MIT discovery suggests a new class of superconductors

You who think superconductivity can't happen without spin polarization? Hold my FeSe

If research from a group of MIT and Argonne boffins is confirmed, then we're one step closer to improved – and possibly entirely novel – superconducting materials.

The scientists worked with iron selenide (FeSe) – a material that reaches a superconductive state at a balmy 70 Kelvins (roughly -203° Celsius), making it the highest-temperature iron-based superconductor currently known. During their experiments the team discovered that FeSE reached the superconducting state – a process known as the nematic phase – not through spin polarization like other iron-based superconductors, but orbitally.

In the words of MIT, "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," said MIT physics professor Riccardo Comin, one of the paper's authors. 

First, a quick nematicity primer

When superconducting materials go through their nematic phase shift, all of the molecules in the material assemble into thread-like lines that are able to transmit electrons without friction, avoiding energy loss.

This has been typically understood to happen as materials are cooled and the molecules' spins shift into alignment with their neighbors – especially in iron-based superconductors.

FeSe doesn't play by those rules. 

In addition to orbital polarization, the material does not display coordinated magnetic behavior, the team said, but it still goes through the nematic phase shift. Therefore "understanding the origin of nematicity requires looking very carefully at how the electrons arrange themselves around the iron atoms," said co-author and MIT post-doc Shua Sanchez. 

To figure out what FeSe is doing when it goes nematic, the researchers took a small strip of the material, stuck it to some titanium, and stretched the whole thing while also cooling it and watching it with a pair of high-powered X-rays. 

As they stretched and cooled the FeSe strip, something interesting happened: The orbitals of its atoms – the energy levels that an atom's electrons can occupy – didn't choose random states. The more they stretched, the more the orbitals all lined up in a single state. "Our observation … suggests that nematicity is instead driven by orbital order," the team explained in their paper.

The results show 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," according to Connor Occhialini, an MIT grad student who assisted on the project. 

With this discovery, the team said chalcogenide superconductors – of which FeSe is one – can be optimized for better performance. And of course there's Occhialini's continuum of materials to be discovered, too. If we're lucky, some might enable superconducting at even warmer temperatures, clearing the way for practical applications. ®

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