Under certain conditions—usually exceedingly cold ones—some materials shift their structure to unlock new, superconducting behavior. This structural shift is known as a "nematic transition," and physicists suspect that it offers a new way to drive materials into a superconducting state where electrons can flow entirely friction-free.
But what exactly drives this transition in the first place? The answer could help scientists improve existing superconductors and discover new ones.
Now, MIT physicists have identified the key to how one class of superconductors undergoes a nematic transition, and it's in surprising contrast to what many scientists had assumed.
The physicists made their discovery studying iron selenide (FeSe), a two-dimensional material that is the highest-temperature iron-based superconductor. The material is known to switch to a superconducting state at temperatures as high as 70 kelvins (close to -300 degrees Fahrenheit). Though still ultracold, this transition temperature is higher than that of most superconducting materials.
The higher the temperature at which a material can exhibit superconductivity, the more promising it can be for use in the real world, such as for realizing powerful electromagnets for more precise and lightweight MRI machines or high-speed, magnetically levitating trains.
For those and other possibilities, scientists will first need to understand what drives a nematic switch in high-temperature superconductors like iron selenide. In other iron-based superconducting materials, scientists have observed that this switch occurs when individual atoms suddenly shift their magnetic spin toward one coordinated, preferred magnetic direction.
But the MIT team found that iron selenide shifts through an entirely new mechanism. Rather than undergoing a coordinated shift in spins, atoms in iron selenide undergo a collective shift in their orbital energy. It's a fine distinction, but one that opens a new door to discovering unconventional superconductors.
"Our study reshuffles things a bit when it comes to the consensus that was created about what drives nematicity," says Riccardo Comin, the Class of 1947 Career Development Associate Professor of Physics at MIT. "There are many pathways to get to unconventional superconductivity. This offers an additional avenue to realize superconducting states."
Comin and his colleagues published their results in a study appearing in Nature Materials. Co-authors at MIT include Connor Occhialini, Shua Sanchez, and Qian Song, along with Gilberto Fabbris, Yongseong Choi, Jong-Woo Kim, and Philip Ryan at Argonne National Laboratory.
International Research Conference on High Energy Physics
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