Researchers have found that atomic-scale rotations inside a crystal can unexpectedly reverse direction while still conserving angular momentum.

In a study led by an international team including scientists from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Fritz Haber Institute of the Max Planck Society, researchers directly tracked how angular momentum is transferred within a crystal lattice for the first time. Using intense terahertz laser pulses, they were able to control these rotational motions and observed a surprising effect: the rotation flips direction due to the crystal’s inherent rotational symmetry.

Figure 1. Terahertz-Driven Crystal Rotation with Reversed Motion Detected Stroboscopically

Published in Nature Physics, the findings provide new insight into how magnetism emerges and may enable more precise control of quantum materials in future technologies. Figure 1 shows Terahertz-Driven Crystal Rotation with Reversed Motion Detected Stroboscopically.

Understanding Angular Momentum in Solid-State Materials

Physical quantities such as energy, linear momentum, and angular momentum are conserved in closed systems, meaning they cannot be created or destroyed but only transferred or transformed. While angular momentum is often associated with everyday rotating objects, it is also a fundamental concept in quantum physics and plays an important role in magnetism.

Over a century ago, Einstein and Wander Johannes de Haas demonstrated that changes in a material’s magnetization can induce observable mechanical rotation, revealing a deep link between magnetic and mechanical angular momentum. Since then, researchers have sought to understand how angular momentum propagates through solid materials at the atomic scale.

A collaborative team of scientists from Berlin, Dresden, Jülich, and Eindhoven has now directly observed this process. Their work shows how angular momentum moves through a crystal lattice via coupled atomic vibrations, providing new insight into how magnetism emerges and remains stable in solid-state systems.

The research team demonstrated that extremely intense terahertz laser pulses can be used to precisely control the rotational motion of atoms inside a crystal. One pulse excites a lattice vibration into circular motion, while a second ultrafast pulse probes a related vibration, allowing the transfer of angular momentum to be observed in real time.

During this transfer, the scientists found an unexpected effect: the direction of angular momentum reverses. They attribute this phenomenon to the crystal’s rotational symmetry, where certain rotational states are physically equivalent even when they rotate in opposite directions. This behavior provides a clear quantum mechanical signature of angular momentum conservation within solids.

The experiments were carried out on the quantum material bismuth selenide, where lattice angular momentum can combine in such a way that the resulting motion has twice the frequency but an opposite rotation direction. The team describes this counterintuitive “1 + 1 = −1” outcome as analogous to an Umklapp process, in which the symmetry of the crystal lattice effectively flips the direction of motion. This marks the first experimental observation of such an effect involving lattice angular momentum.

Researchers involved in the study highlight the fundamental importance of the findings, emphasizing how physical laws are shaped by symmetry. They suggest that the results could eventually enable improved control of ultrafast processes in quantum materials, with potential applications in next-generation information technologies and memory devices.

References

1. https://scitechdaily.com/physicists-observe-strange-quantum-rotation-effect-that-defies-intuition/

Cite this article:

Janani R (2026), Physicists Detect Unusual Quantum Rotation Phenomenon That Challenges Intuition, AnaTechmaz, pp.401