New research reveals dynamics of ultra-fast, fast groups of atoms

Our high-speed, high-bandwidth world constantly needs new ways of processing and storing information. Semiconductors and magnetic materials have been the bulk of device storage 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 ferroelectricity became more interesting with the discovery of polar vertebrae – essentially helical groupings of atoms – within the structure of the material. Now, a team of researchers led by the U.S. Department of Energy (DOE) Argonne National Laboratory has discovered new insights into the behavior of these vortices, which could be the first step in using them for fast, versatile data processing and storage.

What is so important about the behavior of groups of atoms in these materials? First, these polar vortices are intriguing new discoveries, even if they just sit still. For another, this new research, published as a cover story in Earth, reveals how they move. This new form of helical atomic motion can be caused and can be manipulated. This is good news for the potential use of this material in future data processing and storage devices.

“While the motion of individual atoms may not be too exciting, these motions combine to create something new – an example of what scientists call emerging phenomena – that offers possibilities we could not have imagined before,” Haidan said. Wen, a physicist in Argonne’s Division of X-ray Science (XSD).

These vertebrae 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. However, their dynamics cannot be seen in a typical laboratory environment. They should be excited to action by applying an ultra-fast electric field.

All this makes it difficult to observe and characterize. Wen and his colleague, John Freeland, a senior physicist in Argonne’s XSD, studied these vortices for many years, first with the ultra-bright X-rays of the Advanced Photon Source (APS) at Argonne, and more recently with the free-electron laser. -abilities. of the LINAC Coherent Light Source (LCLS) at DOE’s SLAC National Accelerator Laboratory. Both the APS and LCLS are DOE Office of Science User Facilities.

Using the APS, researchers were able to use lasers to create a new state of matter and obtain a comprehensive image of its structure using X-ray diffraction. In 2019, the team, led by Argonne and Pennsylvania State University, presented their findings in a Natural materials cover story, especially that the vertebrae can be manipulated with light pulses. Data were taken on several APS bar lines: 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 thin layers of two different materials with light,” said Venkatraman Gopalan, professor of materials science and engineering and physics to Penn, said. State.

“A lot of work has been done to measure the motion of a small 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 we might be able to characterize with ultra-fast time-scale probes.”

The APS was able to take snapshots of these vortices on nanosecond time scales – a hundred million times faster than blinking your eyes – but the research team discovered that they were not fast enough.

“We knew something exciting was going to happen that we could not detect,” Wen said. “The APS experiments helped us determine where we wanted to measure, on faster time scales that we could not find at the APS. But LCLS, our sister facility at SLAC, provides the exact tools needed to solve this puzzle. loose.”

With their prior research in hand, Wen and Freeland joined colleagues from Lawrence Berkeley National Laboratory of SLAC and DOE (Berkeley Lab) —Gopalan and Long-Qing Chen of Pennsylvania State University; Jirka Hlinka, Head of the Department of Electrical Engineering 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 could tell them how these atoms behave, and whether that behavior can be controlled. Based on what they learned at APS, the team – including lead authors of the new paper, Quan Li of Tsinghua University and Vladimir Stoica of Pennsylvania State University, both postdoctoral researchers at the APS – conducted further investigations into the LCLS. at SLAC followed. .

“LCLS uses X-rays to take pictures of what atoms do on scales that are inaccessible to conventional X-ray machines,” said Aaron Lindenberg, associate professor of materials science and engineering and photon sciences at Stanford University and SLAC. “X-ray scattering can map structures, but it requires a machine like LCLS to see where the atoms are and to detect how they move dynamically at unimaginably fast speeds.”

Using a new ferroelectric material designed by Ramamoorthy Ramesh and Lane Martin at Berkeley Lab, the team was able to generate a group of atoms in a swirling motion through an electric field at terahertz frequencies, the frequency of which is approx. 1000 times faster than the processor in your cell phone. They could then record images of these turns on a femtosecond time scale. A femtosecond is a quadrillionth of a second – it’s such a short time that light can only travel the length of a small bacterium before it’s over.

With this precision, the research team saw a new kind of movement that they had not seen before.

“Despite the fact that theorists were interested in this type of motion, the exact dynamic properties of polar vertebrae remained unclear until the completion of this experiment,” Hlinka said. “The experimental findings helped theorists refine the model and give a microscopic insight into the experimental observations. It was a real adventure to reveal this kind of collective atomic dance.”

This discovery provides a new set of questions that will answer further experiments, and planned upgrades of the APS and LCLS light sources will help push this research further. LCLS-II, now under construction, will increase its X-ray pulses from 120 to 1 million per second, enabling scientists to look at the dynamics of materials with unprecedented accuracy.

And the APS upgrade, which replaces the current electron storage ring with a modern model that will increase the brightness of the cohesive X-rays up to 500 times, will allow researchers to use small objects like these vortices with nanometer resolution.

Researchers can already see the possible applications of this knowledge. The fact that these materials can be customized by applying small changes offers a wide range of possibilities, Lindenberg said.

“From a fundamental perspective, we see a new kind of case,” he said. “From a technological perspective of information storage, we want to take advantage of what is 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 to do it in a dynamic sense, faster than we thought possible. ‘

Wen and Freeland agreed and noted that this material 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’re looking for new phenomena. What aspects can it entail? We’re looking for objects with faster speed. That’s what inspires people. How can we do something else?”