings could help researchers design and develop more efficient and improved electronics. “If computers could be made of electronic devices that are based on hydrodynamic electron flow, they would have significantly reduced heating,” says Prof. Ilani.

Prof. Shahal Ilani’s research is supported by the Helen and Martin Kimmel Award for Innovative Investigation; the Sagol Weizmann-MIT Bridge Program; and the Rosa and Emilio Segre Research Award.