A straightforward superconducting gadget developed by MIT researchers and associates could transport current through electronic devices far more effectively than is currently achievable. The novel diode, a type of switch, might thereby significantly reduce the amount of energy required in high-power computing systems, a significant issue that is anticipated to get much worse.
The diode is more than twice as efficient as comparable ones that have been reported by others, while being in the early phases of research. It could even be integral to emerging quantum computing technologies. The work, which is reported in the July 13, 2023 online issue of Physical Review Letters, is also the subject of a news story in Physics Magazine.
“This paper showcases that the superconducting diode is an entirely solved problem from an engineering perspective,” says Philip Moll, Director of the Max Planck Institute for the Structure and Dynamics of Matter in Germany. Moll was not involved in the work. “The beauty of this work is that Moodera and colleagues obtained record efficiencies without even trying and their structures are far from optimized yet.”
“Our engineering of a superconducting diode effect that is robust and can operate over a wide temperature range in simple systems and potentially opening the door for novel technologies,” says Jagadeesh Moodera, leader of the current work and a senior research scientist in MIT’s Department of Physics. Moodera is also affiliated with the Materials Research Laboratory, the Francis Bitter Magnet Laboratory, and the Plasma Science and Fusion Center (PSFC).
The nanoscopic rectangular diode about 1,000 times thinner than the diameter of a human hair is easily scalable. Millions could be produced on a single silicon wafer.
Toward a superconducting switch
Diodes are a common component in computing systems because they make it easy for electricity to flow in one direction but not the other. The transistors found in today’s semiconductor computer chips number in the billions.
The present work demonstrates that the supercurrent in simple superconducting strips can become non-reciprocal. Moreover, when combined with a ferromagnetic insulator, the diode effect can even be maintained in the absence of an external magnetic field.
Professor Christoph Strunk
The high-power systems in the data centers that support a wide range of contemporary technologies, including cloud computing, must be cooled using enormous quantities of energy since these devices can become quite hot due to electrical resistance.
According to a 2018 news feature in Nature, these systems could use nearly 20% of the world’s power in 10 years.
The development of superconducting diodes has thus become a hot topic in condensed matter physics. This is because superconductors are significantly more efficient than their semiconducting counterparts, which exhibit observable energy loss in the form of heat, in that they transmit current with zero resistance below a specific low temperature (the critical temperature).
However, prior solutions to the issue have required significantly more intricate physics. “The effect we found is due in part to a ubiquitous property of superconductors that can be realized in a very simple, straightforward manner. It just stares you in the face,” says Moodera.
Says Moll of the Max Planck Institute for the Structure and Dynamics of Matter, “The work is an important counterpoint to the current fashion to associate superconducting diodes with exotic physics, such as finite-momentum pairing states. While in reality, a superconducting diode is a common and widespread phenomenon present in classical materials, as a result of certain broken symmetries.”
A somewhat serendipitous discovery
In 2020 Moodera and colleagues observed evidence of an exotic particle pair known as Majorana fermions. These particle pairings might result in the development of a new family of topological qubits, the fundamental units of quantum computing. The team came to the conclusion that the material platform they developed for the Majorana work might also be used to solve the problem of making superconducting diodes as they considered various methods.
They were right. They created various versions of superconducting diodes, each one more effective than the last, using that basic framework. The first, for instance, was made of a superconducting material called vanadium that was shaped into a shape called the Hall bar. This layer was nanoscopically thin. They observed a massive polarity dependence for current flow in the diode effect when they supplied a modest magnetic field that was comparable to the magnetic field of the Earth.
They then created another diode, this time layering a superconductor with a ferromagnet (a ferromagnetic insulator in their case), a material that produces its own tiny magnetic field. They discovered a larger diode effect that remained stable even after the initial magnetic field was switched off after adding a minor magnetic field to the ferromagnet in order to magnetize it so that it produces its own field.
Ubiquitous properties
The team went on to figure out what was happening.
Superconductors are known for their ability to conduct electricity without resistance, but they also possess other less well-known but no less common characteristics. For example, they don’t like magnetic fields getting inside. Superconductors create an internal supercurrent when they are exposed to a weak magnetic field. This internal supercurrent causes its own magnetic flux, which cancels the external field, maintaining the superconducting state.
The Meissner screening effect can be compared to the immune system of our bodies producing antibodies to combat bacterial and other pathogen infections. However, there is a limit to how well this works. Similar to magnetic fields, superconductors are not completely immune to them.
The diodes the team created make use of this universal Meissner screening effect. The material’s screening current mechanism is activated by the modest magnetic field they applied, either directly or through the nearby ferromagnetic layer, and it expels the external magnetic field while sustaining superconductivity.
The scientists also discovered that minute variations between the two sides or edges of the diode devices are a crucial component in refining these superconductor diodes. These differences “create some sort of asymmetry in the way the magnetic field enters the superconductor,” Moodera says.
The team discovered that they could boost efficiency from 20% to more than 50% by designing their own type of edges on diodes to maximize these differences, for instance, one edge with sawtooth features while the other edge is left unaltered. This discovery opens the door for devices whose edges could be “tuned” for even higher efficiencies, Moodera says.
The scientists concluded that the diode effect was a combined result of the edge asymmetries within superconducting diodes, the universal Meissner screening effect observed in all superconductors, and a third property of superconductors known as vortex pinning.
“It is fascinating to see how inconspicuous yet ubiquitous factors can create a significant effect in observing the diode effect,” says Yasen Hou, first author of the paper and a postdoctoral associate at the Francis Bitter Magnet Laboratory and the PSFC. “What’s more exciting is that this work provides a straightforward approach with huge potential to further improve the efficiency.”
Christoph Strunk is a professor at the University of Regensburg in Germany. Strunk, who was not involved in the research, says, “the present work demonstrates that the supercurrent in simple superconducting strips can become non-reciprocal. Moreover, when combined with a ferromagnetic insulator, the diode effect can even be maintained in the absence of an external magnetic field.”
“The rectification direction can be programmed by the remanent magnetization of the magnetic layer, which may have high potential for future applications. The work is important and appealing both from the basic research and from the applications point of view.”
Moodera noted that the two researchers who created the engineered edges did so while still in high school during summer at Moodera’s lab. They are Ourania Glezakou-Elbert of Richland, Washington, who will be going to Princeton this fall, and Amith Varambally of Vestavia Hills, Alabama, who will be entering the California Institute of Technology.
