Fundamental conservation variables such as energy, momentum and angular momentum determine the laws of our nature. These quantities always remain constant in a closed system and can neither be created nor destroyed, but only converted or transferred. While angular momentum can be seen in everyday life, for example on rotating carousels or when riding a bicycle, it plays a central role at the quantum level – including as the origin of magnetism.
A long-standing mystery of physics
Over 100 years ago, Albert Einstein and Wander Johannes de Haas observed in a famous experiment that a measurable rotational movement is triggered when the magnetization of a material changes – and thus that magnetic and mechanical angular momentum are linked. Since then, researchers have been interested in the question of how the resulting angular momentum is distributed inside a solid, i.e. how it is passed on via the crystal lattice – the regular arrangement of atoms.
Now, an international team of physicists from Berlin, Dresden, Jülich and Eindhoven has succeeded in directly observing this process for the first time. The researchers show how angular momentum is transferred between different lattice vibrations – collective movements of the atoms in the crystal. This provides an important basis for understanding how magnetism is set up and stabilized in solids.
Targeted control of angular momentum with terahertz laser light
In addition, the team was able to control the direction of rotation of atomic circular motions using ultra-strong laser pulses in the terahertz spectral range. These invisible laser pulses steer a specific lattice oscillation onto a circular path, while a second ultrashort laser pulse scans another coupled oscillation of the crystal. This revealed a surprising effect: during the transition between these oscillations, the direction of the angular momentum is reversed.
The reason for this is the special rotational symmetry of the crystal lattice: certain rotational states are physically equivalent, even if they have opposite directions of rotation. The experimental observation thus represents a direct quantum mechanical “fingerprint” of the conservation of angular momentum in the solid state.
For the quantum material bismuth selenide under investigation, an unusual picture therefore emerges: the angular momentum bound to the lattice vibrations – so-called lattice angular momentum – can combine in such a way that a rotation with twice the frequency but in the opposite direction of rotation occurs. This “1 + 1 = -1” corresponds to a so-called flipping process, in which the direction of movement is reversed to a certain extent due to the symmetry of the crystal lattice. Such a process has now been experimentally demonstrated for the first time for lattice angular momentum.
“I find it extraordinarily aesthetic how physical laws are directly dictated by the symmetries of nature,” says Olga Minakova, PhD student at the Fritz Haber Institute of the Max Planck Society and lead experimental physicist of the study. Sebastian Maehrlein, head of department at the Institute of Radiation Physics at the HZDR, professor at the TU Dresden and head of the study, adds: “For me, these are extraordinarily exciting results. We have discovered something fundamentally new here that will hopefully go down in the textbooks.”
In the long term, the results pave the way for the targeted control of ultrafast processes in quantum materials and could thus provide new impetus for future information technologies and innovative data storage.
Institutes involved
Fritz Haber Institute of the Max Planck Society (Berlin), Helmholtz-Zentrum Dresden-Rossendorf, Technische Universität Dresden, Forschungszentrum Jülich, Eindhoven University of Technology (Netherlands)
Find out more
Nature Physics – News & Views: Preserved rotations in solids
Publication
O. Minakova, C. Paiva, M. Frenzel, M. S. Spencer, J. M. Urban, C. Ringkamp, M. Wolf, G. Mussler, D. M. Juraschek, S. F. Maehrlein: Observation of angular momentum transfer among crystal lattice modes, in Nature Physics, 2026. (DOI: 10.1038/s41567-026-03274-8)
Further information
Prof. Sebastian F. Maehrlein | Head of High-Field THz Driven Phenomena
Institute of Radiation Physics at the HZDR
Tel: +49 351 260 2240 | E-Mail: s.maehrlein@hzdr.de
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Further links
👉 www.hzdr.de
Photo: O. Minakova/ S.F. Maehrlein/ B. Schröder/ HZDR
