MIT researchers and their colleagues have created a simple superconducting device that can transmit current through electronic devices much more efficiently than is possible today. As a result, the new diode, a kind of switch, can dramatically reduce 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, the diode is more than twice as efficient as similar ones reported by others. It may even be an integral part of emerging quantum computing technologies.
The work, which is reported in the July 13 online number of Physical review lettersis also the subject of a news i Physics journal.
“This paper shows 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 work. “The beauty of (this) work is that (Moodera and colleagues) achieved record efficiencies without even trying (and) their structures are far from optimized yet.”
“Our construction of a superconducting diode effect that is robust and can operate over a wide temperature range in simple systems can potentially open the door to new technologies,” said Jagadeesh Moodera, leader of the current work and senior researcher 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.
Towards a superconducting switch
Diodes, devices that allow current to flow easily in one direction but not in the reverse direction, are ubiquitous in computer systems. Modern semiconductor chips contain billions of diode-like devices called transistors. However, these devices can get very hot due to electrical resistance, requiring huge amounts of energy to cool the powerful systems in the data centers behind countless modern technologies, including cloud computing. According to a news item from 2018 i Naturethese systems could use nearly 20 percent of the world’s power in 10 years.
As a result, the work of creating diodes made of superconductors has been a hot topic in condensed matter physics. This is because superconductors transmit current with no resistance at all below a certain low temperature (the critical temperature), and are therefore much more efficient than their semiconducting cousins, which have appreciable energy loss in the form of heat.
Until now, however, other approaches to the problem have involved much more complicated physics. “The effect we found is (in part) due to a ubiquitous property of superconductors that can be realized in a very simple, straightforward way. It just stares you in the face,” says Moodera.
Says Moll of the Max Planck Institute, “The work is an important counterpoint to the current way of associating superconducting diodes (with) exotic physics, such as finite moment pairing states. Although a superconducting diode is in reality a common and widespread phenomenon found in classical materials, as a result of certain broken symmetries.”
A somewhat incredible discovery
In 2020, Moodera and colleagues observed evidence of an exotic pair of particles known as Majorana fermions. These particle pairs could lead to a new family of topological qubits, the building blocks of quantum computers. While thinking about methods to create superconducting diodes, the team realized that the materials platform they developed for the Majorana work could also be applied to the diode problem.
They were right. Using the general platform, they developed various iterations of superconducting diodes, each more efficient than the last. The first, for example, consisted of a nanoscopically thin layer of vanadium, a superconductor, which was patterned into a structure common to electronics (the Hall bar). When they applied a small magnetic field comparable to the Earth’s magnetic field, they saw the diode effect – a giant polarity dependence of current flow.
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 small magnetic field. After applying a small magnetic field to magnetize the ferromagnet so that it produces its own field, they found an even larger diode effect that was stable even after the original magnetic field was turned off.
Ubiquitous properties
The team proceeded to find out what happened.
In addition to transmitting current without resistance, superconductors also have other, less well-known but equally common properties. For example, they don’t like magnetic fields entering. When subjected to a small magnetic field, superconductors produce an internal supercurrent that induces its own magnetic flux that interrupts the external field, thereby maintaining its superconducting state. This phenomenon, known as the Meissner screening effect, can be seen as similar to our body’s immune system releasing antibodies to fight the infection of bacteria and other pathogens. However, this only works up to a certain limit. Similarly, superconductors cannot completely keep out large magnetic fields.
The diodes the team created make use of this universal Meissner screening effect. The small magnetic field they applied — either directly or through the adjacent ferromagnetic layer — activates the material’s shield current mechanism to drive out the external magnetic field and maintain superconductivity.
The team also found that another key factor in optimizing these superconducting diodes is small differences between the two sides, or edges, of the diode devices. These differences “create some kind of asymmetry in how the magnetic field enters the superconductor,” says Moodera.
By engineering their own shape of edges on diodes to optimize these differences—for example, one edge with sawtooth features, while the other edge was intentionally left unaltered—the team found they could increase efficiency from 20 percent to more than 50 percent. This discovery opens the door to devices whose edges can be “tuned” for even higher efficiency, says Moodera.
In summary, the team discovered that the edge asymmetries within superconducting diodes, the ubiquitous Meissner shielding effect found in all superconductors, and a third property of superconductors known as vortex spinning all came together to produce the diode effect.
“It is fascinating to see how inconspicuous but ubiquitous factors can create a significant effect in observing the diode effect,” said Yasen Hou, first author of the paper and a postdoc at the Francis Bitter Magnet Laboratory and PSFC. “What’s more exciting is that (this work) provides a simple approach with enormous potential to further improve efficiency.”
Christoph Strunk is a professor at the University of Regensburg in Germany. Says Strunk, who was not involved in the research, “the current work shows that the supercurrent in simple superconducting strips can become non-reciprocal. Furthermore, in combination with a ferromagnetic insulator, the diode effect can be maintained even in the absence of an external magnetic field. The rectification direction can be programmed through the residual 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 application point of view.”
Teenage contributors
Moodera noted that the two researchers who created the engineered edges did so while still 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 Varambaly of Vestavia Hills, Alabama, who will enter Caltech.
Says Varambaly, “I didn’t know what to expect when I set foot in Boston last summer, and never expected to (become) a co-author in a Physical review letters paper.
“Every day was exciting, whether I was reading dozens of papers to better understand the diode phenomena, or using machines to make new diodes for study, or participating in conversations with Ourania, Dr. Hou, and Dr. Moodera about our research.
“I am deeply 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 great research partner and friend.”
In addition to Moodera and Hou, the corresponding authors of the paper are Professors Patrick A. Lee from the MIT Department of Physics and Akashdeep Kamra from the Autonomous University of Madrid. Other authors from MIT are Liang Fu and Margarita Davydova from the Department of Physics, and Hang Chi, Alessandro Lodesani and Yingying Wu, all from the Francis Bitter Magnet Laboratory and the Plasma Science and Fusion Center. Chi is also affiliated with the US Army CCDC Research Laboratory.
Authors also include Fabrizio Nichele, Markus F. Ritter and Daniel Z. Haxwell of IBM Research Europe; Stefan Ilicof Materials Physics Center (CFM-MPC); and F. Sebastian Bergeret of CFM-MPC and 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 funders are the European Research Council, the European Union’s Horizon 2020 Research and Innovation Framework Programme, the Spanish Ministry of Science and Innovation, the A. v. Humboldt Foundation and the Department of Energy’s Office of Basic Sciences.
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