Researchers Observe Reversal of Angular Momentum in Crystal Lattice Vibrations

An international team of physicists has directly observed how angular momentum moves through a crystal lattice, revealing a previously unseen quantum effect in which atomic rotations unexpectedly reverse direction. The research, led by scientists from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Fritz Haber Institute of the Max Planck Society, used intense terahertz laser pulses to excite vibrations inside the quantum material bismuth selenide. By tracking the motion with ultrafast laser techniques, the researchers were able to watch angular momentum transfer between coupled lattice vibrations in real time. The experiment uncovered an unusual phenomenon: as angular momentum moved from one vibrational mode to another, the direction of rotation flipped. According to the researchers, this reversal is caused by the rotational symmetry of the crystal structure, where certain rotational states become physically equivalent even if they spin in opposite directions. The team compared the effect to a quantum version of “1 + 1 = -1,” because two rotational motions combined into a new motion rotating in the opposite direction at twice the frequency. The findings provide direct experimental evidence of angular momentum conservation within solids, addressing a longstanding question in condensed matter physics related to the origins of magnetism. The effect also resembles an Umklapp process, a known phenomenon in crystal physics where momentum appears reversed due to lattice symmetry, but this is the first time it has been experimentally demonstrated for lattice angular momentum. Researchers believe the discovery could improve understanding of ultrafast magnetic processes and eventually contribute to future quantum technologies, including advanced memory devices and information-processing systems. The study was published in Nature Physics and involved collaborations between institutions in Germany and the Netherlands.

Researchers Observe Reversal of Angular Momentum in Crystal Lattice Vibrations

An international team of physicists has directly observed how angular momentum moves through a crystal lattice, revealing a previously unseen quantum effect in which atomic rotations unexpectedly reverse direction. The research, led by scientists from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Fritz Haber Institute of the Max Planck Society, used intense terahertz laser pulses to excite vibrations inside the quantum material bismuth selenide. By tracking the motion with ultrafast laser techniques, the researchers were able to watch angular momentum transfer between coupled lattice vibrations in real time. The experiment uncovered an unusual phenomenon: as angular momentum moved from one vibrational mode to another, the direction of rotation flipped. According to the researchers, this reversal is caused by the rotational symmetry of the crystal structure, where certain rotational states become physically equivalent even if they spin in opposite directions. The team compared the effect to a quantum version of “1 + 1 = -1,” because two rotational motions combined into a new motion rotating in the opposite direction at twice the frequency. The findings provide direct experimental evidence of angular momentum conservation within solids, addressing a longstanding question in condensed matter physics related to the origins of magnetism. The effect also resembles an Umklapp process, a known phenomenon in crystal physics where momentum appears reversed due to lattice symmetry, but this is the first time it has been experimentally demonstrated for lattice angular momentum. Researchers believe the discovery could improve understanding of ultrafast magnetic processes and eventually contribute to future quantum technologies, including advanced memory devices and information-processing systems. The study was published in Nature Physics and involved collaborations between institutions in Germany and the Netherlands.

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