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Under certain conditions (typically very cold conditions), some materials change their structure to unlock new superconducting behaviors. This structural shift is known as the “nematic transition,” and physicists suspect it provides a new way to drive materials into a superconducting state where electrons can flow completely frictionlessly.
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 type of superconductor undergoes a nematic transition, in a striking contrast to what many scientists had assumed.
Physicists discovered it by studying iron selenide (FeSe), a two-dimensional material that is the hottest iron-based superconductor. This material is known to transition to a superconducting state at temperatures as high as 70 Kelvin (close to -300 degrees Fahrenheit). Although still cryogenic, this transition temperature is higher than most superconducting materials.
The higher the temperature at which a material can exhibit superconductivity, the more promising it is for use in the real world, such as enabling more precise and lightweight MRI machines or powerful electromagnets for high-speed maglev trains.
For these and other possibilities, scientists must first understand what drives a nematic switch in a high-temperature superconductor such as iron selenide. In other iron-based superconducting materials, scientists have observed that this switch occurs when individual atoms suddenly shift their magnetic spins into one coordinated, preferred magnetic direction.
But an MIT team discovered that iron selenide moves through an entirely new mechanism. Instead of undergoing a coordinated shift in spin, the atoms of iron selenide undergo a collective shift in orbital energy. It’s a nice distinction, but one that opens new doors to discovering unconventional superconductors.
In 1947, MIT Associate Professor of Physics Riccardo Comin said, “Our work slightly restructures the consensus about what causes nematicity.” “There are many routes to reach unconventional superconductivity. This provides additional avenues for realizing the superconducting state.”
Comin and his colleagues published their findings. natural material. MIT co-authors include Gilberto Fabbris, Yongsung Choi, Jongwoo Kim, and Philip Ryan of Argonne National Laboratory, along with Connor Occhialini, Shua Sanchez, and Qian Song.
The word “nematicity” comes from the Greek word “nema”. It means “thread”. An example describes the thread-like body of nematode worms. 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 as coordination lines.
In recent years, physicists have used nematicity to describe the coordinated motion that causes materials to become superconducting. The strong interaction between the electrons causes the entire material to stretch infinitely like a fine taffy in a particular direction that allows the electrons to flow freely in that direction. The biggest question was what kind of interaction caused the stretching. In some iron-based materials, this stretching appears to be driven by atoms spontaneously shifting their magnetic spins to point in the same direction. Scientists therefore hypothesized that most iron-based superconductors exhibit the same spin-driven transition.
However, iron selenide seems to buck this trend. Transitioning to a superconducting state at the highest temperature of any iron-based material, the material appears to lack coordinated magnetic behavior.
“Iron selenide has the most obscure story of all these materials,” says Sanchez, an MIT postdoctoral fellow and NSF MPS-Ascend Fellow. “There is no magnetic order in this case. So, to understand the origin of the nematic, you have to look very carefully at how the electrons are arranged around the iron atoms and what happens when those atoms stretch into each other.”
In their new study, the researchers worked with ultra-thin, millimeter-long samples of iron selenide glued to thin titanium strips. 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. By stretching the sample 1 micron at a time, we looked for properties that moved in a coordinated manner.
The team used ultra-bright X-rays to track not only how the atoms in each sample were moving, but also how the electrons in each atom were behaving. After a certain point, they observed a definite and coordinated movement in the orbits of atoms. An atomic orbital is essentially an energy level that can be occupied by an atom’s electrons. In iron selenide, electrons can occupy one of two orbital states around an iron atom. In general, the choice of state to occupy is random.
But when the team stretched the iron selenide, they found that the electrons began to overwhelmingly prefer one orbital state over the other. This meant clear and coordinated changes with new mechanisms of nematic and superconductivity.
“What we’ve shown is that there are different fundamental physics when it comes to spin-to-orbital nematics,” said MIT graduate student Occhialini, “and there will be a continuum of matter moving between the two.” “Understanding where you are on that landscape will be key to finding new superconductors.”
Occhialini, CA, et al, Spontaneous orbital polarization of the nematic phase of FeSe, natural material (2023). DOI: 10.1038/s41563-023-01585-2. www.nature.com/articles/s41563-023-01585-2
This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site for news about MIT research, innovation and education.
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