MSGID: 1:317/3 6279eae2   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Ultrafast 'camera' captures hidden behavior of potential 'neuromorphic'   
   material    
      
     Date:   
         May 9, 2022   
     Source:   
         DOE/Brookhaven National Laboratory   
     Summary:   
         Imagine a computer that can think as fast as the human brain while   
         using very little energy. That's the goal of scientists seeking   
         to discover or develop 'neuromorphic' materials that can send and   
         process signals as easily as the brain's neurons and synapses. In   
         a paper just published scientists describe surprising new details   
         about vanadium dioxide, one of the most promising neuromorphic   
         materials.   
      
      
      
   FULL STORY   
   ==========================================================================   
   Imagine a computer that can think as fast as the human brain while using   
   very little energy. That's the goal of scientists seeking to discover   
   or develop materials that can send and process signals as easily as   
   the brain's neurons and synapses. Identifying quantum materials with   
   an intrinsic ability to switch between two distinct forms (or more)   
   may hold the key to these futuristic sounding "neuromorphic" computing   
   technologies.   
      
      
   ==========================================================================   
   In a paper just published in the journal Physical Review X, Yimei Zhu,   
   a physicist at the U.S. Department of Energy's (DOE) Brookhaven National   
   Laboratory, and his collaborators describe surprising new details about   
   vanadium dioxide, one of the most promising neuromorphic materials. Using   
   data collected by a unique "stroboscopic camera," the team captured the   
   hidden trajectory of atomic motion as this material transitions from   
   an insulator to a metal in response to a pulse of light. Their findings   
   could help guide the rational design of high-speed and energy-efficient   
   neuromorphic devices.   
      
   "One way to reduce energy consumption in artificial neurons and   
   synapses for brain-inspired computing is to exploit the pronounced   
   non-linear properties of quantum materials," said Zhu. "The principal   
   idea behind this energy efficiency is that, in quantum materials, a small   
   electrical stimulus may produce a large response that can be electrical,   
   mechanical, optical, or magnetic through a change of material state."   
   "Vanadium dioxide is one of the rare, amazing materials that has emerged   
   as a promising candidate for neuro-mimetic bio-inspired devices," he   
   said. It exhibits an insulator-metal transition near room temperature in   
   which a small voltage or current can produce a large change in resistivity   
   with switching that can mimic the behavior of both neurons (nerve cells)   
   and synapses (the connections between them).   
      
   "It goes from completely insulating, like rubber, to a very good metal   
   conductor, with a resistivity change of 10,000 times or more," Zhu said.   
      
   Those two very different physical states, intrinsic in the same material,   
   could be encoded for cognitive computing.   
      
      
      
   ==========================================================================   
   Visualizing ultrafast atomic motions For their experiments, the scientists   
   triggered the transition with extremely short pulses of photons --   
   particles of light. Then they captured the material's atomic-scale   
   response using a mega-electron-volt ultrafast electron diffraction   
   (MeV-UED) instrument developed at Brookhaven.   
      
   You can think of this tool as similar to a conventional camera with   
   the shutter left open in a dark setting, firing intermittent flashes   
   to catch something like a thrown ball in motion. With each flash, the   
   camera records an image; the series of images taken at different times   
   reveals the ball's trajectory in flight.   
      
   The MeV-UED "stroboscope" captures the dynamics of a moving object in a   
   similar way, but at much faster time scale (shorter than one trillionth   
   of a second) and at much smaller length scale (smaller than one billionth   
   of a millimeter).   
      
   It uses high-energy electrons to reveal the trajectories of atoms!   
   "Previous static measurements revealed only the initial and final state   
   of the vanadium dioxide insulator-to-metal transition, but the detailed   
   transition process was missing," said Junjie Li, the first author of   
   the paper. "Our ultrafast measurements allowed us to see how the atoms   
   move -- to capture the short-lived transient (or 'hidden') states -- to   
   help us understand the dynamics of the transition."  The pictures alone   
   don't tell the whole story. After capturing upwards of 100,000 "shots,"   
   the scientists used sophisticated time resolved crystallographic analysis   
   techniques they'd developed to refine the intensity changes of a few dozen   
   "electron diffraction peaks." Those are the signals produced by electrons   
   scattering off the atoms of the vanadium dioxide sample as atoms and   
   their orbital electrons move from the insulator state to metallic state.   
      
      
      
   ==========================================================================   
   "Our instrument uses accelerator technology to generate electrons with an   
   energy of 3 MeV, which is 50 times higher than smaller laboratory-based   
   ultrafast electron microscopy and diffraction instruments," Zhu said. "The   
   higher energy allows us to track electrons scattered at wider angles,   
   which translates to being able to 'see' the motions of atoms at smaller   
   distances with better precision."  Two stage dynamics and a curved path   
   The analysis revealed that the transition takes place in two stages,   
   with the second stage being longer in duration and slower in speed than   
   the first. It also showed that the trajectories of the atoms' motions   
   in the second stage were not linear.   
      
   "You would think the trajectory from position A to B would be a direct   
   straight line -- the shortest possible distance. Instead, it was a   
   curve. This was completely unexpected," Zhu said.   
      
   The curve was an indication that there is another force that also plays   
   a role in the transition.   
      
   Think back to the stroboscopic images of a ball's trajectory. When you   
   throw a ball, you exert a force. But another force, gravity, also pulls   
   the ball to the ground, causing the trajectory to curve.   
      
   In the case of vanadium dioxide, the light pulse is the force that gets   
   the transition going, and the curvature in atomic trajectories is caused   
   by the electrons orbiting around the vanadium atoms.   
      
   The study also showed that a measure related to the intensity of light   
   used to trigger the atomic dynamics can alter atomic trajectories --   
   similar to the way the force you exert on a ball can impact its path. When   
   the force is large enough, either system (the ball or the atoms) can   
   overcome the competing interaction to achieve a near linear path.   
      
   To verify and confirm their experimental findings and further understand   
   the atomic dynamics, the team also carried out molecular dynamics and   
   density functional theory calculations. These modeling studies helped them   
   decipher the cumulative effects of forces to track how the structures   
   changed during the transition and provided time-resolved snapshots of   
   the atomic motions.   
      
   The paper describes how the combination of theory and experimental   
   studies provided detailed information, including how vanadium "dimers"   
   (bound pairs of vanadium atoms) stretch and rotate over time during the   
   transition. The research also successfully addressed some long-standing   
   scientific questions about vanadium dioxide, including the existence of   
   an intermediate phase during the insulator-to-metal transition, the role   
   of photoexcitation-induced thermal heating, and the origin of incomplete   
   transitions under photoexcitation.   
      
   This study sheds new light on scientists' understanding of how   
   photoinduced electronic and lattice dynamics affect this particular   
   phase transition -- and should also help continue to push the evolution   
   of computing technology.   
      
   When it comes to making a computer that mimics the human brain, Zhu said,   
   "we still have a long way to go, but I think we are on the right track."   
   This research was funded primarily by the DOE Office of Science. The 3   
   MeV ultrafast electron diffraction (MeV-UED) instrument was developed   
   with a series of Laboratory Directed Research and Development awards and   
   is operated and maintained at Brookhaven Lab's Accelerator Test Facility   
   (ATF) -- a DOE Office of Science user facility.   
      
      
   ==========================================================================   
   Story Source: Materials provided by   
   DOE/Brookhaven_National_Laboratory. Note: Content may be edited for   
   style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Junjie Li, Lijun Wu, Shan Yang, Xilian Jin, Wei Wang, Jing Tao, Lynn   
         Boatner, Marcus Babzien, Mikhail Fedurin, Mark Palmer, Weiguo   
         Yin, Olivier Delaire, Yimei Zhu. Direct Detection of V-V Atom   
         Dimerization and Rotation Dynamic Pathways upon Ultrafast   
         Photoexcitation in VO2. Physical Review X, 2022; 12 (2) DOI:   
         10.1103/PhysRevX.12.021032   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2022/05/220509150759.htm   
      
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