MSGID: 1:317/3 64a7951a   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Physicists generate the first snapshots of fermion pairs    
    The images shed light on how electrons form superconducting pairs that   
   glide through materials without friction.    
      
     Date:   
         July 6, 2023   
     Source:   
         Massachusetts Institute of Technology   
     Summary:   
         Physicists captured the first images that directly show the   
         pairing of fermions. The snapshots of particles pairing up in a   
         cloud of atoms can provide clues to how electrons pair up in a   
         superconducting material.   
      
      
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   When your laptop or smartphone heats up, it's due to energy that's lost   
   in translation. The same goes for power lines that transmit electricity   
   between cities. In fact, around 10 percent of the generated energy is   
   lost in the transmission of electricity. That's because the electrons   
   that carry electric charge do so as free agents, bumping and grazing   
   against other electrons as they move collectively through power cords   
   and transmission lines. All this jostling generates friction, and,   
   ultimately, heat.   
      
   But when electrons pair up, they can rise above the fray and glide   
   through a material without friction. This "superconducting" behavior   
   occurs in a range of materials, though at ultracold temperatures. If   
   these materials can be made to superconduct closer to room temperature,   
   they could pave the way for zero-loss devices, such as heat-free laptops   
   and phones, and ultraefficient power lines.   
      
   But first, scientists will have to understand how electrons pair up in   
   the first place.   
      
   Now, new snapshots of particles pairing up in a cloud of atoms can   
   provide clues to how electrons pair up in a superconducting material. The   
   snapshots were taken by MIT physicists and are the first images that   
   directly capture the pairing of fermions -- a major class of particles   
   that includes electrons, as well as protons, neutrons, and certain types   
   of atoms.   
      
   In this case, the MIT team worked with fermions in the form of   
   potassium-40 atoms, and under conditions that simulate the behavior of   
   electrons in certain superconducting materials. They developed a technique   
   to image a supercooled cloud of potassium-40 atoms, which allowed them   
   to observe the particles pairing up, even when separated by a small   
   distance. They could also pick out interesting patterns and behaviors,   
   such as a the way pairs formed checkerboards, which were disturbed by   
   lonely singles passing by.   
      
   The observations, reported today in Science, can serve as a visual   
   blueprint for how electrons may pair up in superconducting materials. The   
   results may also help to describe how neutrons pair up to form an   
   intensely dense and churning superfluid within neutron stars.   
      
   "Fermion pairing is at the basis of superconductivity and many phenomena   
   in nuclear physics," says study author Martin Zwierlein, the Thomas   
   A. Frank Professor of Physics at MIT. "But no one had seen this pairing   
   in situ. So it was just breathtaking to then finally see these images   
   onscreen, faithfully."  The study's co-authors include Thomas Hartke,   
   Botond Oreg, Carter Turnbaugh, and Ningyuan Jia, all members of MIT's   
   Department of Physics, the MIT-Harvard Center for Ultracold Atoms,   
   and the Research Laboratory of Electronics.   
      
   A decent view To directly observe electrons pair up is an impossible   
   task. They are simply too small and too fast to capture with existing   
   imaging techniques. To understand their behavior, physicists like   
   Zwierlein have looked to analogous systems of atoms. Both electrons   
   and certain atoms, despite their difference in size, are similar in   
   that they are fermions -- particles that exhibit a property known as   
   "half-integer spin." When fermions of opposite spin interact, they can   
   pair up, as electrons do in superconductors, and as certain atoms do in   
   a cloud of gas.   
      
   Zwierlein's group has been studying the behavior of potassium-40   
   atoms, which are known fermions, that can be prepared in one of two   
   spin states. When a potassium atom of one spin interacts with an atom   
   of another spin, they can form a pair, similar to superconducting   
   electrons. But under normal, room- temperature conditions, the atoms   
   interact in a blur that is difficult to capture.   
      
   To get a decent view of their behavior, Zwierlein and his colleagues   
   study the particles as a very dilute gas of about 1,000 atoms, that they   
   place under ultracold, nanokelvin conditions that slow the atoms to a   
   crawl. The researchers also contain the gas within an optical lattice,   
   or a grid of laser light that the atoms can hop within, and that the   
   researchers can use as a map to pinpoint the atoms' precise locations.   
      
   In their new study, the team made enhancements to their existing technique   
   for imaging fermions that enabled them to momentarily freeze the atoms   
   in place, then take snapshots separately of potassium-40 atoms with   
   one particular spin or the other. The researchers could then overlay   
   an image of one atom type over the other, and look to see where the two   
   types paired up, and how.   
      
   "It was bloody difficult to get to a point where we could actually   
   take these images," Zwierlein says. "You can imagine at first getting   
   big fat holes in your imaging, your atoms running away, nothing is   
   working. We've had terribly complicated problems to solve in the lab   
   through the years, and the students had great stamina, and finally, to   
   be able to see these images was absolutely elating."  Pair dance What   
   the team saw was pairing behavior among the atoms that was predicted by   
   the Hubbard model -- a widely held theory believed to hold they key to   
   the behavior of electrons in high-temperature superconductors, materials   
   that exhibit superconductivity at relatively high (though still very cold)   
   temperatures. Predictions of how electrons pair up in these materials have   
   been tested through this model, but never directly observed until now.   
      
   The team created and imaged different clouds of atoms thousands of   
   times and translated each image into a digitized version resembling a   
   grid. Each grid showed the location of atoms of both types (depicted   
   as red versus blue in their paper). From these maps, they were able to   
   see squares in the grid with either a lone red or blue atom, and squares   
   where both a red and blue atom paired up locally (depicted as white), as   
   well as empty squares that contained neither a red or blue atom (black).   
      
   Already individual images show many local pairs, and red and blue atoms   
   in close proximity. By analyzing sets of hundred of images, the team   
   could show that atoms indeed show up in pairs, at times linking up in a   
   tight pair within one square, and at other times forming looser pairs,   
   separated by one or several grid spacings. This physical separation,   
   or "nonlocal pairing," was predicted by the Hubbard model but never   
   directly observed.   
      
   The researchers also observed that collections of pairs seemed to form   
   a broader, checkerboard pattern, and that this pattern wobbled in and   
   out of formation as one partner of a pair ventured outside its square and   
   momentarily distorted the checkerboard of other pairings. This phenomenon,   
   known as a "polaron," was also predicted but never seen directly.   
      
   "In this dynamic soup, the particles are constantly hopping on top of   
   each other, moving away, but never dancing too far from each other,"   
   Zwierlein notes.   
      
   The pairing behavior between these atoms must also occur in   
   superconducting electrons, and Zwierlein says the team's new snapshots   
   will help to inform scientists' understanding of high-temperature   
   superconductors, and perhaps provide insight into how these materials   
   might be tuned to higher, more practical temperatures.   
      
   "If you normalize our gas of atoms to the density of electrons in a metal,   
   we think this pairing behavior should occur far above room temperature,"   
   Zwierlein offers. "That gives a lot of hope and confidence that such   
   pairing phenomena can in principle occur at elevated temperatures, and   
   there's no a priori limit to why there shouldn't be a room-temperature   
   superconductor one day."  This research was supported, in part, by the   
   U.S. National Science Foundation, the U.S. Air Force Office of Scientific   
   Research, and the Vannevar Bush Faculty Fellowship.   
      
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   Journal Reference:   
      1. Thomas Hartke, Botond Oreg, Carter Turnbaugh, Ningyuan Jia, Martin   
         Zwierlein. Direct observation of nonlocal fermion pairing in an   
         attractive Fermi-Hubbard gas. Science, 2023; 381 (6653): 82 DOI:   
         10.1126/ science.ade4245   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230706152721.htm   
      
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