A pulse of light sets the tempo in the material. Atoms in a crystalline sheet just a few atoms thick begin to move - not randomly, but in a coordinated rhythm, twisting and untwisting in sync like dancers following a beat.
This atomic choreography, set in motion by precisely timed bursts of energy, happens far too fast for the human eye or even traditional scientific tools to detect. The entire sequence plays out in about a trillionth of a second.
To witness it, a Cornell-Stanford University collaboration of researchers turned to ultrafast electron diffraction, a technique capable of filming matter at its fastest timescales. Using a Cornell-built instrument and Cornell-built high-speed detector, the team captured atomically thin materials responding to light with a dynamic twisting motion.
Their findings, recently published in Nature, open new possibilities for understanding and controlling the behavior of moiré materials - stacked 2D structures whose unusual properties can be tuned simply by twisting one layer slightly atop another. The results provide insight into how light might one day be used to manipulate materials in real time, with implications for future technologies in superconductivity, magnetism and quantum electronics.
"People have long known that by stacking and twisting these atomically thin layers, you can change how a material behaves. You can turn it into a superconductor, or make electrons act in strange new ways," said Jared Maxson, professor of physics in the College of Arts and Sciences and co-corresponding author on the paper. "What's new here is that we enhance that twist dynamically with light, and actually watch it happen in real time."
Until now, researchers hadn't been able to directly observe how those layers physically respond to a burst of light. But in this study, the Cornell-Stanford team showed that the atomic layers can briefly twist more tightly together, then spring back, like a coiled ribbon releasing its energy.
"Previously, researchers thought that once you stack these moiré materials at a fixed angle, the whole structure is fixed," said co-corresponding author Fang Liu, project lead at Stanford, who created the moiré materials for this research. "What we have shown is that it is definitely not fixed at all - the atoms will move. In fact, the atoms inside each moiré unit cell will do a kind of circle dance."
To capture this fleeting dance, researchers used the ultrafast electron diffraction instrument built and refined in Maxson's lab, which fires intense bursts of electrons at a sample just after it's been struck by a laser pulse. This pump-and-probe method reveals how the atoms shift over time.
Key to the experiment's success was a high-speed, ultra-sensitive detector developed at Cornell: the Electron Microscope Pixel Array Detector (EMPAD). Originally designed for still images, the EMPAD was used in a new way, essentially becoming a hypersensitive movie camera for atoms.
"Most detectors would have blurred out the signal," Maxson said. "The EMPAD let us capture incredibly subtle features. What we were looking for could have easily been lost in the noise."
While Cornell built the tools and carried out the experiment, the specially engineered materials used in the study came from Liu's lab at Stanford. "There's no way we could have witnessed this phenomenon without combining materials understanding with electron-beam understanding," Maxson said. "We could build the best machine in the world, but without the right materials and the expertise to make them, it wouldn't happen. That's what made this collaboration with Fang's group so powerful."
Liu added: "Jared's ultrafast instrument is the only one that could actually see the moiré pattern, and Maxson's team even modified it in real time to make the experiment possible. This was a true collaboration."
Aaron Lindenberg, professor of materials sciences at Stanford, provided critical insights into the data, Maxson said. The data itself was taken by Cameron Duncan, Ph.D. '22, when he was a doctoral student in Maxson's group. Duncan continued to play a central role in analyzing the data and reconstructing the atomic motion from the complex diffraction patterns.
"We were the first to succeed in finding the ultrafast moiré signal because we customized our home-built hardware specifically to enhance its diffraction-resolving power," said Duncan. "It was satisfying to see our hard work pay off with this result."
For future work, Liu's lab has already produced a new set of moiré samples designed to push the limits of Cornell's ultrafast instrument even further. The teams are planning the next round of experiments to see how different materials and twist angles respond to light, work that could deepen their understanding of how to actively control quantum behavior in real time.
The measurements were carried out at Cornell's Newman Lab, with contributions from the Center for Bright Beams and the Cornell Laboratory for Accelerator-Based Sciences and Education. The project involved students and faculty across physics, applied and engineering physics, and accelerator science.
The EMPAD detector was developed by Cornell researchers David Muller, the Samuel B. Eckert Professor of Engineering; Sol Gruner, professor emeritus of physics (A&S) and colleagues. The work was supported by the Department of Energy, the National Science Foundation and the Defense Advanced Research Projects Agency.
Rick Ryan is a science communicator for the Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE).
