The formation of nematicity in FeSe remains a major open question for understanding unconventional superconductivity in the presence of nematic order.
The term “nematicity” is derived from the Greek word “nema,” which means “thread,” and is used to denote conceptual strands such as coordinated physical phenomena. In recent years, physicists have adopted the term “nematicity” to characterize a coordinated shift that causes a material to become superconducting.
Nematicity is a term that physicists have recently used to describe a coordinated change that propels a substance into a superconducting state. Strong electron-electron interactions cause the material to stretch infinitesimally in one way, like microscopic lips, allowing electrons to flow freely in that direction. What kind of interaction results in the stretching has been a huge mystery.
This stretching is fueled in some iron-based materials by atoms that spontaneously change their magnetic spins to point in the same direction. Therefore, it has been assumed by scientists that the spin-driven transition occurs in the majority of iron-based superconductors.
Extreme conditions can cause materials to go through a “nematic transition” that unlocks new, superconducting behavior. This structural change, called a “nematic transition,” presents a new strategy for bringing materials into the superconducting state, where electrons can move without resistance.
The key to explaining how one kind of superconductor undergoes a nematic transition has been discovered by MIT physicists, and it is very different from what many scientists expected.
Iron selenide (FeSe), a two-dimensional substance that is the highest-temperature iron-based superconductor, was the subject of the researchers’ discovery. Unlike most superconducting materials, this one is reported to become superconducting at temperatures as high as 70 kelvins, or around -300 degrees Fahrenheit.
A material’s potential for usage in the real world, such as for strong electromagnets for more accurate and lightweight MRI scanners or fast, magnetically levitating trains, increases with the temperature at which it may exhibit superconductivity.
However, scientists must determine what causes a nematic transition in high-temperature superconductors such as iron selenide. Scientists have noticed that this flip occurs in various iron-based superconducting materials when individual atoms abruptly alter their magnetic spin towards one coordinated, favored magnetic direction.
The MIT researchers discovered that iron selenide shifts via an altogether new process. Rather than a coordinated shift in spins, iron selenide atoms undergo a collective shift in orbital energy. This distinction opens the door to the discovery of novel superconductors.
Riccardo Comin, the Class of 1947 Career Development Associate Professor of Physics at MIT, said, “Our study reshuffles things a bit regarding the consensus created about what drives nematicity. There are many pathways to get to unconventional superconductivity. This offers an additional avenue to realize superconducting states.”
This stretching appears triggered by atoms that spontaneously alter their magnetic spins to point in the same direction in some iron-based materials. Iron selenide, on the other hand, appears to defy this tendency as it transitions into a superconducting state at the most significant temperature of any iron-based material.
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.”
In their new research, the scientists used millimeter-long, incredibly thin pieces of iron selenide that they adhered to a thin strip of titanium. The iron selenide samples were extended by physically stretching the titanium strip.
They searched for qualities that changed coordinatedly when they stretched the samples by a fraction of a micron at a time. The scientists monitored the motion and behavior of the electrons in each atom in each sample using ultrabright X-rays. They noticed a distinct, coordinated shift in the atoms’ orbitals after a particular point.
The energy levels that an atom’s electrons can occupy are known as atomic orbitals. One of two orbital states can be occupied by electrons around an iron atom in iron selenide. The decision of which state to reside in is typically random. But as they stretched the iron selenide, the team discovered that its electrons overwhelmingly favored one orbital state over the other. This indicated a distinct, coordinated transition, new nematicity, and superconducting process.
According to the new study, several underlying physics govern spin vs. orbital nematicity, and a continuum of materials falls between the two. When searching for new superconductors, knowing where you stand in that landscape will be crucial.
The National Science Foundation, the Air Force Office of Scientific Research, and the Department of Energy funded this study.
Journal Reference:
- Occhialini, C. A., Sanchez, et al. Spontaneous orbital polarization in the nematic phase of FeSe. Nature Materials. DOI: 10.1038/s41563-023-01585-2