Scientists Discover Atoms Suddenly Spinning Backward in Quantum Experiment
Using terahertz lasers, researchers watched angular momentum move through crystals and found rotations can unexpectedly flip direction due to quantum symmetry effects.
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 where atoms suddenly reverse their spinning direction. Using intense terahertz laser pulses, scientists triggered tiny atomic rotations inside a quantum material and discovered that the direction of rotation can unexpectedly flip as momentum transfers between different parts of the crystal structure.
The study was led by scientists from the Helmholtz-Zentrum Dresden-Rossendorf, the Fritz Haber Institute of the Max Planck Society, and collaborators across Germany and the Netherlands. Their findings, published in Nature Physics, provide new insights into the fundamental origins of magnetism and could eventually help researchers better control advanced quantum materials for technological applications.
The team studied bismuth selenide, a material that displayed especially unusual behavior during the experiments. Using ultra-strong terahertz laser pulses, researchers drove one type of lattice vibration into circular motion, then tracked how that motion interacted with other coupled vibrations using a second ultrafast laser pulse. The surprising discovery was that as angular momentum moved from one vibration to another, the direction of rotation completely reversed.
This strange effect stems from the rotational symmetry of the crystal lattice, where certain rotational states are physically equivalent even when they spin in opposite directions. The researchers describe this as a "1 + 1 = -1" quantum effect, where the angular momenta tied to lattice vibrations combined to produce a new rotation moving at twice the frequency but in the opposite direction. This phenomenon resembles an Umklapp process, where motion is effectively reversed because of the crystal's symmetry.
The discovery resolves a longstanding mystery about how angular momentum spreads through the internal structure of solids, a question that has puzzled physicists since Einstein and de Haas demonstrated the connection between magnetism and mechanical rotation over a century ago. The research provides direct quantum mechanical evidence of angular momentum conservation inside crystals and could lead to new ways of controlling magnetic properties in advanced materials.
