Ultrafast ‘camera’ captures hidden behavior of potential ‘neural’ materials

Yimei Zhu and Junjie Li at the 3 MeV ultra-fast electron diffraction instrument at the Brookhaven National Laboratory’s Accelerator Test Facility. This instrument acts like a high-resolution stroboscopic “camera” to track the trajectories of atoms. Credit: Brookhaven National Laboratory

Imagine a computer that can think at the same speed as the human brain while using very little energy. This is the goal of scientists who seek to discover or develop materials that can easily transmit and process signals such as neurons and synapses in the brain. Identification of quantum materials with the intrinsic ability to switch between two (or more) distinct forms may be key to future ‘neural’ computing technologies.

In a paper just published in the magazine physical x review, Yimei Zhu, a physicist at the US Department of Energy’s Brookhaven National Laboratory, and colleagues describe surprising new details about vanadium dioxide, one of the most promising neurotoxins. Using data collected by a unique ‘stroboscopic camera’, the team captured the hidden path of atomic motion as this material travels from insulator to metal in response to a pulse of light. Their findings could help guide the rational design of high-speed, energy-efficient neural devices.

“One way to reduce the power consumption of artificial neurons and synapses for brain-inspired computing is to exploit the apparent nonlinear properties of quantum materials,” Zhou said. “The main idea behind this energy efficiency is that in quantum materials, a small electrical stimulus might produce a large response that could be electrical, mechanical, optical or magnetic by changing the state of matter.”

“Vanadium dioxide is one of the rare and surprising materials that has emerged as a promising candidate for neurobiologically inspired devices,” he said. It shows an insulator-metal transition near room temperature where a small voltage or current can produce a large change in resistance with switching that can mimic the behavior of both neurons (neurons) and synapses (the connections between them).

“It goes from a perfectly insulating material — like rubber — to a very good metallic conductor, with a change in resistance of 10,000 times or more,” Chu said.

These two completely different physical states, rooted in the same material, can be encoded for cognitive computing.

Visualize ultra-fast atomic motions

For their experiments, the scientists fired the transmission with extremely short pulses of photons – particles of light. They then captured the response of the atomic matter using the ultrafast electron diffraction (MeV-UED) instrument developed at Brookhaven.

You can think of this tool as similar to a traditional camera with the shutter left open in a dark place, firing intermittent flashes to capture something like a ball lying in motion. With each flash, the camera records an image; A series of photographs taken at different times reveals the ball’s path in flight.

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This representation of the crystal lattice of stationary vanadium dioxide shows the positions of the vanadium atoms in the insulator phase (solid orange spheres) and the metal phase (hollow red spheres). Inset: A pulse of light (photon) causes a two-stage transition from the insulator to the metal, where the movement of the vanadium atoms in the first stage is linear, and then bends in the second. This curved motion is evidence that another force (exercised by electrons orbiting the vanadium atoms) also plays a role in the transition. Credit: Brookhaven National Laboratory

The MeV-UED ‘stroboscope’ captures the dynamics of a moving object in a similar way, but at a much faster time scale (shorter than a trillionth of a second) and a much smaller length scale (smaller than a billionth of a millimeter). It uses high-energy electrons to detect the trajectories of atoms.

“Previous static measurements revealed the initial and final state of the insulator-to-metal transition of vanadium dioxide, but a detailed transition process was missing,” said Junjie Li, first author of the paper. “Our ultrafast measurements have allowed us to see how atoms move — to capture short-lived (or ‘hidden’) transient states to help us understand the dynamics of the transition.”

Pictures alone do not tell the whole story. After capturing over 100,000 “shots,” the scientists used complex, time-resolved crystallographic techniques they had developed to fine-tune the intensity variations of a few dozen “electron diffraction peaks.” These are the signals produced by the scattered electrons from the atoms of the vanadium dioxide sample as the atoms and their orbital electrons move from the insulator state to the metallic state.

“Our instrument uses acceleration technology to generate electrons with an energy of 3 megaelectronvolts, which is 50 times higher than that of ultrafast and diffraction electron microscopy instruments,” Zhou 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 accuracy.”

Two-stage dynamics and curved trajectory

The analysis showed 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 motions of the atoms in the second stage were not linear.

“You would think that the path from position A to B would be a straight line—the shortest possible distance,” Chu said. “Instead, it was a curve. That was completely unexpected.”

The curve was indicative of another force also playing a role in the transition.

Think again about the stroboscopic images of the ball’s path. When you throw the ball, you are exerting force. But another force, gravity, also pulls the ball to the ground, causing the trajectory to curve.

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This animation shows the change in the positions of the vanadium atoms as vanadium dioxide switches between the dielectric and the metallic state. This rapid transformation can be triggered by small stimuli and alter the material’s electrical resistance by 10,000 times or more—all promising properties for applications of energy-efficient neuromorphs. Credit: Brookhaven National Laboratory

In the case of vanadium dioxide, the light pulse is the force driving the transition, and the bending in the atomic trajectories is caused by electrons orbiting around the vanadium atoms.

The study also showed that a measure of light intensity used to stimulate atomic dynamics can alter atomic trajectories — similar to the way the force you exert on a ball affects its trajectory. When the force is large enough, any system (the ball or the atoms) can overcome competing interactions to achieve a near linear path.

To verify and confirm their experimental results and to further understand atomic dynamics, the team also performed calculations of molecular dynamics and density functional theory. These modeling studies helped them decode the cumulative effects of forces to track how structures changed during the transition and provided time-resolved snapshots of atomic motions.

The paper describes how the combination of theory and experimental studies provided detailed information, including how “vanadium diodes” (linked pairs of vanadium atoms) expand and rotate over time during the transition. The research also successfully addressed some of the old scientific questions about vanadium dioxide, including the presence of an intermediate phase during the insulator-to-metal transition, the role of photoexcitation-induced thermal denaturation, and the origin of incomplete transitions under photocatalysis.

This study sheds new light on scientists’ understanding of how novel electronics and network dynamics affect this particular phase transition—and should also help drive the development of computing technology.

When it comes to making a computer that mimics the human brain, Chu said, “We still have a long way to go, but I think we’re on the right track.”

Switching identities: a revolutionary insulator-like material that also conducts electricity

more information:
Junjie Li et al, Direct detection of VV atom dispersal and dynamic trajectories of spin upon ultrafast photoexcitation in VO2, X . physical review (2022). DOI: 10.103 / PhysRevX.12.021032

Provided by Brookhaven National Laboratory

the quote: Ultrafast ‘camera’ captures hidden behavior of potentially ‘neural’ material (2022, May 9) Retrieved May 10, 2022 from https://phys.org/news/2022-05-ultrafast-camera-captures-hidden-behavior .html

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