Scientists Stretch Metal to Discovery Superconductivity Switch

Superconducting materials are essential in cutting-edge technology like MRI machines and particle accelerators, but these materials are expensive and finicky, making many of the proposed applications, like magnetic levitation, impossible for now. That could be changing thanks to a team at MIT that reports discovering the mechanism that causes some materials to become superconductors.

Superconductors are aptly named—they conduct electrons super-well because they have no electrical resistance. Most superconductors only work under very specific conditions, like extremely low temperatures. Room-temperature superconductors exist, but they’re rare and usually come with drawbacks of their own.

The research, led by MIT physicist Riccardo Comin, seeks to understand how materials shift from having electrical resistance to being superconductors. It’s almost like flipping a switch; one minute you have a typical chunk of metal, and then resistance drops to zero, and electrons can flow freely. It’s not magic, but the science behind this “nematic transition” is deviously complex.

This metallic compound exhibited superconductivity at room temperature when pressured between two diamond anvils.
Credit: J. ADAM FENSTER/UNIVERSITY OF ROCHESTER

In past experiments on superconductors, physicists have observed nematicity, or a coordinated shift in atomic states. Strong interactions between electrons cause the material to stretch on a microscopic scale, which in turn drives electrons to flow in that direction. Scientists have speculated that the magnetic spin of superconducting materials could be a nematicity mechanism, but then there’s iron selenide. The researchers focused on this material because it transitions to a superconductor at higher temperatures than other iron superconductors, but it doesn’t have any coordinated magnetic traits.

The experiment consisted of attaching small strips of iron selenide to bits of titanium. The titanium served as a frame, allowing the scientists to mechanically stretch the iron selenide to mimic the stretching seen during nematic transition. The team went a fraction of a micrometer at a time, scanning the iron selenide samples with high-energy X-rays for any hint of superconducting transition.

Eventually, the iron selenide samples put on a show. There are two electron orbitals in this molecule, and electrons usually appear in them randomly. However, as the metal stretched, atoms began shifting to prefer one orbital over the other. The change was overwhelming and coordinated across the entire sample, revealing this as a new mechanism of nematicity.

“What we’ve shown is that there are different underlying physics when it comes to spin versus orbital nematicity, and there’s going to be a continuum of materials that go between the two,” says co-author Connor Occhialini. This world could help scientists develop new superconductors with more useful properties.