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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Physicists discover a new switch for superconductivity    
    The results could help turn up unconventional superconducting materials   
      
      
     Date:   
         June 22, 2023   
     Source:   
         Massachusetts Institute of Technology   
     Summary:   
         A study sheds surprising light on how certain superconductors   
         undergo a 'nematic transition' -- unlocking new, superconducting   
         behavior. The results could help identify unconventional   
         superconducting materials.   
      
      
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   FULL STORY   
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   Under certain conditions -- usually exceedingly cold ones -- some   
   materials shift their structure to unlock new, superconducting   
   behavior. This structural shift is known as a "nematic transition,"   
   and physicists suspect that it offers a new way to drive materials into   
   a superconducting state where electrons can flow entirely friction-free.   
      
   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 class of   
   superconductors undergoes a nematic transition, and it's in surprising   
   contrast to what many scientists had assumed.   
      
   The physicists made their discovery 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 kelvins (close to -300 degrees Fahrenheit). Though still   
   ultracold, 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 can be for use in the real   
   world, such as for realizing powerful electromagnets for more precise and   
   lightweight MRI machines or high- speed, magnetically levitating trains.   
      
   For those and other possibilities, scientists will first need   
   to understand what drives a 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 toward one   
   coordinated, preferred magnetic direction.   
      
   But the MIT team found that iron selenide shifts through an entirely   
   new mechanism. Rather than undergoing a coordinated shift in spins,   
   atoms in iron selenide undergo a collective shift in their orbital   
   energy. It's a fine distinction, but one that opens a new door to   
   discovering unconventional superconductors.   
      
   "Our study reshuffles things a bit when it comes to the consensus that   
   was created about what drives nematicity," says Riccardo Comin, the Class   
   of 1947 Career Development Associate Professor of Physics at MIT. "There   
   are many pathways to get to unconventional superconductivity. This offers   
   an additional avenue to realize superconducting states."  Comin and his   
   colleagues will publish their results in a study appearing in Nature   
   Materials. Co-authors at MIT include Connor Occhialini, Shua Sanchez,   
   and Qian Song, along with Gilberto Fabbris, Yongseong Choi, Jong-Woo Kim,   
   and Philip Ryan at Argonne National Laboratory.   
      
   Following the thread The word "nematicity" stems from the Greek word   
   "nema,"meaning "thread" -- for instance, to describe the thread-like body   
   of the nematode worm. Nematicity is also used to describe conceptual   
   threads, such as coordinated physical phenomena. For instance, in the   
   study of liquid crystals, nematic behavior can be observed when molecules   
   assemble in coordinated lines.   
      
   In recent years, physicists have used nematicity to describe a coordinated   
   shift that drives a material into a superconducting state. Strong   
   interactions between electrons cause the material as a whole to stretch   
   infinitesimally, like microscopic taffy, in one particular direction that   
   allows electrons to flow freely in that direction. The big question has   
   been what kind of interaction causes the stretching. In some iron-based   
   materials, this stretching seems to be driven by atoms that spontaneously   
   shift their magnetic spins to point in the same direction. Scientists   
   have therefore assumed that most iron-based superconductors make the same,   
   spin-driven transition.   
      
   But iron selenide seems to buck this trend. The material, which happens   
   to transition into a superconducting state at the highest temperature   
   of any iron- based material, also seems to lack any coordinated magnetic   
   behavior.   
      
   "Iron selenide has the least clear story of all these materials," says   
   Sanchez, who is an MIT postdoc and NSF MPS-Ascend Fellow. "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."   
   A super continuum In their new study, the researchers worked with   
   ultrathin, millimeter-long samples of iron selenide, which they glued   
   to a thin strip of titanium. They mimicked the structural stretching   
   that occurs during a nematic transition by physically stretching the   
   titanium strip, which in turn stretched the iron selenide samples. As   
   they stretched the samples by a fraction of a micron at a time, they   
   looked for any properties that shifted in a coordinated fashion.   
      
   Using ultrabright X-rays, the team tracked how the atoms in each sample   
   were moving, as well as how each atom's electrons were behaving. After a   
   certain point, they observed a definite, coordinated shift in the atoms'   
   orbitals.   
      
   Atomic orbitals are essentially energy levels that an atom's electrons   
   can occupy. In iron selenide, electrons can occupy one of two orbital   
   states around an iron atom. Normally, the choice of which state to occupy   
   is random. But the team found that as they stretched the iron selenide,   
   its electrons began to overwhelmingly prefer one orbital state over   
   the other. This signaled a clear, coordinated shift, along with a new   
   mechanism of nematicity, and superconductivity.   
      
   "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," says Occhialini,   
   an MIT graduate student.   
      
   "Understanding where you are on that landscape will be important in   
   looking for new superconductors."  This research was supported by the   
   Department of Energy, the Air Force Office of Scientific Research,   
   and the National Science Foundation.   
      
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   ==========================================================================   
   Story Source: Materials provided by   
   Massachusetts_Institute_of_Technology. Original written by Jennifer   
   Chu. Note: Content may be edited for style and length.   
      
      
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   Journal Reference:   
      1. Occhialini, C.A., Sanchez, J.J., Song, Q. et al. Spontaneous orbital   
         polarization in the nematic phase of FeSe. Nat. Mater., 2023 DOI:   
         10.1038/s41563-023-01585-2   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/06/230622120822.htm   
      
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