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Misha Friedman/The New York Times

January 27, 2020

Superconductivity? Stress is the Answer For Once

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We’ve been told all of our lives to avoid stress – but in physics, stress might just be the key to unlocking the secret of superconductivity.

Superconductivity, the phenomenon in which the electrical resistance of a material suddenly drops to zero when cooled below a certain temperature, has been a scientific curiosity ever since its discovery in the early 20th century.

A group of Cornell researchers led by Prof. Katja Nowack, physics, published a paper on Oct. 11 in Science that investigates how physically deforming a material can cause it to show traits of partial superconductivity.

The interest first arose in the work of collaborator Philip Moll, a researcher at the Institute of Material Science and Engineering at École Polytechnique Fédéral de Lausanne in Switzerland, during his investigation of the superconductive properties of the metal cerium iridium indium-5.

In an attempt to establish superconductivity, Moll discovered that the critical temperature was changing depending on the placement of the wire contacts. This collides directly with the conventional belief of superconductivity, which is that the entire material must be either completely, uniformly superconductive, or not.

Nowack learned of these strange results from Prof. Brad Ramshaw, physics, and decided to investigate them using a device called a superconducting quantum interference device, which can measure local resistivities of small areas.

“What we found in the end was that in these little microstructures, superconductivity doesn’t uniformly form in the device, but forms in a very spatially modulated, nonuniform fashion. So there’s these little puddles of superconductivity in some parts of the device, and other parts stay non-superconductive down to much lower temperatures,” Nowack said.

They also discovered that these superconductive puddles correlated to the varying amounts of physical stress produced from the creation of the samples. Moll’s team had created the samples by gluing CeIrIn5 crystals to a sapphire substrate and etching patterns into them using a focus ion beam, similar to a mini-sandblower.

According to Nowack, CeIrIn5 shrinks by about 0.3 percent as it cools due to its metallic properties, whereas sapphire does not shrink at all. The resulting strain seemed to be causing the irregular superconductivity noticed by Moll.

“Actually in the literature, it was known that the superconducting transition temperature of the material must depend on strain,” Nowack said. However, only some simple strains, like a single stretch along one axis, had been tested. Using this theory, the Cornell group developed a model for relating strain to superconductivity, and upon comparing their model’s predictions to the more complex deformations of the CeIrIn5 samples, found that the findings correlated exactly.

These findings open up a whole host of possible applications. This correlation between strain and superconductivity may become a new way of investigating the superconductive properties of other metals, which in turn could help refine physicists’ understanding of this relationship even further.

The group hopes to investigate how these new discoveries could affect existing devices, like the Josephson junction, a device which utilizes two superconductors and has applications in quantum computing. “We’re [also] thinking we can apply this to interesting magnetic systems that have interesting magnetic order, and change the properties of the magnetic order using strain,” Nowack said.