Simple superconducting devices could dramatically cut energy use in computing and other applications

MIT scientists and their colleagues have created a simple superconducting device that can pass electricity through electronics much more efficiently than is currently possible. As a result, the new diode, a type of switch, could significantly cut the amount of energy used in high-power computer systems, a major problem that is estimated to get much worse. Although in the early stages of development, this diode is still more than twice as efficient as similar diodes reported by others. It may even be integral to emerging quantum computing technologies.

The piece is reported in the magazine’s July 13 online issue Physical assessment letterwas also the subject of a news story in Journal of Physics.

“This paper demonstrates that the superconducting diode is a completely solved problem from an engineering perspective,” said Philip Moll, director of the Max Planck Institute for the Structure and Dynamics of Matter in Germany. Moll was not involved in the incident. “The beauty of [this] That’s the job [Moodera and colleagues] Achieve record performance without even trying [and] Their structure has not yet been optimized.”

“Our technique for the superconducting diode effect is robust and can operate across a range of temperatures,” said Jagadeesh Moodera, who led the current work and is a senior research scientist in the MIT Department. Scalability in simple systems can open the door to new technologies.” Physics. Moodera is also affiliated with the Materials Research Laboratory, the Francis Bitter Magnet Laboratory, and the Plasma Science and Fusion Center (PSFC).

The nano-sized rectangular diode – about 1,000 times thinner than the diameter of a human hair – can be easily expanded. Millions can be produced on a single silicon wafer.

Towards a superconducting switch

Diodes, devices that allow electricity to flow easily in one direction but not the other, are ubiquitous in computer systems. Modern semiconductor computer chips contain billions of diode-like devices called transistors. However, these devices can get very hot due to resistance, requiring large amounts of energy to cool the high-power systems in the data centers behind countless modern technologies, including cloud computing. . According to a 2018 news feature in NatureThese systems could use nearly 20% of the world’s energy in 10 years.

As a result, research into creating diodes made of superconductors has become a hot topic in condensed matter physics. That’s because superconductors conduct current with no resistance below a certain low temperature (the critical temperature) and are therefore much more efficient than their semiconductor cousins, which have losses. noticeably escapes energy in the form of heat.

However, until now, other approaches to this problem have involved much more complex physics. “The effect we found is due [in part] to a common property of superconductors that can be realized in a very simple, easy-to-understand way. It just looks you in the face,” Moodera said.

“This work is an important counterpoint to the current trend toward incorporating superconducting diodes,” said Moll of the Max Planck Institute. [with] Exotic physics, such as finite momentum coupled states. While in reality, superconducting diodes are a common and ubiquitous phenomenon present in classical materials, which are the result of some broken symmetry.”

A rather accidental discovery

In 2020, Moodera and colleagues observed evidence of a pair of exotic particles called Majorana fermions. These pairs of particles could lead to a new family of topological qubits, the building blocks of quantum computers. While considering methods for creating superconducting diodes, the team realized that the materials platform they developed for the Majorana study could also be applied to the diode problem.

They were right. Using that common foundation, they developed different versions of superconducting diodes, each more efficient than the last. For example, the first layer consists of a nano-thin layer of vanadium, a superconductor, shaped into a structure common to electronics (Hall bars). When they applied an extremely small magnetic field comparable to Earth’s magnetic field, they saw the diode effect – an extremely large dependence of current.

They then created another diode, this time coating the superconductor with a ferromagnetic substance (in their case, a ferromagnetic insulator), a material that creates its own tiny magnetic field. After applying an extremely small magnetic field to magnetize the iron magnet so that it created its own magnetic field, they found an even larger diode effect that was stable even after the initial magnetic field was turned off.

Popular properties

The group continues to find out what’s going on.

In addition to the ability to transmit electric current without resistance, superconductors have other properties that are less known but just as common. For example, they don’t like magnetic fields getting inside. When exposed to a small magnetic field, superconductors create an internal superconducting current that creates its own magnetic flux that cancels the external magnetic field, thus maintaining their superconducting state. This phenomenon, known as the Meissner screening effect, can be thought of as our body’s immune system releasing antibodies to fight infection by bacteria and other pathogens. However, this only works to a limited extent. Similarly, superconductors cannot completely block large magnetic fields.

The diodes the team created make use of this universal Meissner screening effect. The tiny magnetic field they applied – directly or through an adjacent ferromagnetic layer – activates the material’s screening flow mechanism to cancel out external magnetic fields and maintain superconductivity.

The team also discovered that another important factor in optimizing these superconducting diodes is the small differences between the two faces or edges of the diode device. These differences “create some form of asymmetry in the way the magnetic field enters the superconductor,” Moodera said.

By designing their own edge shape on the diodes to optimize these differences – for example, one edge has a jagged characteristic, while the other edge is not intentionally altered – the team found that they could increased performance from 20% to over 50%. Moodera says this discovery opens the door to devices with edges that can be “tuned” to achieve even greater performance.

In summary, the team discovered that edge asymmetry in superconducting diodes, the common Meissner screening effect found in all superconductors, and the third property of superconductors are called vortex pins, they all combine to create the diode effect.

“It is exciting to see how inconspicuous but common factors can produce significant effects,” said Yasen Hou, first author of the paper and a postdoc at the Francis Bitter Magnet Laboratory and PSFC. in observing the diode effect”. “What’s even more interesting is [this work] provides a simple approach with huge potential to further improve efficiency.”

Christoph Strunk is a professor at the University of Regensburg in Germany. “The present work demonstrates that supercurrents in simple superconducting strips can become irreversible,” said Strunk, who was not involved in the research. Furthermore, when combined with ferromagnetic insulators, the diode effect can even be maintained in the absence of an external magnetic field. The rectification direction can be programmed by the residual magnetization of the magnetic layer, which may have high potential for future applications. This work is important and fascinating from both a basic research and an applied standpoint.”

Youth collaborator

Moodera notes that the two researchers who created the engineered edges did so while in high school during a summer in Moodera’s lab. They are Ourania Glezakou-Elbert of Richland, Washington, who will attend Princeton University this fall, and Amith Varambally of Vestavia Hills, Alabama, who will attend Caltech.

“I didn’t know what to expect when I arrived in Boston last summer, and I certainly never expected it,” Varambally said. [be] one co-author in one Physical assessment letter paper.

“Every day is exciting, whether I am reading dozens of articles to better understand the diode phenomenon, or operating machinery to make new diodes for research, or engaging in conversation with Ourania, Dr. Hou and Dr. Moodera for our research.

“I am extremely grateful to Dr. Moodera and Dr. Hou for giving me the opportunity to work on such a fascinating project and to Ourania for being a wonderful research partner and friend.”

In addition to Moodera and Hou, the paper’s corresponding authors are professors Patrick A. Lee of the MIT Department of Physics and Akashdeep Kamra of the Autonomous University of Madrid. Other MIT authors are Liang Fu and Margarita Davydova of the Department of Physics, and Hang Chi, Alessandro Lodesani, and Yingying Wu, all of the Francis Bitter Magnet Laboratory and the Center for Plasma Science and Fusion. Chi is also affiliated with the U.S. Army’s CCDC Research Laboratory.

Authors also include Fabrizio Nichele, Markus F. Ritter and Daniel Z. Haxwell of IBM Research Europe; Stefan Ilić^{of the Center for Materials Physics (CFM-MPC); and F. Sebastian Bergeret of CFM-MPC and the Donostia International Physics Center.}

This work was supported by the Air Force Office of Sponsored Research, the Office of Naval Research, the National Science Foundation, and the Army Research Office. Additional sponsors are the European Research Council, the European Union’s Horizon 2020 Research and Innovation Framework Programme, the Spanish Ministry of Science and Innovation, the AV Humboldt Foundation and the Office of Basic Sciences of the Department of Energy.