December 12, 2025 -- A joint team of scientists from Cornell University, Stanford University and the Max-Planck-Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg have shown that moiré patterns can move coherently when illuminated with light. Their study reveals ultrafast atomic twisting in 2D materials, overturning the existing view that light only causes random heating of the material. This work opens a new pathway for controlling electronic behavior with light in next-generation opto-electronic devices. This work has now been published in Nature.

A moiré is the wavy, shifting effect that appears when two patterns overlap. You’ve seen it when window screens create ripples, when striped fabrics shimmer, or when a phone camera makes a TV screen look strange. Remarkably, these moiré patterns can also be created using materials nearly a million times thinner than a human hair. At this tiny scale, they allow us to probe the strange quantum world using light. Understanding such phenomena at the atomic level - and on timescales as short as a thousandth of a billionth of a second - opens new pathways for harnessing quantum effects in future technologies.

Now, a joint team from Cornell, Stanford, and MPSD has used ultrafast electron diffraction together with atomic-scale simulations to observe atoms within a moiré twisting and untwisting in perfect synchrony. This marks an important step toward using light to achieve coherent, ultrafast control of collective electronic and atomic behavior in moiré systems. The results advance our understanding of how moiré patterns can be rapidly manipulated and how the resulting moiré potential shapes electronic properties - key insights for developing future technologies ranging from energy applications to novel magnetic devices.

Using ultrafast electron diffraction with femtosecond resolution, the researchers observed that the moiré diffraction peaks of MoSe₂/WSe₂ intensified within the first picosecond after light excitation, revealing a transient enhancement of the moiré distortion before it gradually subsided. Detailed analysis and atomistic simulations uncovered a terahertz-frequency twist-untwist oscillation of about half a degree - a remarkably large atomic-scale motion driven by light-induced interlayer forces. This finding defies conventional wisdom, which says that photoexcited lattice deformations should cause incoherent atomic motion - leading to heating and disorder - rather than coherent twist-angle modulation. By transiently modulating the moiré potential, this coherent, light-driven atomic motion opens a new route for dynamically controlling excitons, as well as polarons. It also to create correlated electron behavior in moiré materials on ultrafast timescales that were previously out of reach.

Cameron Duncan, the first-author experimentalist, said, “We were the first to detect the ultrafast moiré signal because we customized our home-built hardware to boost its diffraction-resolving power.” Prof. Jared Maxson from Cornell ads: “What’s new here is that we can enhance the twist dynamically with light and actually watch it happen in real time.” Prof. Fang Liu, project lead at Stanford, who created the moiré materials for this research joins in: “What we have shown is that the moiré pattern is not fixed at all – the atoms will move. In fact, the atoms inside each moiré unit cell will do a kind of circle dance.” Dr. Indrajit Maity, a former MPSD postdoc now at Newcastle, reflected: “It was extremely satisfying to see our prediction of atomic motion confirmed by this remarkable experiment.” Prof. Ángel Rubio, who supervised the atomic-scale analysis, added: “We’ve been studying various electronic properties of moiré materials - from superconductivity to magnetism. But until now, once a moiré forms, we assumed it couldn’t be dynamically twisted or untwisted. This study changes that.” He further noted that “controlling moiré dynamics with light could allow scientists to switch quantum states on and off or rapidly tune their properties.”