For the first time, scientists have directly observed angular momentum moving through a crystal lattice and watched atomic rotations suddenly flip backward. The discovery, made in Germany, challenges simple assumptions about how motion behaves at the quantum scale.
A laser pulse set atoms spinning like a Ferris wheel
Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), together with teams from the Fritz Haber Institute and partners in Berlin, Dresden, Jülich, and Eindhoven, used ultra-powerful terahertz laser pulses to drive atoms inside a crystal into precise circular motion. A second ultrafast laser pulse then tracked how that motion spread to neighboring vibrations. The experiment gave scientists a direct view of angular momentum moving through the material for the first time.
Then the rotation reversed for no obvious reason
As angular momentum transferred from one atomic vibration to another, something unexpected happened. The direction of rotation flipped. Two coordinated rotations combined into one spinning the opposite way. The reversal is not random. It comes from the underlying rotational symmetry of the crystal lattice itself. The effect creates a situation where what looks like two motions going one way produce a single motion going the other.
What this means for understanding magnetism
The finding connects to a famous experiment by Albert Einstein and Wander Johannes de Haas more than a century ago. They showed that changing a material's magnetization could physically cause it to rotate, proving that magnetic and mechanical angular momentum are linked. Since then, scientists have tried to understand how angular momentum spreads through solids. This experiment finally shows that process in action. The results were published in Nature Physics.
A new window into quantum materials
By watching angular momentum travel through lattice vibrations and reverse direction, researchers now have a clearer picture of how magnetism emerges from atomic motion. The discovery gives scientists a new tool for studying and potentially controlling advanced quantum materials. It does not rewrite the laws of physics. It reveals a strange and previously unseen step in a chain that connects spinning atoms to the magnetic properties of the world around us.
