RE: Vanadium - another use for it?10 May 2022 08:55
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.
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