Researchers who participated in the study say that it is a novel discovery, and that no other material behaves in this way. It is almost as if it had memory.
Scientists at EPFL have developed a compound that remembers the history of previous external stimulations. When relaxed at room temperature, vanadium dioxide (VO2) has an insulating phase, and when heated to 68 degrees Celsius, it undergoes a steep transition from insulator to metal. In 2018, scientists found out why: atoms’ lattice patterns change as temperature rises.
Eventually, the material returns to its original state of insulation as the temperature drops.
Like neurons
According to researchers, the compound vanadium dioxide has no living cells and no structure even close to that of the brain, but it ‘remembers’ past external stimuli.
According to classical VO2 theory, the material immediately returns to its insulating state after excitation has been removed. Researchers wanted to know how long it takes for VO2 to change states. A memory effect was discovered in the material’s structure after many measurements, which changed the course of their research.
A sample of VO2 was electrically charged by scientists.
Mohammad Samizadeh Nikoo, a Ph.D. student at EPFL’s Power and Wide-band-gap Electronics Research Laboratory (PowerLab), explained that “the current moved across the material, following a path until it exited on the other side.”
The VO2 in the sample changed state as a result of the current heating up the sample. A short while later, the material returned to the state it was in before the current passed.
In this experiment, scientists applied a second current pulse to the material and found that the time required for the material to change state is directly associated with its history.
According to Prof. Elison Matioli, the VO2 “remembered” the first phase transition and anticipated the next. A memory effect like this was not expected, and it has nothing to do with electronic states but rather with the material’s physical structure.
A strange material
“It’s a novel discovery: no other material behaves this way,” Professor Matioli said.
In studies, scientists discovered that VO2 could retain information for up to three hours after being exposed to external stimuli. There may be a lasting memory effect, but scientists are unable to measure it at the moment.
“The memory effect could, in fact, persist for several days, but we don’t currently have the instruments needed to measure that,” says Matioli.
Since this memory effect was observed in the material itself, it is an extremely significant discovery.
Memory is used by engineers to perform calculations, and materials with increased capacity, speed, and compactness could improve computation performance. These are all the criteria that VO2 meets. A persisting, structural memory further distinguishes it from conventional materials that store binary information depending on electrical state changes.
As a result of the study, the researchers concluded that VO2 switches behave very similarly to neurons in the brain, where memory and processing are combined into one unit, perfectly mimicking their behavior.
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