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

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

Imagine a computer that can think as fast as the human brain while using very little energy. This is the goal of scientists who seek to discover or develop materials capable of sending and processing signals as easily as neurons and synapses in the brain. Identifying quantum materials with an intrinsic ability to switch between two (or more) distinct forms could hold the key to these futuristic-sounding “neuromorph” computing technologies.

In an article just published in the journal physical examination X, Yimei Zhu, a physicist at the US 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 “strobe camera”, the team captured the hidden path of atomic motion as this material changes 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 energy 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 which can be electrical, mechanical, optical or magnetic by a change in the state of the material.”

“Vanadium dioxide is one of the rare and amazing 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 neurons (nerve cells) and synapses (the connections between them) .

“It changes from a complete insulator – like rubber – to a very good metallic conductor, with a resistivity change of 10,000 times or more,” Zhu said.

These two very different physical states, intrinsic to the same material, could be coded for cognitive computing.

Visualization of ultrafast atomic movements

For their experiments, the scientists triggered the transition with extremely short pulses of photons, particles of light. Next, they captured the atomic-scale response of the material using an ultrafast mega-electron-volt 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 environment, firing off intermittent flashes to catch something like a moving thrown ball. With each flash, the camera records an image; the series of images taken at different times reveal the trajectory of the bullet in flight.

The

This representation of the vanadium dioxide crystal lattice in its equilibrium 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-step phase transition from insulator to metal, where the motion of vanadium atoms in the first step is linear, then curved in the second step. This curved motion is proof that another force (exerted by the 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 the same way, but on a much faster time scale (less than a trillionth of a second) and on a much smaller length scale (less than one billionth of a millimeter). It uses high-energy electrons to reveal the trajectories of atoms.

“Previous static measurements only revealed the initial and final state of vanadium dioxide insulator-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 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 “hits”, the scientists used sophisticated time-resolved crystallographic analysis techniques they had developed to refine the intensity changes of a few dozen “electron diffraction peaks”. These are the signals produced by the electrons that scatter across the atoms in the vanadium dioxide sample as the atoms and their orbital electrons change from the insulating state to the metallic state.

“Our instrument uses accelerator technology to generate electrons with an energy of 3 MeV, which is 50 times higher than small electron microscopy and ultrafast diffraction instruments in the laboratory,” Zhu said. “The higher energy allows us to track scattered electrons at wider angles, which translates to the ability to ‘see’ the movements of atoms at smaller distances with better precision.”

Two-step dynamic and curved path

The analysis revealed that the transition takes place in two stages, the second 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.

“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. It 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 stroboscopic images of a bullet’s trajectory. When you throw a ball, you exert a force. But another force, gravity, also pulls the ball towards the ground, causing the trajectory to curve.

The

This animation shows the change in position of vanadium atoms when vanadium dioxide changes from an insulating state to a metallic state. This rapid switching can be triggered by tiny stimuli and changes the electrical resistance of the material by 10,000 times 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 curvature of the atomic trajectories is caused by the electrons orbiting the vanadium atoms.

The study also showed that a measurement related to the intensity of light used to trigger atomic dynamics can alter atomic trajectories, much the same way the force you put on a ball can impact its trajectory. . When the force is large enough, either system (the ball or the atoms) can overcome the competing interaction to achieve an almost linear trajectory.

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

The article describes how the combination of theory and experimental studies provided detailed information, including how vanadium “dimers” (bonded pairs of vanadium atoms) stretch and rotate in 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 electron and lattice dynamics affect this particular phase transition – and should also help to continue pushing 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 dimerization and rotation of VV atoms during ultrafast photoexcitation in VO2, Physical examination X (2022). DOI: 10.1103/PhysRevX.12.021032

Provided by Brookhaven National Laboratory

Quote: Ultrafast ‘camera’ captures hidden behavior of potential ‘neuromorph’ 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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