Stressing metallic material controls superconductivity

No strain, no gain – that’s the credo for Cornell researchers who have helped find a way to control superconductivity in a metallic material by stressing and deforming it.

The researchers, led by Katja Nowack, assistant professor of physics in the College of Arts and Sciences, collaborated with a team led by Philip Moll from the Institute of Material Science and Engineering at École Polytechnique Fédéral de Lausanne in Switzerland. Their paper, “Spatial Control of Heavy-Fermion Superconductivity in CeIrIn5,” published Oct. 11 in Science.

The project began as a puzzle. Moll’s team had been measuring superconductivity in microstructure devices made from cerium iridium indium-5 (CeIrIn5), a heavy fermion metal. The team was perplexed to find that the device’s critical temperature – the point at which electrical resistance vanishes and superconductivity occurs, also known as the transition temperature – changed when the devices were measured in different configurations. Typically, the critical temperature should be the same throughout the structure.

Moll’s frequent collaborator, Brad Ramshaw, Cornell assistant professor of physics and a co-author of the paper, told Nowack about the results, and Nowack decided to investigate the issue. Her lab had constructed a scanning probe microscope for highly sensitive magnetic imaging that operates at temperatures as low as 10 millikelvin (approximately minus 459 degrees Fahrenheit) and is ideally suited for imaging the structures Moll was studying. The probes are superconducting quantum interference devices, or SQUIDs.

When Nowack and her team imaged the small structures, they realized that deformations in the material were enabling the superconductivity to form in a spatially patterned way.

“The pattern we observed looked like something was pulling on the four corners of the square-shaped sample,” Nowack said.

To fabricate the devices, Moll’s team had glued CeIrIn5 crystal structures to a layer of sapphire and patterned them with a focus ion beam, which functions like a miniature sandblaster. Like most metals, CeIrIn5 contracts when cooled. But sapphire, an insulator, hardly shrinks at all. When the two materials were cooled together, the CeIrIn5was deformed by the mechanical tension between the two layers.

“The critical temperature in CeIrIn5 responds to strain,” Nowack said. “So pulling on the crystal in one direction will make that temperature go a little higher, and then pulling on the crystal in another direction will make it go lower.”

These strains can shift the superconducting transition temperature by nearly a factor of four, from approximately 200 to 800 millikelvin.

“The structures have trenches and features cut into them with high spatial control,” Nowack said. “All those details in the geometry heavily influence how the deformation looks at the end. This is really exciting because by modifying the geometry, we can spatially control the superconductivity in these little structures accordingly.”

Using this method allows the researchers to modulate superconductivity without relying on chemical augmentation, known as doping, which can compromise how clean the crystal is and its electronic properties.

“Now that we understand how to use microstructuring to tune the electronic properties of CeIrIn5, we can extend our approach to other materials, or we can design more sophisticated structures and fine-tune our control over the superconducting transition in heavy fermion compounds,” said doctoral student Matt Ferguson, who performed the imaging and served as co-lead author along with Maja Bachmann from the Max Planck Institute in Germany and the University of St. Andrews, Scotland.

“In addition, we want to see if we can do something similar to other types of electronic order, such as magnetism,” Nowack said.

One of the most immediate applications, according to Nowack, is the creation of so-called Josephson junction devices, which are essentially “superconductor sandwiches” – thin insulators or metals tucked between two superconductors that can enable nonlinear electrical behavior. Josephson junctions are the building blocks of superconducting logic and quantum circuits, which may provide a big boost for high-speed electronics and computing in the future.

A Josephson junction based on this work would be made within the same clean piece of crystal, with the thin metallic section created through focused strain.

“Sometimes stressing can produce amazing results,” Nowack said.

Co-authors include doctoral students David Low, Sayak Ghosh and Florian Theuss, and researchers at the Max Planck Institute for Chemical Physics of Solids; University of St. Andrews; Los Alamos National Laboratory, New Mexico; and Technical University Dresden, Germany.

The Cornell research was primarily supported by the U.S. Department of Energy, and the Cornell Center for Materials Research, with funding from the National Science Foundation’s Materials Research Science and Engineering Center.

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