Physicists discover a new switch for superconductivity

Certain circumstances — typically icy ones — certain materials change their structure to allow for new, superconducting properties. This shift in structure is referred to as a “nematic transition,” and scientists believe it could provide an innovative way of driving materials into a superconducting condition where electrons can flow without friction.

What exactly is driving this transformation initially? This could be the answer scientists need to improve or create new superconductors.

In the present, MIT physicists have identified the mechanism by which one type of superconductor undergoes a nematic change, and it is the complete opposite of what scientists believed to be the case.

The physicists discovered the discovery by studying the properties of iron selenide (FeSe). It is the two-dimensional substance that is the highest temperature iron-based superconductor. It is believed to change to a superconducting form at temperatures of up to 70 Kelvins (close to -300°F). While it is still ultracold, transition temperature is much higher than the average superconducting materials.

The greater the temperatures that a substance can show superconductivity, the more promising it will be for real-world applications, like developing powerful electromagnets that can be used to build more precise and light MRI machines or high-speed magnetically moving trains.

To answer these and other questions, scientists must first determine what causes the nematic switch in superconductors with high temperatures, such as iron selenide. In other superconducting materials made of iron, scientists have noticed that this switch happens when atoms change their magnetic spin to an aligned preferred magnetic direction.

However, the MIT team discovered that iron selenide changes using a new mechanism. Instead of undergoing the coordinated shift of spins, the atoms of iron selenide go through the collective shift of the energy of their orbits. It is a distinct distinction but opens the door to finding new superconductors.

“Our study reshuffles things a bit when it comes to the consensus that was created about what drives nematicity,” says Riccardo Comin from the Class of 1947 career development associate professor of Physics in the Physics Department at MIT. “There are many avenues for achieving unconventional superconductivity. This is a further avenue for achieving the superconducting state.”

Comin, his coworkers, and his colleagues have published their findings this morning in a study published in Nature Materials. Co-authors from MIT comprise Connor Occhialini, Shua Sanchez, Qian Song, Gilberto Fabbris, Yongseong Choi Jong-Woo Kim, and Philip Ryan at Argonne National Laboratory.

The thread is being followed.

The term “nematicity” stems from the Greek word “nema,” meaning “thread” — for example, to describe the body that resembles a thread of the nematode, the worm. Nematicity can also define conceptual threads, like the coordinated phenomena of physical science. For instance, when studying the properties of liquid crystals, this behavior is observed when molecules form organized lines.

Recently, scientists have employed the term nematicity to define the coordinated shift that propels the material to superconductivity. Electrons interact strongly, causing the material to stretch infinitely, much like tiny taffy in a specific direction, allowing electrons to flow freely. The most important question has been what type of interaction triggers the stretching. In certain iron-based substances, the stretching appears to be caused by atoms that naturally change their magnetic spins to be in that same direction. Researchers have concluded that all iron-based superconductors go through the exact spin-driven change.

However, iron selenide can defy this trend. It transforms into superconducting form at the highest temperatures of any iron-based substance and does not appear to have coordination of magnetic properties.

“Iron selenide has the least clear story of all these materials,” says Sanchez, an MIT postdoc and an NSF MPS Ascend Fellow. “In this instance there’s none of the magnetic orders. Understanding the nature of nematicity is to look attentively at the way electrons are arranged around the iron atoms and what happens when those elements stretch out.”

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