Physicists and material scientists have created a way to interact with "polar vortex" nanostructures at a rate of nearly 400 billion times per second, a fundamental scientific approach that could someday provide data storage and processing that is more compact and more than 100 times faster than many modern central processing units.
The research, published April 14 in Nature, was conducted on a polarized material in which atoms spun in a series of circular formations and were excited by X-ray laser pulses. The system could be designed to adopt an approach to computing similar to that of older hardware that used magnets, according to the study's authors.
Before semiconductors became ubiquitously used to store information in computers with electrical voltage, magnets were once commonly employed for the same purpose. Early ROM and RAM systems were among the components that stored "0s" and "1s" by magnetizing cores in a clockwise or counterclockwise direction.
Although magnetic core memory faded decades ago as semiconductors became a less-expensive alternative, today's researchers hope to create a similar system using only electricity. Direct manipulation of a polarization-based system using electric fields can theoretically allow a processing speed in the realm of terahertz, or trillions of interactions per second, though such speeds have not been reached experimentally.
The search for these ultrafast speeds has led to ferroelectric materials, which are permanently polarized with different positive and negative sides and are the electric analogs to natural magnets, also known as ferromagnets. Devices based on these materials would be much faster than their magnetic counterparts, said senior author Venkatraman Gopalan, a professor of materials science and physics at Pennsylvania State University.
Ferroelectric materials are also being researched for other electronic applications, such as telecommunications, though issues such as physical strain on the material while in use have acted as obstacles to their development.
"The idea was, can we play with these ferroelectric materials, and can they be useful for computation for electronic devices or data storage?" Gopalan said.
To create their test material, researchers stacked alternating sheets of lead titanate and strontium titanate — both ferroelectric materials — that were 16 atomic layers thick. The two sheets were both rigid materials but were able to avoid the strain of the electric activity they were hosting when combined into the superlattice.
In the experiment, the lead layers produced electric dipoles — areas with a net positive charge on one end, and a negative charge on the other — that the strontium layers forced to spin in circular shapes known as polar vortices, which were discovered in the same superlattice in 2016. The researchers were able to make the vortices move up and down by striking them with multifrequency ray laser pulses, revealing how they can be manually interacted with.
The polar vortices responded to laser pulses of 340 and 380 gigahertz, meaning they can be controlled with signals a few trillionths of a second long. This frequency is more than 100 times faster than the clock speed in many standard laptops, and even the best central processing units have yet to break 10 gigahertz.
At a lower laser frequency of about 80 gigahertz, a new phenomenon appeared in the material that the authors called "vortexons." The researchers saw that a vortexon is a circular formation that overlaps two polar vortices. The lead titanate atoms in the vortexon are moved in a straight line, but they collectively produce a spiral shape in their movement. The authors describe the vortexons as another avenue through which to interact with polar vortices.
"That itself is a beautiful achievement of this work, that we have created something that doesn't just naturally exist in nature," Gopalan said. "We have made these electrical dipoles behave in a way that doesn't naturally happen."
The researchers found that the vortexons reacted to laser pulses of higher frequencies when the temperature of the ferroelectric superlattice was increased. Corresponding author Haidan Wen, a physicist at Argonne National Laboratory, said the unusual heat dependence of the moving atoms' "dances" makes them easy to control, as does the to operate the system at room temperature.
The authors envisioned that the polar vortices and vortexons could be the basis for faster, denser data storage and processing. Gopalan said a polar vortex could be used to store a bit of information — clockwise or counterclockwise could be assigned as "0" or "1," much like the old magnet-based computing hardware — while the vortexons would be involved in reading and writing the information.
But such a technology would be made years in the future, said Gopalan, who is wrapping up research on "bubbles" of electric charges known as polar skyrmions that may have a use for information storage in computing.
Wen, who plans to further investigate the functionality of vortexons, said he would like to see engineers pick up where their research left off to begin making these ultrafast computers a reality.
"Hopefully, the discovery of these peculiar dances will attract the electrical engineers' interest," Wen said, "and they're probably gonna help us to design a next generation [of] devices."
The study, "Subterahertz collective dynamics of polar vortices," published April 14 in Nature, was authored by Qian Li, Argonne National Laboratory and Tsinghua University; Vladimir Stoica, Argonne National Laboratory and Pennsylvania State University; Marek Paściak, Christelle Kadlec and Jirka Hlinka, Institute of Physics of the Czech Academy of Sciences; Margaret McCarter, Sujit Das and Ajay Yadav, University of California, Berkeley; Cheng Dai, Shukai Yi, Long-Qing Chen and Venkatraman Gopalan, Pennsylvania State University; Hyeon Jun Lee, Youngjun Ahn, Samuel Marks and Paul Evans, University of Wisconsin-Madison; Suji Park, Brookhaven National Laboratory; Takahiro Sato, Matthias Hoffmann, Matthieu Chollet, Michael Kozina, Silke Nelson and Diling Zhu, SLAC National Accelerator Laboratory; Aaron Lindenberg, SLAC National Accelerator Laboratory and Stanford University; Ramamoorthy Ramesh and Lane Martin, University of California, Berkeley and Lawrence Berkeley National Laboratory; and John Freeland and Haidan Wen, Argonne National Laboratory.
Correction: A previous version of this story misidentified the journal in which the study was published. Also, Haidan Wen's role in the study was misidentified. The story also mischaracterized the laser pulses used. The errors have been corrected. The story has also been updated to clarify the nature of the polar vortices.