A new switch for superconducting

MIT physicists discovered that iron selenide, a high-temperature superconductor, achieves its superconductivity through a unique mechanism, contradicting assumptions about iron-based superconductors. Instead of a coordinated shift in atoms’ magnetic spins, atoms in iron selenide undergo a collective shift in their orbital energy, offering new pathways for creating superconducting materials.

MIT physicists have unlocked a new superconductivity mechanism in iron selenide, a material known for turning into a superconductor at relatively high temperatures, around -300 degrees Fahrenheit (70 kelvins). Contrary to previous beliefs, iron selenide does not rely on coordinated magnetic spins of atoms for superconductivity. Instead, atoms undergo a collective shift in their orbital energy.

Iron selenide’s superconducting behavior holds potential for real-world applications, including powerful electromagnets in MRI machines and high-speed magnetically levitating trains.

Riccardo Comin, the Class of 1947 Career Development Associate Professor of Physics at MIT, explained, “Our study reshuffles things a bit when it comes to the consensus that was created about what drives nematicity. There are many pathways to get to unconventional superconductivity. This offers an additional avenue to realize superconducting states.”

The term “nematicity” originates from the Greek word “nema,” meaning “thread.” It describes a coordinated shift that drives a material into a superconducting state, allowing electrons to flow friction-free. The key interaction causing this shift was assumed to be atoms changing their magnetic spins to align in the same direction in most iron-based materials.

Iron selenide deviates from this pattern. Sanchez, an MIT postdoc and NSF MPS-Ascend Fellow, said, “Iron selenide has the least clear story of all these materials. In this case, there’s no magnetic order. So, understanding the origin of nematicity requires looking very carefully at how the electrons arrange themselves around the iron atoms, and what happens as those atoms stretch apart.”

By using ultrabright X-rays to observe ultrathin iron selenide samples, the team detected a definite, coordinated shift in atoms’ orbitals when stretched. Orbitals represent the energy levels available for an atom’s electrons. In iron selenide, electrons usually randomly choose one of two orbital states around an iron atom. The team found that as they stretched iron selenide, electrons started favoring one orbital state, indicating a coordinated shift and a new form of superconductivity.

Occhialini, an MIT graduate student, added, “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. Understanding where you are on that landscape will be important in looking for new superconductors.”

The study will be published in Nature Materials. The research received support from the Department of Energy, the Air Force Office of Scientific Research, and the National Science Foundation.

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