(Nanowerk News) Under certain conditions — usually very cold — some materials shift their structure to reveal new superconducting behaviors. This structural shift is known as the “nematic transition,” and physicists suspect that it offers a new way to push materials into a superconducting state in which electrons can flow completely without friction.
But what exactly is driving this transition? The answers could help scientists improve existing superconductors and discover new ones.
Now, MIT physicists have identified a key to how one class of superconductors undergo nematic transitions, in stark contrast to the assumptions of many scientists.
The physicists made their discovery by 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 kelvin (close to -300 degrees Fahrenheit). Although still very cold, 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 is for real-world use, such as for realizing powerful electromagnets for more precise and lightweight MRI machines or high-speed magnetic drift trains.
For that and other possibilities, scientists first need to understand what drives the 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 in the direction of one coordinated, favored magnetic direction.
But the MIT team found that iron selenide shifted by an entirely new mechanism. Instead of undergoing coordinated shifts in spin, the atoms in iron selenide undergo collective shifts in their orbital energies. It’s a nice distinction, but one that opens a new door for discovering unconventional superconductors.
“Our research changes things a bit when it comes to the consensus being made about what drives nematicity,” said Riccardo Comin, Associate Professor of Physics Class of 1947 Career Development at MIT. “There are many pathways to achieve unconventional superconductivity. This offers an additional avenue for realizing the superconducting state.”
Comin and his colleagues have published their results in a study that appears in Natural Ingredients (“Spontaneous orbital polarization in the nematic FeSe phase”). His co-authors at MIT include Connor Occhialini, Shua Sanchez, and Qian Song, with Gilberto Fabbris, Yongseong Choi, Jong-Woo Kim, and Philip Ryan at Argonne National Laboratory.
Following the thread
The word “nematicity” comes from the Greek word “nema,” which means “thread” — for example, to describe the threadlike body of a nematode worm. Nematicity is also used to describe conceptual threads, such as coordinated physical phenomena. For example, in the study of liquid crystals, nematic behavior can be observed when molecules come together in coordinated lines.
In recent years, physicists have used nematicity to describe the coordinated shifts that push materials into a superconducting state. The strong interaction between electrons causes matter as a whole to stretch infinitesimally, like microscopic taffy, in one particular direction allowing electrons to flow freely in that direction. The big question is what kind of interaction causes stretching. In some iron-based materials, this stretching appears to be driven by atoms spontaneously shifting their magnetic spins in the same direction. Therefore, scientists assume that most iron-based superconductors make the same spin-driven transitions.
But iron selenide seems to buck this trend. The material, which transitions to a superconducting state at the highest temperatures of iron-based materials, also does not appear to have a coordinated magnetic behavior.
“Iron selenide has the most obscure story of all this material,” said Sanchez, who is an MIT postdoc and NSF MPS-Ascend Fellow. “In this case, there is no magnetic sequence. So understanding the origin of nematicity requires very careful observations of how electrons arrange themselves around iron atoms, and what happens when those atoms stretch.
A super continuum
In their new study, the researchers worked with extremely thin, millimeter-long samples of iron selenide, which they glued onto thin strips of titanium. They mimicked the structural stretching that occurs during the nematic transition by physically stretching the titanium strip, which in turn stretched the iron selenide sample. As they stretched the sample by a fraction of a micron at a time, they looked for any properties to shift in a coordinated manner.
Using ultrabright X-rays, the team tracked how the atoms in each sample moved, as well as how each atom’s electrons behaved. After a certain point, they observed definite and coordinated shifts in the atomic orbitals. Atomic orbitals are basically the energy levels that an atomic electron can occupy. In iron selenide, electrons can occupy one of two orbital states around the iron atom. Usually, the choice of which country to occupy is random. But the team found that when they stretched the iron selenide, its electrons started to favor one orbital state more strongly than the other. This signals a clear and coordinated shift, along with new mechanisms of nematicity, and superconductivity.
“What we’ve shown is that there are fundamental physics that differ in terms of spin versus orbit nematicity, and there will be a continuum of matter that sits somewhere in between,” said Occhialini, an MIT graduate student. “Understanding where you are in that landscape will be important in the search for new superconductors.”