When atoms dance: Researchers observe angular momentum in crystals

A carousel turns. If you try to stop it, you feel a resistance — angular momentum. The same physical principle also applies inside crystals, at the level of individual atoms. Yet for a long time, physicists could not directly observe how angular momentum is distributed and transferred there. Now researchers have made this process visible for the first time.

More than 100 years ago, Albert Einstein and his colleague Wander Johannes de Haas made a curious observation: when the magnetization of a metal rod changes, the rod itself begins to rotate. The experiment revealed a deep connection between magnetism and rotational motion. But how angular momentum is transferred between different atomic vibrations inside a crystal remained unclear.

An international research team involving the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and Technische Universität Dresden has now succeeded in directly observing and controlling this process for the first time. The results were published in the journal Nature Physics.

Laser pulses set atoms in motion

The researchers used special laser pulses in the terahertz range — electromagnetic waves invisible to the human eye that lie between infrared radiation and microwaves. These pulses drove atoms inside a crystal of bismuth selenide into circular motion. A second ultrashort laser pulse then measured how these motions evolved. The researchers observed something unexpected: when the crystal switched between two different vibrational states, the rotational direction reversed. Clockwise motion became counterclockwise motion.

The reason lies in the crystal’s internal symmetry. Certain opposite rotational states are physically equivalent because of the rotational symmetry of the crystal lattice. Physicists refer to this as an “Umklapp process” — a phenomenon in which the symmetry of the crystal effectively reverses the direction of angular momentum.