An international team of physicists has directly observed a counterintuitive quantum phenomenon where atomic rotations within a crystal lattice flip direction while strictly obeying the conservation of angular momentum. This discovery, published in Nature Physics, provides new insights into the fundamental origins of magnetism and opens pathways for controlling quantum materials with unprecedented precision.
The Challenge of Tracking Angular Momentum
In classical physics, angular momentum is intuitive: a spinning bicycle wheel or carousel maintains its rotation unless acted upon by an external force. In quantum physics, however, angular momentum is a fundamental property that governs magnetism and atomic behavior. While conservation laws dictate that this momentum cannot be created or destroyed in a closed system, tracking how it moves and transforms within the complex structure of a solid crystal has remained elusive.
More than a century ago, Albert Einstein and Wander Johannes de Haas demonstrated a link between magnetization and mechanical rotation. Since then, scientists have sought to understand exactly how angular momentum transfers through the crystal lattice —the organized, repeating arrangement of atoms in a solid. Until now, this transfer process was largely theoretical, making it difficult to manipulate or predict.
Direct Observation via Terahertz Lasers
Researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Fritz Haber Institute of the Max Planck Society, and institutions in Jülich and Eindhoven successfully bridged this gap. By using powerful terahertz laser pulses, they were able to directly observe and manipulate the flow of angular momentum within a crystal of bismuth selenide.
The experiment involved a two-step process:
1. Excitation: One ultrafast laser pulse drove specific lattice vibrations into a circular motion, injecting angular momentum into the system.
2. Measurement: A second pulse measured the resulting connected vibrations in the crystal lattice.
This setup allowed the team to track how angular momentum passes between different vibrational modes of the atoms, revealing the mechanics of magnetism at a fundamental level.
The “1 + 1 = -1” Phenomenon
The most striking finding was that during the transfer of angular momentum between lattice vibrations, the direction of rotation flipped. This reversal occurs due to the crystal’s rotational symmetry. In certain quantum states, rotations in opposite directions are physically identical, allowing the system to conserve total angular momentum while reversing the apparent direction of spin.
The researchers describe this as a quantum “1 + 1 = −1” effect. When two units of lattice angular momentum combine, they create a new rotation with twice the frequency but the opposite rotational direction. This behavior is akin to an Umklapp process, where the symmetry of the crystal lattice effectively reverses the motion.
“I find it extraordinarily elegant how the laws of physics are directly dictated by the symmetries of nature,” says Olga Minakova, doctoral researcher at the Fritz Haber Institute and central experimental physicist of the study.
Implications for Future Technology
This observation serves as a direct quantum mechanical signature of angular momentum conservation in solids. It confirms that the symmetry of the material’s structure plays a critical role in determining how quantum properties behave.
Sebastian Maehrlein, head of department at HZDR and leader of the study, notes the significance of the finding: “For me, these are exceptionally exciting results. We have discovered something fundamentally new that will hopefully make its way into the textbooks.”
Beyond theoretical interest, this capability to control rotational direction using laser pulses could lead to:
* Better control of ultrafast processes in quantum materials.
* Advanced memory devices that leverage quantum states for data storage.
* Next-generation information technologies based on precise manipulation of angular momentum.
Conclusion
By revealing how angular momentum can reverse direction while remaining conserved, this study deepens our understanding of magnetism and quantum symmetry. It marks a significant step toward mastering the control of quantum materials for future technological applications.
