Physicists Observe Strange Quantum Effect in Crystal Atoms
Scientists directly watch angular momentum move through crystal for first time, discovering unexpected reversal effect.
An international team of researchers has directly observed how angular momentum moves through a crystal lattice for the first time, revealing an unexpected quantum effect that causes the direction of rotation to reverse. The discovery, made using intense terahertz laser pulses, gives scientists a new view into the fundamental origins of magnetism and could eventually help researchers better control advanced quantum materials. The study was led by scientists from the Helmholtz-Zentrum Dresden-Rossendorf, the Fritz Haber Institute of the Max Planck Society, and collaborators in Berlin, Dresden, Jülich, and Eindhoven.
In physics, quantities such as energy, momentum, and angular momentum are conserved, meaning they cannot disappear or be created from nothing. Instead, they move between different parts of a system. Angular momentum is familiar in everyday life through spinning objects like bicycle wheels or merry-go-rounds, but at the atomic scale it is deeply connected to magnetism. More than a century ago, Albert Einstein and Wander Johannes de Haas demonstrated that changing the magnetization of a material could physically cause it to rotate, showing that magnetic and mechanical angular momentum are linked together.
The team studied how angular momentum travels between lattice vibrations, which are coordinated motions of atoms inside a crystal. To observe this, the scientists used ultra-strong terahertz laser pulses to drive one vibration into a circular motion. A second ultrafast laser pulse then tracked how that motion interacted with another coupled vibration in the material. During the experiment, the researchers observed something surprising: as angular momentum moved from one vibration to another, the direction of rotation flipped.
The effect comes from the rotational symmetry of the crystal lattice. In this system, certain rotational states are physically equivalent even when they spin in opposite directions. According to the researchers, the result acts as a direct quantum mechanical signature of angular momentum conservation inside solids. The material used in the experiment, bismuth selenide, displayed especially unusual behavior where the angular momenta tied to its lattice vibrations combined in a way that produced a new rotation moving at twice the frequency but in the opposite direction.
Researchers describe this as a kind of "1 + 1 = −1" effect. In physics, this phenomenon resembles an Umklapp process, where motion is effectively reversed because of the symmetry of the crystal structure. The findings, published in Nature Physics, represent a significant advance in understanding how quantum effects manifest in solid materials and could lead to new approaches for controlling magnetic properties in advanced technologies.
