Scientists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have achieved a groundbreaking feat by creating the first-ever atomic movies.
These movies show how atoms rearrange in a quantum material as it shifts from being an insulator to a metal.
Their discovery, published in Nature Materials, could lead to the design of new materials with commercial applications.
The researchers demonstrated that a technique called atomic pair distribution function (PDF) analysis can be used successfully at X-ray free-electron laser (XFEL) facilities.
Typically, PDF is used in synchrotron light source experiments, where samples are bombarded with X-rays.
By studying how these X-rays scatter, scientists can understand the material’s properties. However, these experiments are limited by the shortest X-ray pulses that can be generated.
“It’s like a camera’s shutter speed,” explained Jack Griffiths, co-lead author of the paper. “If something changes faster than your camera’s shutter speed, the photo will be blurry. Similarly, shorter X-ray pulses let us see fast changes in materials in more detail.”
Griffiths, a former postdoctoral researcher at Brookhaven’s Condensed Matter Physics & Materials Science (CMPMS) Department, conducted the research while at Brookhaven and is now at the National Synchrotron Light Source II (NSLS-II).
Synchrotron light sources are excellent for studying materials that change slowly, over minutes or hours, like batteries charging and discharging. However, the scientists aimed to observe changes happening in mere picoseconds.
“A picosecond is incredibly fast. In one second, light travels around the Earth seven and a half times. But in one picosecond, it only travels one-third of a millimeter,” Griffiths explained.
To capture these fast changes, the scientists brought the PDF technique to an XFEL called the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. LCLS generates extremely bright and short X-ray pulses, like a high-speed camera shutter.
“When trying something new, there’s always an element of the unknown. It can be nerve-racking but also very exciting,” said Emil Bozin, the other co-lead author and a physicist in the CMPMS X-ray Scattering Group. The fast “shutter speed” of LCLS allowed the team to create movies showing atomic movements during the transition between metal and insulator.
“I was blown away by how well it worked,” said Simon Billinge, a physicist in the X-ray Scattering Group and a professor at Columbia University. “It’s like needing a navigation app. You know your starting point and destination, but you need the app to show you the route. Ultrafast PDF was our navigation app.”
Understanding these atomic routes is crucial for designing new materials. For example, in computer memory, materials need to switch phases efficiently without requiring too much energy and must resist unwanted changes over time.
Achieving PDF analysis at an XFEL involved a significant organizational effort. The team collaborated closely with experts at LCLS to determine the best beamlines for the PDF technique.
This collaborative effort included physicists from Columbia University, University of Wisconsin, Madison, DOE’s Argonne National Laboratory, and the UK’s Science and Technology Facilities Council.
Their successful experiments revealed a new phase of the quantum material. The scientists discovered that when they excited the material with a laser pulse, it transitioned to a previously unknown state. This finding resolved a long-standing debate about what happens when certain quantum materials are excited by a laser. The material was briefly disordered, even though it started and ended in an ordered state.
“The discovery of a transient state represents a new phase of the material, which lives for just a short time,” said Robinson. This indicates that an undiscovered, fully stable material might be nearby in composition.
Scientists are eager to explore these “hidden” materials and the potential of the ultrafast PDF technique. Future projects will continue to rely on multidisciplinary collaboration. As LCLS is upgraded to LCLS-II-HE, enabling even higher resolution molecular movies, the Brookhaven team is ready to optimize and expand the use of ultrafast PDF.
“There is international interest in making this a routine and successful technique,” said Bozin. “And we are looking forward to being a part of it.”
The preparation of samples was done at the Center for Functional Nanomaterials at Brookhaven Lab, and additional measurements were taken at the Advanced Photon Source at Argonne.
