Mild causes atomic layers to do the twist

A pulse of sunshine units the tempo within the materials. Atoms in a crystalline sheet just some atoms thick start to maneuver – not randomly, however in a coordinated rhythm, twisting and untwisting in sync like dancers following a beat.

This atomic choreography, set in movement by exactly timed bursts of vitality, occurs far too quick for the human eye and even conventional scientific instruments to detect. The whole sequence performs out in a few trillionth of a second.

To witness it, a Cornell-Stanford University collaboration of researchers turned to ultrafast electron diffraction, a way able to filming matter at its quickest timescales. Using a Cornell-built instrument and Cornell-built high-speed detector, the crew captured atomically skinny supplies responding to mild with a dynamic twisting movement.

Their findings, lately revealed in Nature, open new prospects for understanding and controlling the conduct of moiré supplies – stacked 2D buildings whose uncommon properties may be tuned just by twisting one layer barely atop one other. The outcomes present perception into how mild would possibly someday be used to control supplies in actual time, with implications for future applied sciences 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,” stated Jared Maxson, professor of physics within the College of Arts and Sciences and co-corresponding creator on the paper. “What’s new right here is that we improve that twist dynamically with mild, and truly watch it occur in actual 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).