Our high-speed, high-bandwidth world constantly requires new ways of processing and storing information. Semiconductors and magnetic materials have been the backbone of data storage devices for decades. In recent years, however, researchers and engineers have turned to ferroelectric materials, a type of crystal that can be manipulated with electricity.
In 2016, the study of ferroelectrics became more interesting with the discovery of polar eddies – essentially spiral-shaped groups of atoms – within the structure of the material. Now, a team of researchers led by the US Department of Energy’s (DOE) Argonne National Laboratory have discovered new information about the behavior of these vortices, information that could be the first step towards their use for treatment. and fast and versatile data storage.
What is so important about the behavior of groups of atoms in these materials? On the one hand, these polar eddies are intriguing new discoveries, even when they are just standing still. On the other hand, this new research, published on the cover of Nature, reveals how they move. This new kind of spiral-shaped atomic motion can be made to occur and can be manipulated. This is good news for the potential use of this material in future data processing and storage devices.
“While the movement of individual atoms might not be too exciting, these movements come together to create something new – an example of what scientists call emerging phenomena – that may harbor abilities we never imagined. before, “said Haidan Wen, a physicist at Argonne’s Division of X-ray Sciences (XSD).
These vortices are indeed small – about five or six nanometers wide, thousands of times smaller than the width of a human hair, or about twice as wide as a single strand of DNA. Their dynamics, however, cannot be seen in a typical lab environment. They must be excited into action by applying an ultra-fast electric field.
All of this makes them difficult to observe and characterize. Wen and his colleague John Freeland, a senior physicist in the Argonne XSD, have spent years studying these vortices, first with ultra-bright x-rays from the advanced photon source (APS) at Argonne, and more recently with free electron laser capabilities. of the LINAC Coherent Light Source (LCLS) at the DOE’s SLAC National Accelerator Laboratory. Both APS and LCLS are facilities for users of the DOE Office of Science.
Using APS, researchers were able to use lasers to create a new state of matter and get a complete picture of its structure using X-ray diffraction. In 2019, the team, jointly led by Argonne and Pennsylvania State University, reported its findings in a Nature Materials cover story, including the fact that vortices can be manipulated with pulses of light. Data were taken at several APS beamlines: 7-ID-C, 11-ID-D, 33-BM and 33-ID-C.
“Although this new state of matter, a so-called supercrystal, does not exist naturally, it can be created by illuminating carefully designed thin layers of two separate materials using light,” said Venkatraman Gopalan. , professor of materials science and engineering and physics at Penn State.
“A lot of work has gone into measuring the motion of a tiny object,” Freeland said. “The question was, how do we see these phenomena with x-rays? We could see that there was something interesting with the system, something that we might be able to characterize with timescale probes. super-fast. “
APS was able to take snapshots of these vortices at nanosecond timescales – a hundred million times faster than it takes to blink – but the research team found that it wasn’t. was not fast enough.
“We knew something exciting was going to happen that we couldn’t detect,” Wen said. “The APS experiments helped us figure out where we want to measure, at faster timescales that we couldn’t access the APS. But LCLS, our sister facility to SLAC, provides the exact tools needed to solve this puzzle. “
With their previous research in hand, Wen and Freeland joined colleagues at SLAC and the DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab) – Gopalan and Long-Qing Chen of Pennsylvania State University; Jirka Hlinka, head of the dielectric department at the Institute of Physics of the Czech Academy of Sciences; Paul Evans of the University of Wisconsin, Madison; and their teams – to design a new experiment that would be able to tell them how these atoms behave and whether that behavior can be controlled. Building on what they learned at APS, the team – including the lead authors of the new paper, Qian Li and Vladimir Stoica, both APS postdocs at the time of this work – a continued his research at the LCLS of SLAC. .
“LCLS uses x-ray beams to take snapshots of what atoms do at timescales not accessible to conventional x-ray machines,” said Aaron Lindenberg, associate professor of materials science and engineering and science photons at Stanford University and SLAC. “X-ray scattering can map structures, but it takes a machine like LCLS to see where atoms are and to track how they are moving dynamically at incredibly fast speeds.”
Using a new ferroelectric material designed by Ramamoorthy Ramesh and Lane Martin at the Berkeley Lab, the team were able to excite a group of moving atoms swirling by an electric field at terahertz frequencies, the frequency that is about 1,000 times faster than your cell processor. call. They were then able to capture images of these spins at femtosecond timescales. A femtosecond is a quadrillionth of a second – it’s such a short period of time that light can only travel the length of a small bacteria before it is terminated.
With this level of precision, the research team saw a new type of movement that they had never seen before.
“Although theorists were interested in this type of motion, the exact dynamic properties of polar vortices remained unclear until the end of this experiment,” Hlinka said. “The experimental findings helped theorists refine the model, providing microscopic insight into the experimental observations. It was quite an adventure to reveal this kind of concerted atomic dance.”
This discovery opens up a whole new set of questions that will require further experimentation to answer, and planned upgrades to APS and LCLS light sources will help push this research further. LCLS-II, currently under construction, will increase its x-ray pulses from 120 to 1 million per second, allowing scientists to examine the dynamics of materials with unprecedented precision.
And the APS upgrade, which will replace the current electron storage ring with a state-of-the-art model that will increase the brightness of coherent x-rays up to 500 times, will allow researchers to image small objects like these vortices with nanometric resolution.
Researchers can already see the possible applications of this knowledge. The fact that these materials can be adjusted by applying small changes opens up a wide range of possibilities, Lindenberg said.
“Fundamentally, we see a new kind of question,” he said. “From an information storage technology perspective, we want to take advantage of what’s happening at these frequencies for high-speed, high-bandwidth storage technology. I’m excited about controlling the properties of this material, and this experiment shows possible ways to do this in a dynamic sense, faster than we thought possible. “
Wen and Freeland agree, noting that these materials may have applications that no one has thought of yet.
“You don’t want something that does what a transistor does, because we already have transistors,” Freeland said. “So you are looking for new phenomena. What aspects can they bring? We are looking for objects with faster speed. That’s what inspires people. How can we do something different?”