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   Message 7,888 of 8,931   
   ScienceDaily to All   
   Semiconductor lattice marries electrons    
   22 Mar 23 22:30:26   
   
   MSGID: 1:317/3 641bd5fa   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Semiconductor lattice marries electrons and magnetic moments    
      
     Date:   
         March 22, 2023   
     Source:   
         Cornell University   
     Summary:   
         A model system created by stacking a pair of monolayer   
         semiconductors is giving physicists a simpler way to study   
         confounding quantum behavior, from heavy fermions to exotic quantum   
         phase transitions.   
      
      
         Facebook Twitter Pinterest LinkedIN Email   
   FULL STORY   
   ==========================================================================   
   A model system created by stacking a pair of monolayer semiconductors is   
   giving physicists a simpler way to study confounding quantum behavior,   
   from heavy fermions to exotic quantum phase transitions.   
      
      
   ==========================================================================   
   The group's paper, "Gate-Tunable Heavy Fermions in a Moire' Kondo   
   Lattice," published March 15 in Nature. The lead author is postdoctoral   
   fellow Wenjin Zhao in the Kavli Institute at Cornell.   
      
   The project was led by Kin Fai Mak, professor of physics in the College   
   of Arts and Sciences, and Jie Shan, professor of applied and engineering   
   physics in Cornell Engineering and in A&S, the paper's co-senior   
   authors. Both researchers are members of the Kavli Institute; they came   
   to Cornell through the provost's Nanoscale Science and Microsystems   
   Engineering (NEXT Nano) initiative.   
      
   The team set out to address what is known as the Kondo effect, named   
   after Japanese theoretical physicist Jun Kondo. About six decades ago,   
   experimental physicists discovered that by taking a metal and substituting   
   even a small number of atoms with magnetic impurities, they could scatter   
   the material's conduction electrons and radically alter its resistivity.   
      
   That phenomenon puzzled physicists, but Kondo explained it with a model   
   that showed how conduction electrons can "screen" the magnetic impurities,   
   such that the electron spin pairs with the spin of a magnetic impurity   
   in opposite directions, forming a singlet.   
      
   While the Kondo impurity problem is now well understood, the Kondo lattice   
   problem -- one with a regular lattice of magnetic moments instead of   
   random magnetic impurities -- is much more complicated and continues   
   to stump physicists. Experimental studies of the Kondo lattice problem   
   usually involve intermetallic compounds of rare earth elements, but   
   these materials have their own limitations.   
      
   "When you move all the way down to the bottom of the Periodic Table,   
   you end up with something like 70 electrons in an atom," Mak said. "The   
   electronic structure of the material becomes so complicated. It is very   
   difficult to describe what's going on even without Kondo interactions."   
   The researchers simulated the Kondo lattice by stacking ultrathin   
   monolayers of two semiconductors: molybdenum ditelluride, tuned to a   
   Mott insulating state, and tungsten diselenide, which was doped with   
   itinerant conduction electrons.   
      
   These materials are much simpler than bulky intermetallic compounds,   
   and they are stacked with a clever twist. By rotating the layers at   
   a 180-degree angle, their overlap results in a moire' lattice pattern   
   that traps individual electrons in tiny slots, similar to eggs in an   
   egg carton.   
      
   This configuration avoids the complication of dozens of electrons jumbling   
   together in the rare earth elements. And instead of requiring chemistry   
   to prepare the regular array of magnetic moments in the intermetallic   
   compounds, the simplified Kondo lattice only needs a battery. When a   
   voltage is applied just right, the material is ordered into forming a   
   lattice of spins, and when one dials to a different voltage, the spins   
   are quenched, producing a continuously tunable system.   
      
   "Everything becomes much simpler and much more controllable," Mak said.   
      
   The researchers were able to continuously tune the electron mass and   
   density of the spins, which cannot be done in a conventional material,   
   and in the process they observed that the electrons dressed with the   
   spin lattice can become 10 to 20 times heavier than the "bare" electrons,   
   depending on the voltage applied.   
      
   The tunability can also induce quantum phase transitions whereby heavy   
   electrons turn into light electrons with, in between, the possible   
   emergence of a "strange" metal phase, in which electrical resistance   
   increases linearly with temperature. The realization of this type   
   of transition could be particularly useful for understanding the   
   high-temperature superconducting phenomenology in copper oxides.   
      
   "Our results could provide a laboratory benchmark for theorists,"   
   Mak said. "In condensed matter physics, theorists are trying to deal   
   with the complicated problem of a trillion interacting electrons. It   
   would be great if they don't have to worry about other complications,   
   such as chemistry and material science, in real materials. So they often   
   study these materials with a 'spherical cow' Kondo lattice model. In the   
   real world you cannot create a spherical cow, but in our material now   
   we've created one for the Kondo lattice."  Co-authors include doctoral   
   students Bowen Shen and Zui Tao; postdoctoral researchers Kaifei Kang and   
   Zhongdong Han; and researchers from the National Institute for Materials   
   Science in Tsukuba, Japan.   
      
   The research was primarily supported by the Air Force Office of Scientific   
   Research, the National Science Foundation, the U.S. Department of Energy   
   and the Gordon and Betty Moore Foundation.   
      
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   ==========================================================================   
   Story Source: Materials provided by Cornell_University. Original written   
   by David Nutt, courtesy of the Cornell Chronicle. Note: Content may be   
   edited for style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Wenjin Zhao, Bowen Shen, Zui Tao, Zhongdong Han, Kaifei Kang, Kenji   
         Watanabe, Takashi Taniguchi, Kin Fai Mak, Jie Shan. Gate-tunable   
         heavy fermions in a moire' Kondo lattice. Nature, 2023; DOI:   
         10.1038/s41586- 023-05800-7   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/03/230322140331.htm   
      
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