Compound that 'remembers' phase transitions could have uses in computer memory

Researchers ran a current through vanadium dioxide and were never forgiven

Researchers in Switzerland have discovered a compound that can "remember" its previous phase transition states, offering potential applications in computer memory and information processing.

Vanadium dioxide's (VO2) crystalline structure has long intrigued scientists. It changes structure (phase transition) at around 67°C (152.6°F) and below that threshold acts as an electronic insulator, while above it as a conductor.

But Mohammad Samizadeh Nikoo, a PhD student, discovered that the material can somehow remember its earlier transitions, opening up the potential for the material to complement silicon in computer memory and processing.

Working with a team at public Swiss university École polytechnique fédérale de Lausanne, Nikoo applied an electric current to a sample of VO2. The current moved across the material until it exited on the other side, at which point the sample heated up, causing the VO2 to change to its conducting state. When the current was removed, it reverted to the earlier state.

Things got weird when the team realized the material was not quite the same. It somehow remembered the previous transition. When applying a second current pulse to the material, the team found the time it took to change state was directly linked to the history of the material.

Professor Elison Matioli, who oversaw the study, said: "The VO2 seemed to 'remember' the first phase transition and anticipate the next. We didn't expect to see this kind of memory effect, and it has nothing to do with electronic states but rather with the physical structure of the material. It's a novel discovery: no other material behaves in this way."

The team found that VO2 seems to recall its most recent external stimulus for up to three hours.

"The memory effect could in fact persist for several days, but we don't currently have the instruments needed to measure that," Matioli said in a university article.

The findings have been written up in the science journal Nature Electronics. The paper says the dynamics in VO2 can be excited in sub-nanosecond timescales and monitored for several orders of magnitudes in time, from microseconds to hours.

"A two-terminal switch undergoes complex but predictable and reversible dynamics, induced by a series of excitations," the study said.

"From a technological point of view, our results show that the response of these dynamics to a sequence of excitations can enable new schemes for data storage and processing. Our functional devices can, thus, potentially meet the continuous demands of electronics in terms of downscaling, fast operation and decreasing the voltage-supply level." ®

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