A new switch for superconductivity is found by physicists.
Physics news: Some materials change their structure under specific circumstances—usually ones that are extremely cold—to reveal new, superconducting behaviour. This structural change, known as a “nematic transition,” may provide a novel route to superconductivity, a state in which electrons can move without any resistance at all.
What, though, exactly motivates this change in the first place? The solution might enable researchers to develop new superconductors and enhance those that already exist.
The secret to how one kind of superconductors undergoes a nematic transition has now been discovered by MIT physicists, and it is startlingly different from what many scientists had anticipated.
Iron selenide (FeSe), the highest temperature iron-based superconductor, is a two-dimensional substance that the physicists studied in order to make their discovery. At temperatures as low as 70 kelvins (about -300 degrees Fahrenheit), the substance is known to transition to a superconducting state. This transition temperature is higher than that of the majority of superconducting materials even though it is still extremely cold.
A material’s potential for usage in the real world, such as creating strong electromagnets for more accurate and lightweight MRI scanners or fast, magnetically levitating trains, increases with the temperature at which it can exhibit superconductivity.
Scientists must first comprehend what causes a nematic transition in high-temperature superconductors like iron selenide in order to rule out those and other alternatives. Scientists have seen that this flip happens in various iron-based superconducting materials when individual atoms abruptly change their magnetic spin towards one coordinated, favoured magnetic direction.
However, the MIT researchers discovered that iron selenide moves via a completely novel mechanism. Atoms in iron selenide collectively change their orbital energy rather than undergo a coordinated change in spin. It’s a small distinction, but it creates a new avenue for the study of unusual superconductors.
“Our study reshuffles things a little bit when it comes to the consensus that was created about what drives nematicity,” explains Riccardo Comin, the Class of 1947 Career Development Associate Professor of Physics at MIT. The journey to unconventional superconductivity can take many different forms. This provides a different way to achieve superconducting states.
In a paper that was published in Nature Materials, Comin and his colleagues reported their findings. Connor Occhialini, Shua Sanchez, and Qian Song are co-authors at MIT, and Gilberto Fabbris, Yongseong Choi, Jong-Woo Kim, and Philip Ryan are co-authors at the Argonne National Laboratory.
The word “nematicity” derives from the Greek word “nema,” which means “thread” and is used, for example, to describe the nematode worm’s thread-like body. Another term for conceptual links, such as coordinated physical phenomena, is nematicity. For instance, nematic behaviour can be seen in the study of liquid crystals when molecules arrange themselves into coordinated lines.
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 the huge mystery. This stretching appears to be 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.
Iron selenide, however, appears to defy this pattern. The substance also appears to have no coordinated magnetic behaviour, despite the fact that it exhibits the highest temperature superconducting transition of any iron-based substance.
The least understood of all these compounds is iron selenide, according to Sanchez, an NSF MPS-Ascend Fellow and postdoc at MIT. “There is no magnetic order in this situation. Therefore, paying close attention to how the electrons arrange themselves around the iron atoms and what happens as those atoms stretch apart is necessary to comprehend the origin of nematicity.
The super continuum
In their latest discovery, the scientists used millimeter-long, incredibly thin pieces of iron selenide that they adhered to a thin strip of titanium. By physically stretching the titanium strip, they were able to simulate the structural stretching that takes place during a nematic transition, which in turn stretched the iron selenide samples. They searched for any qualities that changed in a coordinated manner when they stretched the samples by a fraction of a micron at a time.
The scientists monitored the motion of each sample’s atoms and the behaviour of each atom’s electrons 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 started to overwhelmingly favour one orbital state over the other. This indicated a distinct, coordinated transition as well as a new nematicity and superconducting process.
“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,” MIT graduate student Occhialini adds. Physics news In order to find new superconductors, it will be crucial to know where you are in relation to that terrain.