Capable of "remembering" the entire history of previous external stimuli, just like the brain (Image source: POWERlab/2022 EPFL)
Researchers at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have discovered that vanadium dioxide (VO2), a compound used in electronic products, is capable of "remembering" the entire history of previous external stimuli. This is the first material identified as possessing this property, although there may be others.
MOHAMMAD Samizadeh Nikoo, a PhD student at the Power and Wide Bandgap Electronics Research Laboratory (POWERlab) at EPFL, stumbled upon this discovery while studying the phase transition of VO2. He and his colleagues reported their findings in a paper published in Nature Electronics.
When relaxed at room temperature, VO2 is insulating, but it undergoes a sharp insulator-to-metal transition at 68°C, accompanied by changes in its lattice structure. However, this state does not persist for long: "The material immediately reverts to an insulating state upon removal of excitation," said Nikoo.
In his thesis, Nikoo began exploring how long it takes for VO2 to transition from one state to another. However, his research led him down a different path - after conducting hundreds of measurements, he observed a memory effect in the material's structure.
In his experiment, Nikoo applied an electric current to a sample of VO2. "The current passes through the material, moving along a certain path until it exits from the other side," he explained.
When the current heats the sample, it causes VO2 to change state. Then, once the current is removed, the material returns to its original state. However, when Nikoo applied a second current pulse to the material, he found that the time required for the state change is directly related to the material's history.
"VO2 seems to 'remember' the transition in the first stage and predict the transition in the next stage," said Elison Matioli, head of POWERlab. "We didn't expect to see this memory effect, which is not related to electronic states but to the physical structure of the material. This is a novel discovery: no other materials behave in this way.".
Researchers continue to discover that VO2 is capable of remembering its recent external stimuli for up to three hours. "The memory effect may actually persist for several days, but we currently do not have the necessary instruments for measurement," said Matioli.
The findings of the research team are significant because the memory effect they observed is an inherent property of the material itself. Engineers rely on memory to perform various computations, and materials that enhance the computational process by providing greater capacity, speed, and miniaturization are in high demand. VO2 ticks all three boxes. More importantly, its continuous structural memory distinguishes it from traditional materials, which store data as binary information relying on the manipulation of electronic states.
The researchers conducted numerous measurements to arrive at their findings. They also validated their results by applying the new method to different materials in other laboratories worldwide. This discovery provides a good replication of what happens in the brain, as the VO2 switch can function like a neuron.