Ultra-fast ‘camera’ captures hidden behavior of potential ‘neuromorphic’ material

Yimei Zhu and Junjie Li at the 3 MeV Ultrafast Electron Diffraction instrument at the Brookhaven National Laboratory Accelerator Test Facility. This instrument acts as a high-resolution stroboscopic “camera” to track the trajectories of atoms. Credit: Brookhaven National Laboratory

Imagine a computer that can think as fast as the human brain using very little power. That is the goal of scientists seeking to discover or develop materials that can send and process signals as easily as neurons and synapses in the brain. Identifying quantum materials with an intrinsic ability to switch between two (or more) different forms may hold the key to these futuristic-sounding “neuromorphic” computing technologies.

In an article just published in the journal Physical examX, Yimei Zhu, a physicist at the US Department of Energy’s (DOE) Brookhaven National Laboratory, and colleagues describe surprising new details about vanadium dioxide, one of the most promising neuromorphic materials. Using data collected by a unique “strobe camera,” the team captured the hidden path 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, energy-efficient neuromorphic devices.

“One way to reduce power consumption in artificial neurons and synapses for brain-inspired computing is to exploit the pronounced nonlinear properties of quantum materials,” Zhu said. “The main idea behind this energy efficiency is that in quantum materials, a small electrical stimulus can 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 and surprising materials that has emerged as a promising candidate for bio-inspired neuromimetic devices,” he said. It exhibits an insulator-to-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 electrical connections). between them).

“It goes from being 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 to the same material, could be encoded for cognitive computing.

Visualization of ultrafast atomic motions

For their experiments, the scientists triggered the transition with extremely short pulses of photons, particles of light. They then captured the material’s atomic-scale response using a megaelectronvolt ultrafast electron diffraction (MeV-UED) instrument developed at Brookhaven.

You can think of this tool as similar to a conventional camera with the shutter open in a dark environment, 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 trajectory of the ball in flight.

Ultra-fast 'camera' captures hidden behavior of potential 'neuromorphic' material

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

The MeV-UED “strobe” captures the dynamics of a moving object in a similar way, but on a much faster time scale (less than a trillionth of a second) and a much smaller length scale (less than a billionth of a second). millimeter). ). It uses high-energy electrons to reveal the paths of atoms.

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

Pictures alone don’t tell the whole story. After capturing more than 100,000 “shots,” the scientists used sophisticated time-resolved crystallographic analysis techniques they had developed to refine the intensity changes of a few dozen “electron diffraction spikes.” Those are the signals produced by electrons scattering off the atoms in the vanadium dioxide sample as the atoms and their orbital electrons transition from the insulating state to the metallic state.

“Our instrument uses acceleration technology to generate electrons with an energy of 3 MeV, which is 50 times higher than the smallest laboratory ultrafast electron microscopy and diffraction instruments,” Zhu said. “The higher energy allows us to track scattered electrons at wider angles, which translates into being able to ‘see’ the motions of atoms at smaller distances with greater precision.”

Two-stage dynamics and a curved path

The analysis revealed that the transition takes place in two stages, the second stage being longer and slower than the first. He also showed that the trajectories of the movements of the atoms in the second stage were not linear.

“One would think that the path from position A to position B would be a direct straight line, the shortest distance possible. 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 of the stroboscopic images of the trajectory of a ball. When you throw a ball, you exert a force. But another force, gravity, also pulls the ball toward the ground, causing the ball to curve.

Ultra-fast 'camera' captures hidden behavior of potential 'neuromorphic' material

This animation shows the change in position of the vanadium atoms as the vanadium dioxide changes between the insulating and metallic states. This rapid change can be triggered by a small stimulus and changes the material’s electrical resistance 10,000-fold or more, all promising properties for energy-efficient neuromorphic applications. Credit: Brookhaven National Laboratory

In the case of vanadium dioxide, the light pulse is the force that initiates the transition, and the bending in the atomic paths is caused by electrons orbiting the vanadium atoms.

The study also showed that a measure related to the intensity of light used to trigger atomic dynamics can alter atomic trajectories, similar to the way the force you exert on a ball can affect its trajectory. When the force is large enough, either system (the ball or the atoms) can overcome the competing interaction to achieve a nearly linear path.

To verify and confirm their experimental findings and better understand 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 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 “dimers” (bonded pairs of vanadium atoms) stretch and twist over time during the transition. The research also successfully addressed some longstanding scientific questions about vanadium dioxide, including the existence of an intermediate phase during the transition from insulator to metal, 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 electron and lattice dynamics affect this particular phase transition, and should also help continue to drive 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’re on the right track.”

Switching identities: Revolutionary insulator-like material also conducts electricity

More information:
Junjie Li et al, Direct Detection of Dynamic Pathways of VV Atom Dimerization and Rotation Following Ultrafast Photoexcitation at VO2, Physical examX (2022). DOI: 10.1103/PhysRevX.12.021032

Provided by Brookhaven National Laboratory

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

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