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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