An international team of researchers has for the first time directly observed how angular momentum moves through a crystal lattice, revealing an unexpected quantum effect that causes the direction of rotation to reverse. The discovery, made using powerful terahertz laser pulses, gives scientists a new perspective on the fundamental origins of magnetism and could ultimately help researchers better control advanced quantum materials.
The research was led by scientists from Dresden-Rossendorf-Helmholtzzentrum (HZDR), the Fritz Haber Institute of the Max Planck Society, and collaborators in Berlin, Dresden, Jülich and Eindhoven. Their discovery is natural physics.
A long-standing mystery about magnetism
In physics, quantities such as energy, momentum, and angular momentum are conserved. That is, they do not disappear or are created out of nothing. Instead, move between different parts of the system. Angular momentum is familiar in everyday life through rotating objects such as bicycle wheels and merry-go-rounds, but at the atomic scale it is deeply related to magnetism.
More than a century ago, Albert Einstein and Wonder Johannes de Haas demonstrated that changing the magnetization of a material could physically cause it to rotate. Their famous experiment showed that magnetic and mechanical angular momentum are interrelated. Since then, scientists have been trying to understand exactly how angular momentum is spread through the internal structure of solids.
Now, the researchers have directly observed the process unfolding inside the crystal.
Powerful laser reveals hidden atomic movements
The research team studied how angular momentum is transferred between lattice vibrations, the coordinated motion of atoms within a crystal. To observe this, scientists used ultra-powerful terahertz laser pulses to drive a single vibration into a circular motion. Then, with a second ultrafast laser pulse, they tracked how that motion interacted with other coupled vibrations in the material.
During the experiment, the researchers observed something surprising. When angular momentum is transferred from one vibration to another, the direction of rotation is reversed.
This effect arises from the rotational symmetry of the crystal lattice. In this system, certain rotational states are physically equivalent even if they are rotated in opposite directions. According to the researchers, this result serves as a direct quantum mechanical signature of conservation of angular momentum inside solids.
Strange “1 + 1 = −1” quantum effect
The material used in the experiment, bismuth selenide, exhibited particularly unusual behavior. The angular momentum coupled to that lattice vibration combined to produce a new rotation moving in the opposite direction at twice the frequency.
Researchers describe this as a type of “1 + 1 = −1” effect. In physics, this phenomenon is similar to the Umklap process, where the symmetry of the crystal structure effectively reverses the motion. Although the Umkrapp process is already known in other fields of condensed matter physics, this is the first experimental demonstration involving lattice angular momentum.
“I think it’s very elegant that the laws of physics are directly defined by the symmetries of nature,” says Olga Minakova, a postdoctoral fellow at the Fritz Haber Institute of the Max Planck Society and the lead experimental physicist on the study.
“For me, these are very exciting results. We have discovered something fundamentally new, which will hopefully be published in textbooks,” added Sebastian Meerlein, professor at the Technical University of Dresden, head of the HZDR Institute of Radiation Physics, and leader of the research.
Future applications of quantum technology
The discovery not only answers a long-standing physics question, but may also have practical implications. The researchers say the work could give scientists more control over ultrafast processes in quantum materials, potentially contributing to future information technology and next-generation memory devices.
Participating institutions include the Fritz Haber Institute of the Max Planck Society (Berlin), the Helmholtz Zentrum Dresden-Rossendorf, the Dresden University of Technology, the Jülich Center for Forced Learning, and the Eindhoven University of Technology (Netherlands).
