The mature human brain weighs three pounds, and despite the amazing parallel processing power and still-indistinguishable-from-magic sorcery it possesses, it adheres to the same oxygen requirement as other living tissue.
Evgeny Tsymbal’s explanation of a technological marvel movable, data-covered walls that are only a few atoms wide that may eventually enable computers to function more like brains was delivered with a hint of humour.
“There was unambiguous evidence that oxygen vacancies are responsible for this,” said Tsymbal, George Holmes University Professor of physics and astronomy at the University of Nebraska-Lincoln.
In partnership with colleagues in China and Singapore, Tsymbal and a few Husker alumni have demonstrated how to construct, control and explain the oxygen-deprived walls of a nanoscopically thin material suited to next-gen electronics.
These walls can communicate in several electronic dialects, which would enable the devices housing them to store even more data than the majority of digital data-writing and reading processes, which only speak the binary of 1s and 0s. Electrical spikes sent through the walls can pass depending on which impulses have already gone through them, much like synapses in the brain, giving them an adaptability and energy-efficiency more equivalent to human memory.
The walls can retain their data states even if their gadgets turn off, much like how brains preserve memories even while their users are sleeping. This is a prelude to electronics that turn back on with the speed and ease of a light.
The team investigated the barrier-smashing walls in a nanomaterial, named bismuth ferrite, that can be sliced thousands of times thinner than a human hair. Bismuth ferrite also boasts a rare quality known as ferroelectricity: The polarization, or separation, of its positive and negative electric charges can be flipped by applying just a pinch of voltage, writing a 1 or 0 in the process.
That 1 or 0 remains even when the voltage is removed, giving it the equivalent of long-term memory that DRAM lacks in contrast to normal DRAM, a dynamic random-access memory that needs to be refreshed every few milliseconds.
I think that this mechanism is very important, because what most people are doing including us, theoretically is looking at pristine materials, where polarization switches up and down, and studying what happens with the resistance. All the experimental interpretations of this behavior were based on this simple picture of polarization. But here, it’s not only the polarization. It involves some chemical processes inside of it.
Professor Evgeny Tsymbal
In a section of the material known as a domain, the polarization is typically read as a 1 or 0 and flipped to rewrite it as a 0 or 1. The wall that is created when two domains with opposing polarities come together only takes up a small portion of the space allotted to the domains.
They have been identified as primary suspects in the search for novel strategies to cram ever-more functionality and storage into ever-smaller gadgets due to the few-atom thickness of those walls and the peculiar qualities that occasionally occur in or around them.
However, it has been challenging to locate, let alone control or produce, walls that run perpendicular to a ferroelectric material’s surface and net an electric charge useable in data processing and storage. But about four years ago, Tsymbal began talking with Jingsheng Chen from the National University of Singapore and He Tian from China’s Zhejiang University.
At the time, Tian and some colleagues were pioneering a technique that allowed them to apply voltage on an atomic scale, even as they recorded atom-by-atom displacements and dynamics in real time.
The researchers discovered that even 1.5 volts applied to a bismuth ferrite film produced a domain wall parallel to the material’s surface with a particular electrical resistance whose value could be used to read a data state. When voltage was withdrawn, the wall, and its data state, remained.
The domain wall started to migrate down the material as the voltage was increased, a behavior observed in other ferroelectrics. But this wall stayed parallel, but the walls in those other materials had then propagated perpendicular to the surface.
And unlike any of its predecessors, the wall migrated just one atomic layer at a time, moving at a glacial speed. Its position, in turn, corresponded with changes in its electrical resistance, which dropped in three distinct steps three more readable data states that emerged between the application of 8 and 10 volts.
The researchers had nailed down a few W’s the what, the where, the when critical to eventually employing the phenomenon in electronic devices. But they were still missing one. Tsymbal, as it happened, was among the few people qualified to address it.
“There was a puzzle,” Tsymbal said. “Why does it happen? And this is where theory helped.”
Most domain walls are electrically neutral, possessing neither a positive nor a negative charge. That’s with good reason: Neutral walls are the default since they take less energy to maintain their electric state. Contrarily, the domain wall in the ultra-thin bismuth ferrite that the scientists discovered had a significant charge.
And that, Tsymbal knew, should have kept it from stabilizing and persisting. Yet somehow, it was managing to do just that, seeming to flout the rules of condensed-matter physics.
There had to be an explanation. In his prior research, Tsymbal and colleagues had found that the departure of negatively charged oxygen atoms, and the positively charged vacancies they left in their wake, could impede a technologically useful outcome.
This time, Tsymbal’s theory-backed calculations suggested the opposite that the positively charged vacancies were compensating for other negative charges accumulating at the wall, essentially fortifying it in the process.
Later measurements by the team’s experimental apparatus revealed that, as expected by the calculations, the distribution of charges in the material nearly perfectly coincided with the position of the domain wall. If oxygen vacancies turn up in other ferroelectric playgrounds, Tsymbal said, they could prove vital to better understanding and engineering devices that incorporate the prized class of materials.
“From my perspective, that was the most exciting,” said Tsymbal, who undertook the research with support from the university’s quantum-focused EQUATE project. “This links ferroelectricity with electrochemistry. We have some kind of electrochemical processes namely, the motion of oxygen vacancies which basically control the motion of these domain walls.”
“I think that this mechanism is very important, because what most people are doing including us, theoretically is looking at pristine materials, where polarization switches up and down, and studying what happens with the resistance. All the experimental interpretations of this behavior were based on this simple picture of polarization. But here, it’s not only the polarization. It involves some chemical processes inside of it.”
The team detailed its findings in the journal Nature. Tsymbal, Tian and Chen authored the study with Ze Zhang, Zhongran Liu, Han Wang, Hongyang Yu, Yuxuan Wang, Siyuan Hong, Meng Zhang, Zhaohui Ren and Yanwu Xie, as well as Husker alumni Ming Li, Lingling Tao and Tula Paudel.
